615 Cycles & Still Grinnin'

Has it really been over a year since I last posted?

Well, I'm still here and so are the lithium cells.  615 cycles in and they are still working.  Ran a capacity test back at the end of June (487 cycles) and got 8.4kWh AC power out of the pack from full.  About 7% down on the 9.0kWh of the previous test at 180 cycles.  I did change the charging parameters in between though; charging to a higher Voltage but doing a very short absorption cut-off.  The net effect is less time held at a Voltage above the float level and I also reduced the float level a bit to prevent any "slow cooking" of the cells in the long sunny summer days (we finally had a summer in the UK - Yay!).

What prompted this posting was my latest toy and a return to storage deficits problems in the winter.

This bad boy is a switch mode 1-30V, 0-30A power supply.  It was about £317 delivered from Germany (but turns out it's a re-branded Chinese Manson unit).

It's survived the tests that have destroyed three other Manson sourced switch mode PSUs (branded as Maplins in the UK).  The 80W Maplins PSUs that you saw me using last year really don't like to have their DC outputs connected to a 24V battery when the output is turned off (or the AC power is off).  The spark of reverse current in-rush to the capacitors in the PSU destroys something in the Voltage control feedback loop and the next time you turn the PSU on, it immediately explodes with white smoke pouring out of the back (over-Voltage on the big capacitors, causing catastrophic dielectric failure and explosion of the caps).

Happily (luckily) the Maas HCS-3602 seems to have passed the two critical tests.  It was subjected to an AC power failure while connected to a 24V battery (the house inverter cut out as there wasn't enough solar power) and today I forgot to turn the PSU on before connecting it to the battery and a big spark of in-rush current went into its DC output.  I turned it on with trepidation, but all was well and it's sitting in the sun charging my spare Ritar lead acid AGM battery at 30 Amps as I write.

I did ask the supplier and the Manson tech support if the PSU was stable with reverse Voltage on its DC output and in-rush currents, but got stonewalled by both of them.  They'll be happy to discover my test findings but I don't see why I should be the one to test their product in the real world and have to trog back to the shop when it unexpectedly explodes.

The Ritar battery pack (formed of two 180Ah 12V blocks in series) has come in handy in the last few November gloomy days.  Just like in Star Trek, when the di-lithium crystals are depleted and they need a bit of power to get out of a pinch, Jim shouts, "Tie in the auxiliary power!".  I decided to "make it so" with these Ritars (mixing my Star Trek generations up now...).

They route emergency power through my old 1kW inverter and then via the new Maas PSU to hold the lithium battery pack at a level just above its low charge point.  When the sun comes up the next day, the solar charger only has to tickle the main battery for its Voltage to rise a bit and then the auxiliary power system cuts out.

I saunter along after breakfast and swap the PSU round to feed from the house 3kW inverter and charge the Ritar pack at a rate the solar array can cope with, using the current limiter on the PSU.  That's how I 'tested' the PSU with AC power failure.  I'd set the charge current too high and wandered off while a lonely bank of cloud came and spoiled the party.

The Ritars might be only good for 600 cycles, if I'm lucky, but as I only use them for a few odd days a week in the winter, that could be a useful way to save on just buying more massive lithium cells.  It's nice to have a "reserve tank" when you cock up your weather prediction and energy use and need to "run on fumes" for a bit.  The Ritar pack can even take a bit more abuse as I don't need to slavishly stick to the 50% DoD rule if only discharging it infrequently.  It will shorten the life, for sure, but even if I halve the life to 300 cycles; at 30 cycles a year (say) that's still 10 years use.

I'd previously tried a scheme like this but the problem was not using a 24V battery (I was tinkering with 12V batteries and a 150W inverter) and not having a high power 24V charger that could shift energy efficiently.

This Maas PSU measured over 91% efficient at 25.5V and 17A output.  It is also power factor corrected, so is inverter friendly.  I measured the DC-AC-DC conversion throughput from Ritar battery to lithium battery at about 78% efficiency (the 1kW inverter only being about 86% efficient, despite its claimed 92% rating).  The PSU barely breaks into a sweat.  It has a variable speed fan but doesn't become a fan heater in the room.

The rear has half decent binding posts to take M6 lugs, but annoyingly the screw tops are captive so I could not use the M6 ring lugs bought and had to resort to cutting off the ends to turn them into fork lugs.  I could only find M6 forks locally that would only accept 6mm2 cable.  I'm using 16mm2 cable to keep the voltage drop down at 30A, speeding up charging and keeping efficiency up (low heat loss in the cables and connections).

This power supply has some interesting features in that it has 3 memories for Voltage and current (that disable the front knobs - prevents accidental changes) and also a remote control terminal that would make it suitable as an AC charger controlled by a BMS that can remotely program the output Voltage, current and enable/disable the output.  The remote terminal takes 0-5VDC control signals and the maker kindly provided the special multi-way plug to fit the socket on the back.

6 Month Capacity Test

So, after 188 daily charge and discharge cycles, what's the score with these Lithium cells?

I decided to find out.

I waited until 3pm on Friday afternoon, when the bank had finished charging and had just gone into float stage and then hit the big red breaker on the PV array.  No charge input.  I then started recording the battery Voltage and AC kWh of energy consumed by the house from the Ofgem AC meter on the output of the inverter.  This meter is calibrated and modified to show down to 100Wh (0.1kWh).

I then did nothing special, other than going about daily life to see how long the battery would last until empty.  It never really gets empty as the protection cuts out the load before it gets below 3.00V per cell.  This leaves enough reserve for the battery to power the charge controllers and the battery monitors.

We did some laundry, which runs the inverter to over 2kW output and we did some vacuum cleaning and the fridge freezer did it's thing.  We even had the folks round for dinner and ran the power hungry video projector for a movie.

After 24 hours, the battery was down from the initial 27.00V to 26.09V and had delivered 5.6kWh of AC power.  So we kept going... And finally to bed.

Woke up the next morning (Sunday) and the power was off.  The logger laptop was still running on its internal battery and showed the inverter had shut down just after 7am.  So the house ran on battery power only with no charge input for just over 40 hours.  The battery was sitting at 24.40V off load and the AC meter read 9.0kWh.

Impressive for a system that I only rate as usable for 80% of the nominal 10.24kWh total capacity (if you take 400Ah x 3.2V x 8 cells = 10.24kWh DC power available).

Quite a bit of power is wasted by the inverter at low power output levels and at best it has an efficiency of about 92%.  So to get 9.0kWh out in terms of AC energy after all the conversion losses is pretty good.

A Lifetime of Power? (Part 2)

Ok, so the question, "How long do these Lithium ion batteries last?" was partially answered in Part 1 a few days ago.

But there's a second aspect to a battery's longevity...  Cycle life.

Cycle life is independent of calendar life.  You can leave a cell on a shelf and never use it once and it will die.  That's its calendar life... The time it takes to die of old age from the day it was "born".

Cycle life is how many cycles of discharge and recharge the cell can do before it wears out from working hard.  Work a cell harder and it wears out faster.  Treat it to an easy life, and it will die of old age.

Batteries are somewhat akin to people in that respect.

So...  People often ask then, "Well how many cycles can this battery do?". That's also a question that has the answer, "It depends...".

All cells are quoted as living for so many cycles if you treat them right.

The biggest impact on cycle life is from how much you discharge them in each cycle.  If you discharge good gel Lead acid batteries to 50% of their capacity (50% DoD), they will last for maybe 850 cycles.  Push them harder, by discharging to 80% DoD, and the same battery may only last 500 cycles.  In each case the battery does not suddenly die, but it's ability to hold and deliver power is eroded.  When the battery capacity has dropped to 80% of it's original rating, it is considered "near dead".  This is the case for Lead acid batteries because they then rapidly get worse after that level of damage.

Lithium ion cells vary in their quoted cycle life depending on the particular chemistry.  The Winston Battery (LiFeYPO4) cells are supposed to be the longest living, with the Yttrium as the added ingredient that extends their life, even beyond the generally long life of general Lithium Iron Phosphate (LiFePO4) cells.  Claimed life for the Winston cells is up to 8,000 cycles at 70% DoD.

If used in a solar system that naturally has a daily charge and discharge behaviour, that would suggest over 22 years of daily use.  This may be longer than the calendar life of the cells though.  But it certainly suggests that, unlike Lead acid batteries, the Lithium ion cells I'm using now should die of old age before they expend their cycle life from over work.

Tests done by the department of control and telematics at the Czech Technical University in Prague have demonstrated over 13,000 actual cycles on an automated test rig that charged and discharged these cells to 10% DoD and 1.5C discharge and charge rates with no degradation of performance.

See the report here.

A Lifetime of Power? (Part 1)

How long do these Lithium ion cells last?

Good question and not a straightforward answer. It depends on quite a few things...

Firstly it depends on what type of Lithium chemistry you are talking about.  Some types of cell are optimised for power delivery (highest power from the smallest, lightest cell).  Some are optimised for energy delivery (the most energy stored per cell).  Some are optimised for safety (no metallic Lithium or dangerous oxides that promote incendiary-bomb-like fires). 

Many of these optmisations have an impact on the lifespan of the cells, both in their "calendar life" and their "cycle life".  Today, let's start with the issue of calendar life.

Calendar life is a measure of how long it takes for a cell to self-destruct due to chemical impurities that cause side reactions to the desired ones.  These side reactions gradually cause the cell to die and they start happening the day the cell is made.  Nothing you can do to stop it, but you can slow it down.  That's why AA cells and so on in the shops have a "best before" date stamped on them.  Alkaline cells have a calendar life of about 4 years.

The very best industrial Lead acid batteries have a calendar life of up to 13 years, but only if you keep them cool (under 25C).  For every 5C warmer you store them, you HALVE their calendar life!

Lithium ion cells generally have quite long calendar life of over 10 years.  But it depends on the chemistry and how they are stored.  Lead acid batteries like to be stored in a cool place, fully charged and topped up regularly.  Lithium ion cells like to be stored in a cool place and at a low state of charge (30-50%).  But you have to be careful that they do not become over discharged (by attached monitoring chips or other circuits).  They don't like being stored on charge all the time and in a hot place.

Unfortunately, that's often the kind of treatment they get.  Laptop batteries are notorious for short calendar life of just 2-3 years.  This is due to poor storage conditions.  They are installed in the machine when used on the mains all the time.  The cheap chargers keep the battery on charge the whole time (even when the laptop is turned off) and the heat from the motherboard can be extreme (45-55C locally inside the unit, near one or more cells of the battery pack).  It would be better to half discharge the battery, take it out and keep it somewhere cool until needed.  But then who does that with a laptop?  It's very inconvenient to put the battery back in and charge it before needing to use the machine away from the mains.  So people "abuse" the battery for convenience sake and complain when it dies even though they haven't used it for many real discharge cycles.

Some Lithium ion cells are more sensitive to storage conditions (especially temperature) than others.  Lithium Iron Phosphate cells (like the Winston ones I'm using) are relatively "ok" with warmer temperatures.  It does not have as big an impact on their life as for some Lithium metal oxide cells, which degrade more severely with heat.  These metallic oxide cells are historically the types of cells used in laptops, so not a good combination...  Cells that don't like heat, stored permanently in a warm / hot place.

It's also why the sealed lead acid batteries in my computer UPS (uninterruptable power supply) used to die every two years... It gets hot (35C) in the box where they are housed.  If I keep the batteries outside the box and extend the wires into the box, the batteries last for at least 4-5 years.  The culprit was temperature.  After all, the batteries never discharged as they were for emergencies only.

So... My Lithium home energy battery bank is kept in a cool part of the house and I keep it shaded from the sun.  The inverter is positioned so that it does not blow warm air at it.  At temperatures ranging from 17-23C there (recorded by the charge controller) the battery pack should last for well over 10 years.  Nobody knows for sure, because they've not been in production long enough to find out but I'll be one of the first to know!

Most testing of calendar life is done by deliberately running the battery at higher temperatures in "accelerated testing" and then estimating the life at lower temperatures, assuming that all the chemical reactions slow down in a linear way with temperature.

The other aspect to calendar life is storage charge level.  As mentioned, different types of cells like to be stored at different charge levels.  Lead acid likes to be stored fully charged and Lithium likes to be stored at 30-50% charge.  This makes Lithium ion cells more suited to renewable energy storage, especially solar power.

Solar batteries are by nature only charged during the day and then discharged at night.  The ideal off grid solar power system has a battery that is never fully empty but also never kept fully charged.  If it did reach full charge, it would mean no more energy could be stored and so sunlight would be wasted.  In that case, either the solar array is too big for the battery or the battery is too small for the array.  In Winter, most solar batteries struggle to maintain anything close to full charge and may spend days or even weeks at low to middle charge levels.

As you can see, Lithium ion cells fit this model much better than Lead acid cells.  Lead acid cells are quickly damaged by not being kept fully charged 24 hours a day.  Lithium ion cells PREFER not to be kept fully charged for longest calendar life.

Next time I'll discuss cycle lifespan.

Or why not talk to us at Sustainables4U about your energy storage needs!

Still No Trouble at t'Mill

131 daily cycles completed on my lithium battery bank.  No drama, no problem.  Not very interesting subject matter for a blog.

After tinkering with various settings of charging that only seemed to provoke one cell to want to wander off towards "higher ground", I reverted back to the original scheme of charging to 3.50V per cell (28.0V for the pack) but only for 30 minutes instead of up to 60 minutes.  All charging for longer seemed to achieve was to make one cell go high and cause the rest of the pack to drop (as the charger wound back the power).

Charging to the original Voltage and keeping the time short seems to be the best way.  It also has the side benefit that the water heater comes on sooner.

I'm now going into business making these things!

I'm working with a friend who runs Sustainables4U and we're packaging these storage systems for stationary (not moving from the coal shed) and mobile (on a trailer or in the back of a van) applications.

We've taken delivery of a batch of 200Ah Winston cells and will make a prototype portable generator.  Something you can use at a building site or at a festival or even an eco show in a field that will make mains electricity to use without the noise, smoke and smell of a petrol generator - the sort that are always burbling behind burger vans at car boot sales...

We'll have a sort of flight case on wheels that will hold either 4x 200Ah cells or 4x 400Ah cells to give 2kWh or 4kWh of usable energy storage and a 1kW or 3kW pure sine inverter respectively.

I've even invested in some PCB CAD software to turn out a proper version of the inverter interface board so it won't even be bodged together with stripboard and bits of old string.

More Charging Experiments

Not much to report for a few weeks... Thankfully.

The Winston battery pack continues to do nothing other than sit there and do it's job without fuss, chemical smells, or sudden death.

April's final generation figures were interesting in that they were the same as March's figures.  That's interesting because March was one of the sunniest on record and April was one of the rainiest on record.

May is turning out to be a mixed bag and a bit disappointing.  We had a long run of gloomy days and the main lithium bank hit the bottom a few times.  On one run of bad weather I also depleted both of the backup Ritar lead acid batteries as well, using an old 1kW inverter to run a pair of the lab power supplies to charge the lithium bank during the night.

The lithium battery protection works well, sounding the police car siren at low battery and then shutting down the inverter when the weakest cell gets to 2.999V.

Then during the day after using the Ritar battery, I turn the lab power supplies the other way round and use the solar to recharge the fragile lead acid batteries first.  It doesn't matter if the lithium bank sits at the bottom for a few days without being charged much but the lead acid ones need charging as soon as possible.

I might set up a more permanent hybrid battery system to make use of the lead acid batteries as a "battery of last resort".  Used that way they might last for many years, if only discharged once in a few weeks, rather than every day.

I've also been playing with the lithium bank charge settings.  Cell no. 8 has taken to reaching full charge a bit before the others.  This is a feature of the bank being bottom balanced and the longer absorption charge times I've been playing with (now up to 50 minutes).  At 28.0V absorption level, the cell was getting up to 3.58V before charge end, while the others were getting up to 3.51V.  But cell no.8 rockets up to this high in a few minutes at the end of the 50 minutes, causing the other cells to actually fall a bit.  So I've been winding the absorption Voltage down, a bit at a time, to see at what point the cells will remain close together.

At 27.7V, or 3.462V per cell, this seems to be the case.  Today the pack sat at absorption for the full 50 minutes and all the cells remained fairly close together in Voltage.

Bumping Off the Limiter

Just pushing the boundary of the Winston Battery and charging system.

The system has been in and running stable for a over a month now, so I thought I'd try to see if the end Voltage could be raised a bit from the conservative 3.50V per cell.

I have been increasing the time that the charger spends in the constant Voltage absorption phase from 10 minutes up to 30 minutes now.  But I wondered if it might be possible to increase the final Voltage and shorten the charge time.

So I increased the final Voltage to 3.525V per cell (28.00V to 28.20V) and decreased the absorption charge time from 30 minutes to 25 minutes.

Today was extremely sunny again and the battery reached the absorption Voltage by 11:20am.  The CellLog8s was set to slightly higher cell and pack Voltage upper alarm limits (3.57V and 28.60V).  But the alarm tripped all of a sudden at about 20 minutes of absorption.

A couple of cells seemed to be climbing up the steep part of the charge curve and the alarm tripped on the cell differential being greater than 60mV.  I increased the alarm points to 3.58V per cell and 28.70V pack Voltage and 70mV differential.  After that the alarm did not trip again, but the the charger soon flipped to the lower float mode so I couldn't observe if the high cells were going to continue to run away.

So it seems that the original 3.50V per cell Voltage is about as high as you can go and still see the cells track each other fairly closely (under 60mV difference).  A longer time at that constant Voltage at a tapering charge current seems to keep things in line while allowing the pack to get close to full. 

Pushing the pack to any higher Voltage just causes the cells to amplify their differences in the final phase of charging and could risk one cell getting too far ahead.  This would falsely increase the pack Voltage and ironically prevent the other cells from being more fully charged as the charger would cut back the current too sharply, too early.

So the original settings are back on the chargers and I'll watch them some more to see if the current 30 minute charge is long enough.

Emergency Reserve Battery

Well, after all this time, it was bound to happen...

We had a run of partly cloudy days that then ended with a gloomy rainy day.

I didn't notice that the lithium battery was getting very low until the "5 minute warning" police siren sounded at after midnight.  The fridge had just clicked off on the thermostat and we were about 20 minutes away from a forced shut down (the fridge start up current would push the battery down below the CellLog8s alarm set point).

So I switched the fridge and a few other things back to the mains and had a rummage in the "lab" for an old 200W 12V inverter.  This still had some crock clips attached permanently to its inputs (hadn't got round to cannibalising it for the wire, fuse and clips for anything else!).  Then I plugged one of the spare 80W lab power supplies into it and set it for 3A limit at 24.5V.  Just enough to hold the reduced house load up, without trying to charge the lithium bank at all.

I connected the lab power supply to the left over charger inputs on the terminal board between the lithium bank and the house inverter and hoped that it would be enough until dawn.

Despite being a quite big lead acid battery (180Ah).  I was using only one at 12V to power a 24V system, so its effective capacity is only 90Ah at 24V.  And you can only really use about half of that, so about 45Ah at 24V.  Still, that's 15 hours reserve at a 3A load from the house inverter.

Not efficient at all, being triple converted, but it worked for 5 hours of emergency load support before dawn.

With the sun up, I reversed the lab supply, plugging it into the unmetered AC output on the house inverter (so as to not count the kWhs put back into the reserve battery) and charged it back up at a constant 5A.  This way the reserve battery takes priority over the lithium bank.  The lithium cells don't mind being left at low charge for ages (they actually rather like it), but the lead ones need to be fully recharged as soon as possible.

This could be a start of a hybrid battery system, where the lead acid battery is only used infrequently to back up the house battery in an emergency and then recharged as a priority.  Used like that, a lead acid battery will last for many years.

Having three of these lab power supplies available, I could support a load of up to 240W for a short time (up to 5 hours per 180Ah 12V battery).  Or I could dig out the 1kW 24V inverter and use both of the 180Ah batteries at the same time.  But I rather like the ermm... "compact" arrangement of a single battery and a tiny inverter that can just be moved around without having to mess about with bolting batteries together.

In theory, the SmartGauge alarm relay could turn the emergency reserve chargers on, but the lab supplies have a safety feature that means they default to the outputs being off when they first power up.  So someone would still have to be there to press the "go" button on them.

Mercifully, this new Ritar lead acid battery that I used is behaving better than the one I sent back to the supplier a couple of weeks ago. 

After much e-mail to-ing and fro-ing, and measuring and testing, the supplier gave in and agreed that the battery that was gurgling and farting under only moderate charge and making a terrible noxious stink was faulty and so swapped it for a new one.

I didn't have the heart to tell him that I wasn't planning on using either battery any more, as I'd gotten a lithium battery bank in the meantime :D.

Balance Tracking Data

Well, the pack has been in for a little over 3 weeks now and has been cycled to various depths from full to nearly empty.  It's been bottom balanced at 3.000V once, by hand with nothing more advanced than a test meter and a big light bulb.

I set the CellLog8s doing what it does... Logging data at 15 second intervals from all 8 cells plus the pack Voltage.  And here's the trace from the night of the 7th March to the night of 13th March.  Again, you can click on the graph to open a bigger view.

 

You'll notice that on the first night, the pack almost bottomed out before starting to charge on the 8th.  Just briefly it got down to about 3.1V.  Below you can see the zoomed in view of that discharge "spike".

The cells show good tracking with a spread that is just 15mV from the highest to the lowest cell in the pack.  This differential shrinks to about 7mV when under lower load.

The chart above shows the opposite end of state of charge at the 12th March.  Here you can see the pack reaching just shy of 27.80V and the spread of cell Voltages from 3.465V to 3.505V, some 40mV.

Remember that the pack is bottom balanced, so there will be more variation at the top of charge.  As long as we always undercharge the pack, this isn't a problem and requires no active balancing or Voltage limiting.  If we tried to do this, we'd be top balancing the pack and then would mess up the bottom balance.

As the charging current at the 28.00V target has been pretty massive (over 70A), I think the maximum regulation Voltage on the chargers was a bit too low.  They always seem to stop at 27.85V, measured on the CellLog8s and my DVM.  So I've tweaked the settings a bit.  The SSMPPT-15 and TSMPPT-60 have had their maximum regulation limit raised from 28.40V (3.55Vpc) to 28.80V (3.60Vpc).  Hopefully this will allow the terminal Voltage (at the charger end of the cables) to go high enough to raise the battery terminal Voltage to the desired 28.00V level.

I also tweaked the timer on the SSMPPT-15 so that it charges for 10 minutes (rather than 1 minute) before cutting out on the extended absorption timer.  You can see from the high charge chart that the SSMPPT-15 quit assisting the charge early and the TSMPPT-60 wasn't quite able to hold up the Voltage.  The big charger is still aiming to charge for 20 minutes, but now the small charger will support it for half the time.  Of course, the 10 minutes is not concurrent with the 20 minutes of the big charger, as the Voltage set point on the SSMPPT-15 is 0.1V lower at 27.90V.  It reaches this point while the TSMPPT-60 is still in bulk charge mode, trying to get to 28.00V.

Bottom Balancing the Pack

Having got the low Voltage disconnect protection sorted, it was time to finish the initial preparation of the battery pack.

The strategy here is to have the cells balanced closely at their bottom or empty state.  That way you can use them closer to empty while not tripping the protection.  With this method, no active balancer for controlling the top / full Voltage of the cells is required, provided that you normally under charge the cells.  Only a way to disconnect the load at the bottom of charge is needed.

So I turned off the solar charger for a couple of days and ran the house as normal on battery power to run them down (without wasting the energy).

It's incredible how efficient they are at absorbing energy.  They are always operated in the bulk part of their charge curve (ignoring the short 20 mins at the "top" constant Voltage of 28.00V).  And over the last few days have sat for quite some time soaking up all the power the chargers could throw at them - up to 76 Amps without the cells getting the slightest bit warm or even changing terminal Voltage much.

No need for the "battery protection" dump loading that I used to do with the old lead acid bank when the charge current was too high during bulk and absorption charging.  It does mean that less energy is diverted to the water tank now though.

Here you can see a trace from the Morningstar logger, showing the dramatic cliff-edge that lithium batteries fall off when nearing empty.  Click on the graphs for bigger versions.

 

That was the pack Voltage.  A close up of the data from the CellLog8s shows the detail of each cell at the end point.  You can see where the cells started to nose-dive and then the alarm tripped on one cell reaching 2.999V.  The pack then recovers a bit and I then start the bottom balancing, using nothing but a DVM, the CellLog8s display and a 60W 12V light bulb to hand drain each cell to the same level (3.000V plus or minus about 3mV).

Then I left the inverter off for a day with the chargers on and then another very sunny day with low inverter load, finally putting a total of 11.3kWh into the system (some went to the fridge freezer, and a bit more to the water heater, late on the second day). I counted about 7.7kWh into the battery bank itself.  You can see how it soaked it up relentlessly on the second day.

The top trace is pack Voltage, rising to 27.8V on the second day (still not quite reaching the "full" charge Voltage of 28.0V).  Middle trace is charger combined power output.  Bottom trace is solar strength % (red), TSMPPT-60 charge Amps (blue) and SSMPPT-15 charge Amps (green).  Charge current maxing out at over 70 Amps for quite a lot of the day.

One final graph shows the dramatic "hockey stick" charge curve as you get to the very full state of a cell. It was taken during logging of one of the cells during the initial charge, where I monitored the final 40 minutes of charging from 3.65V to 3.97V and then the current taper at that constant Voltage.

Charging to 4.00V is not recommended for regular charging as it is very close to saturated charge and then the cells get damaged quickly after that.  This is why I have set the target charge Voltage much lower at 3.50V per cell.  It's the start of the saturation zone.  To charge much higher than this requires an active top balancing charger but only gains you a small additional storage % of capacity.

The "5 Minute Warning" Alarm

The CellLog8s LVD is fine and works well at protecting the battery bank from over discharge but it doesn't give you any warning that the lights are about to go out.

So I started thinking about some kind of pre-alarm that would give me a few minutes warning (under heavy load) that the battery was nearly depleted.  I could then turn off some big loads and buy some time.

The old SmartGauge Voltmeter has a programmable relay in it too.  But it only works on pack Voltage.  I've been using it as an obvious visual reminder of pack Voltage (it's no good at reading SoC for non lead acid batteries).

Then I started looking around the house for something to use as a buzzer or alarm sounder that the relay could activate so that you'd get the message...

I found a small toy sound effect thingy that you press the top and it makes a cool police car sound.  I actually found another one that my wife had that made steam train noises and in fact a whole load of similar sound chip enabled things, like a Dr Who Darlek bottle opener that says "EXTERMINATE!" when you close a circuit (with the bottle top) and a Christmas card that plays George Michael's "Last Christmas"...

But I decided to go with the police car :D

Taking it to bits was very easy and then all I had to do was solder wires on to the existing switch contact and run these out to the alarm relay on the SmartGauge and program the chosen low Voltage alarm.

The toy still uses the same two 1.5V button cells to make the noise.  The SmartGauge does not provide any power to the relay contacts so external power for the alarm or whatever is needed.  If the batteries in the alarm go dead, it does not affect the safety of the battery bank as this alarm is just for information.

When the relay contacts close, the police siren only goes off once for a few seconds and then stops.  This is a good thing!  Saves the batteries in the alarm and prevents bricks being thrown at the thing for sounding too much once the message has gotten through to whoever is within ear-shot of it :D

CellLog8s "One-Shot" LVD

With little prospect of the firmware being fully fixed, I decided to implement a work around to make the CellLog8s at least work as a "one shot" Low Voltage Disconnect (LVD) for the inverter.

The problem was that without proper hysteresis in the CellLog8s firmware, the alarm output would flip-flop in an unstable way near the alarm set point value.  So I had to devise a way to iron out this transition behaviour and make it trigger once only.

I found a little DPDT latching relay in Maplins that does the trick, but I had to rebuild the interface board that I'd made previously.  In the video you can see the new circuit.

In this new version, the inverter receives an "Enable" signal from the interface.  This just connects to the common pin on the Remote/Off/On select switch on the inverter front panel.  The new relay is stable in both positions of its double throw output and has two coils, one to select each output mode.  It only needs a single short pulse to cause the state change and then further pulses have no effect (as you have to energise the opposite coil to change the state).

So, you press a button to "Enable" the inverter (or reset it, if it had tripped).  This just flips the relay "on".

The 680 Ohm resistors in series are because the relay has 12V coils with a measured DC resistance of about 700 Ohms.  They weren't quite equal though and (by luck more than judgement) I happened to pick the coils in such a way that the alarm state coil is the "stronger" one, so that when the alarm state is "true", the "reset" button does not work... Useful that.  You can't force the inverter to start up when something is wrong.

The second pole on the relay is just used for the LED indicator.

The output of the CellLog8s alarm port (now set to Normally Open) sits and does nothing until the set point is reached, at which point it will trigger.  The alarm port goes to closed state and triggers the "Disable" coil on the relay.  The LED goes out and the inverter is forced to shut down.  It cannot restart until the alarm condition has cleared and the reset button is pressed on the CellLog8s interface (and of course after you've investigated why it tripped!).

As programmed in the CellLog8s now, either a pack LVD or a cell imbalance alarm can cause it.

Next, all I had to do was hack the inverter to accept the Enable signal...

Here's another video of me "hacking" the inverter to get at the switch on the front panel and wiring in the connection to the new interface.  A bit of testing, too.

Now the battery is fully protected from any low Voltage drain from the inverter (the main load).

The advantage the new system has is that the relay consumes no power to hold the inverter in the enabled state.  Just a pulse of current from the reset button and then nothing.

In the alarm state, the other coil consumes 20mA for as long as the alarm is triggered. In practice, the load from the inverter is usually such that the pack or cell Voltage sags to the limit and triggers the alarm.  Instantly, the load is disconnected and the pack/cell Voltage recovers enough to rise above the alarm set point, which cancels the alarm.  Now the relay consumes no power again but is latched in the "Off" state.

In theory, the charge controllers, the SmartGauge, and even the CellLog8s itself could cause the pack to drain down and be damaged. But as I've set the cut-off Voltages quite high (24.0V pack and 3.00V per cell), it would probably take several days with no solar charge (the PV disconnect breaker thrown) to drain the last few Ampere.hours from the pack and damage it.

More Settings Tinkering

No pictures or video today... Shock, horror!

A bit more tinkering with charging settings, increasing the float Voltage again to 27.00V (3.375Vpc) seems to null out discharging with the long sunny afternoons and water heater running.

I also changed the timers a bit.  If there was not much load on the battery for a few days, even charging for 10 minutes per day to the upper Voltage limit might start to cumulatively over charge the cells.  So I'm now making use of the absorption extension timer.

If the pack Voltage never drops below 26.60V (3.325Vpc) during the night, the next days absorption timer is set for only 1 minute top charge.  This effectively eliminates the possibility of cumulative over charging.  If we went away for a long time, I'd shut the whole system down.

If "normal" amounts of charge are taken from the pack, the Voltage drops to under 26.60V and the next days absorption timer assumes an extended absorption timer setting of 20 minutes.

I had noticed that 10 minutes wasn't resulting in a very large fall-off in input power to the battery during the constant Voltage phase, so extending the time seemed appropriate (with the new safeguard of a much shorter default timer).

The small SunSaver 15 Amp charger has now had its absorption timer defaulted to 1 minute for any condition.

With the clear blue days we've been having, I've seen total charge rates as high as 72 Amps, and at the top of charge, the battery pack doesn't need the assistance of the small charger for long before the Tristar charger can hold the constant Voltage.

Progress on the CellLog8s Front

Some general tuning and tinkering over the last couple of days.

After watching the battery pack charge and float, I noticed that it was starting to discharge a bit more than I'd like when floating.  So I increased the float level by 0.1V to 26.90V and then observed the next charge day.  This time, rather than discharging at 3 Amps, it settled into a discharge of around 1 Amp.

Meanwhile, things are moving forward with the CellLog8s problem and development of an interface to my inverter for low Voltage disconnect.

Junsi, on the OEM RC Groups thread, managed to replicate the unstable alarm port output problem in his lab and set about looking for a remedy for it.

Not 24 hours later, I received a PM on the forum and then a new beta firmware code to test!  Now that's FAST.

Uploaded the firmware (v2.09) to the Cellog8s, now connected to all the cells in the pack, and played about with the battery until 3am to see how it was now.

Much better, is the answer.  But still a ways off being useful without bodging some external electronics to fix the remaining problem...

At least now the alarm triggers reliably when some way above / below the set points.  But there's still a lot of instability at the set point.  The alarm trigger has no hysteresis in it.  With a big battery you get VERY slow changes in Voltage and then the battery can spend a long time transitioning across the set point (and I mean a few minutes spent dropping the pack Voltage by 1-2mV at a 150W load!).

Anticipating that they would fix the software, I built a follower relay module (that just follows the sense of the alarm output of the CellLog8s).  The relay itself came from an old broken mains timer switch and was convenient as it had a 24V DC coil.

The instability of the CellLog8s alarm output made the relay chatter noisily near the set point, with the transition instability.

If I used it "as is", it would probably do what Jack Rickard's VDR (voltage dependent relay) did to his test load and A123 battery pack.  The load and pack cycled on and off furiously at the switching set point, and then both of them exploded with the stress of a few hundred Amps being pulsed rapidly.

The addition of a programmable variable for alarm set point hysteresis would eliminate this problem.  If you have a low Voltage alarm trigger point at 24.00V and hysteresis of 0.5V, then the alarm will trigger ONCE at 23.99V, and then stay triggered in the alarm state until the pack Voltage rises to 24.51V.

With no hysteresis, your alarm triggers multiple times as the Voltage floats around the 23.895 to 23.995 zone, in exactly the same way you see a DVM last digit toggle randomly between two values when it is close to the threshold of the next digit. Fine for a DVM display (even desirable as you can interpret the toggling as meaning the value is very close the the transition point)... VERY BAD for a load controller.

Initial Tests & Programming the Morningstars

First day of solar charging the new Winston Battery pack.

Got the CellLog8s wired up into the individual cells during the night, ready to monitor them for any over charging.

It was quite simple in the end.  I'd thought about making lugs from copper strip.  I'd thought about soldering wires on to the cell terminal straps.

In the end, all I had to do was splice on thin stranded bell wire to the 9 way header cable (cutting off the other plug), and then insert the thin stranded wire into the laminated cell strap sandwiches. Each strap is made of several copper plates held together with heat shrink tube (and the terminal bolts of course). When bolted back down, the wires are securely held and aren't going anywhere.  Well, maybe at least until one of the cats decides the spaghetti is a toy and pulls it out or trips over it!  I wrapped the cable in some spare cable tidy spiral-wrap stuff.  I might shorten the leads later but was toying with the idea of having the monitor mounted somewhere above the battery pack.

Got the two Morningstar charge controllers re-programmed with very conservative settings to start to get a feel for how the pack would behave.  It was very sunny from the get-go, with the chargers putting out a combined 55 Amps into the battery.  No magic smoke ensued.  For that matter, nothing even got the slightest bit warm; none of the cables; the new "200" Amp battery disconnect breaker; any of the cells.

I checked the live data feeds from the two chargers with the MSView software to double check that they were adhering to the expected target Voltages and temperature compensation slopes.  Some months ago, I'd previously had an issue with one of the programming wizards that had a bug in the temperature compensation settings, but Morningstar were quick to provide a bug fix after I mailed their software support team.

Basic settings are as follows (per cell values in brackets):
Absorption Target Voltage: 28.00 (3.50)
Absorption (Constant Voltage) Cumulative Time : 10 mins
Absorption Time Extended: none
Float Target Voltage: 26.80 (3.35)
Float Exit Time: 12 hours
HVD (trigger): 29.00 (3.625)
HVD (release): 26.40 (3.30)
Max Regulation Voltage: 28.40 (3.55)
Temperature Compensation: Pack -60mV/C (25C to 80C)

The smaller SunSaver charger has the same settings with the exception that the absorption target Voltage is set to 27.90V, so that this charger quits before the main one and then the main one is in the driving seat to control the finishing charge.  This is especially important because the SunSaver charger does not have remote terminal Voltage sensing.

An overview of the programming wizard in the MSView software is included in the second video below.  Speaking of which, today's video is a monster.  It was so long (23 minutes) that I had to split it into two as YouTube complained it was too long. Soon I'll be making blog videos almost as long as episodes of EVTV :D

Get some popcorn...

...And We're Back On!

And so the initial charging comes to a conclusion (finally!) and it was time to put the battery pack together in its new home, behind the sofa :D

Here's the video.

Not many photos, as I got carried away with video recording each step.

First problem was to fix the M14 cable lugs on the positive and negative battery pack flier leads that would go to the existing M8 stud terminal block.

Without the aid of a £200 industrial crimper to crimp the 120mm2 lug on to the 35mm2 cable, I resorted to using one claw hammer as an anvil and another to beat the sleeve flat on the copper.  I folded the end of the cable over in two, so that it was chunky enough to fit snugly in the sleeve.  Then I used my plumbers Pipemaster soldering iron to heat the sleeve up and silver solder the cable in place to guarantee a good connection.

I also soldered a couple of small wires into the lugs to use for the Smartgauge, the charge controller remote Voltage sensor and the CellLog8s logger.

Then it was just a case of using a bit of fine wet and dry sanding paper to clean off the oxides on the cell terminals and the copper link straps and bolt the thing up.  I held the wrench close to the socket so that it wasn't possible to apply too much force and only really tightened the bolts to the point where the spring washer was sitting flat and gripping the straps.  It's not like this pack will be bouncing down the road in the boot of an electric car, so there's no worry about vibration loosening the terminals.

I was more tense than usual when doing this bolting up.  These cells can muster 2,400A output without really blinking.  A ring or watch strap or 1/2" socket wrench with no insulation on it could make for some impressive fireworks.  Be afraid...  Be very afraid.  It's not the Volts that kill you, it's the Amps.

And so a while later... Actually, a lot later, because my mate James came round for the afternoon, I got round to finishing up the sanding, bolting up and wiring.  Here she is:

And a close up of the terminal block with the addition of the old home made shunt in the negative battery leg (the horseshoe shaped bit of 35mm2 cable).

The shunt is pretty accurate (well good enough to see what's going on) and using an extra 8mm stud gives me the flexibility to change the shunt for something else... Maybe a real shunt for a proper amp-hour counter or just a better home made shunt (now that I have an 80,000 count DVM to calibrate it with).

Just clipped the CellLog8s on the battery pack plus and minus to see what it would do with the full 27V on it.  Tomorrow I'll make up the special 9 pin lead to connect it to the individual cells.

I also programmed the two Morningstar charge controllers with my custom settings for charging this unusual battery bank.  More on that later...

The battery was at a high state of static charge after its initial charge.  It sat at 27.59V with no loads on it yet.  Turning on the inverter and loading it up to 35A briefly, brought the pack Voltage tumbling down to a steady 26.53V.  Releasing the load saw it recover to about 26.75V.  I make that about 6 milli-Ohms for the whole pack, including the cell straps terminal connections!

Mysterious Liquid

While lugging the cells around for their initial charge, I noticed something odd about them.  Three of them had free liquid electrolyte sloshing around in them, while the other five appeared to be free of the free liquid (if that makes sense).  That is, some cells appeared totally solid when you picked them up and tilted them, while others made a sloshing noise of liquid running from one end of the cell to the other.

Two of the ones I'd already charged had this liquid, but one that was waiting to be charged also had some liquid, so they came out of the crate like that...

So I weighed all the cells to see if the "wet" ones were any heavier than the "dry" ones.  Maybe they'd been overfilled?  But no, they were all the same weight to within 100g and some of the dry ones were heavier than the wet ones.

I mailed GWL, and asked them about the liquid and whether it meant that I should use the cells only in an upright attitude.  I know lots of people have mounted them on their sides in cars and the cells are often promoted as being "sealed" and suitable for mounting in any orientation except upside down.

The reply came back that some free liquid is normal in new cells.  It may have been evolved during the pre-charge at the factory or in pre-delivery "testing".  But the liquid should all be absorbed back into the cell plates and the cell become "dry" over a number of charge and discharge cycles.  Over charging or over discharging the cells will lead to lots of liquid and gas being evolved, which would cause the cell to swell up and vent. If the cell is on its side, it could spray the liquid out of the vent.  If it's upside down, it will almost certainly spray any liquid inside, out.

So... Nothing to worry about and the recommended installation position is "right way up", to allow emergency venting without loss of liquid in the event of serious fault or abuse of the cells.

Still Testing the CellLog8s

After the previous experiments with the CellLog8s alarm output, I posted a query on the RC Groups forum where the manufacturer provides support for the range of Junsi chargers and cell monitors.

They looked at my video and blog entry and suggested that the problem might have been a grounding problem in my wiring of the LED to the alarm port.  The alarm port negative needs to be referenced to the cell negative.

So here's my update on the situation.  I wired the LED +ve into the cell +ve terminal.  The negative of the LED goes to the +ve alarm port and the -ve alarm port goes to the cell -ve terminal.  The chargers are all connected to the same points too.

You can see the LED and its connection points in the photo above.

In the photo below you can see the problem.

 The alarm was set to trigger when the pack Voltage exceeded 3.65V.  It's now 3.95V as shown on the charger and the logger display is showing 3.973V, alternating with the alarm status "over".  But at the same time, it's not beeping an alarm and the LED connected to the normally closed alarm output is ON (indicating no alarm condition).

Here's today's video showing the erratic behaviour of the alarm port and beeper.

Flakey CellLog8s alarm

Today's episode:

Still charging up the cells one at a time...

When no.4 got to be full, I decided to play with the CellLog8s and its alarm output.  I'd noticed that when it gets to (and beyond) the set point of the over Voltage alarm, it would flash "over" on the display but not always beep.  It goes though random phases of beeping every 4 seconds (like it should) and then not beeping for a while.

So I set it up with the provided alarm output cable (with a tiny plug and tiny thin wires...) to a 12V LED.  This  has the current limiting built in for use as a panel lamp in cars. You can see the alarm port on the device and the external LED circled in green in the photo.

The plan is that the CellLog8s will be my low Voltage monitor, both for the pack as a whole and individual cells (as any cell that goes below 2.0V will be permanently damaged).

The 3kW inverter is my only load and has a LVD cut-off built in, but there are two problems with this.  The first is that it has a fixed cut-off Voltage of 21.0V, which is too low.  It's 2.62V per cell.  For high drain applications (0.5C / 200A discharge) 2.8V is the recommended cut-off.  For lower currents, the cut-off Voltage is actually higher.  The cell can be considered "empty" when it gets to 3.0V.  This means that for the many hours of a day where the inverter is drawing a mere 3-4A while doing not a lot, the worst case applies and I need to shut the thing down when it gets to 24.0V pack Voltage.

Then it's also possible for the pack to be out of balance and one cell get below 3.0V before the others as the pack nears "empty".  If I accurately bottom balance the cells, this shouldn't happen but I want to catch it if it does.

The CellLog8s does both of these things.  It monitors each cell Voltage, and it monitors the pack Voltage.  And the alarm output can be triggered at any programmable level.

Now, back to the test...  Because I'm charging the cells, I set the over Voltage alarm to 3.65V so that as the cell gets near to the end, it would alarm so that I could watch it finish and shut the charger down.  Just to see the alarm work.

Ideally, the alarm output would trigger and latch.  That way, if the threshold is crossed, the load will be disabled and then require manual reset before it could be enabled again.

Anyway, for the test, I had the alarm set to "Normally Closed" output.  This means that the LED comes ON when there is NO alarm condition.  This would mean the CellLog8 has power (to drive the output transistor) and the wiring is working.  This would provide a fail-safe "inhibit" signal to the inverter remote port.  When there is no alarm, it's safe for the load to run.

When the alarm is triggered (by low Voltage), the LED should turn OFF.  This would signal to the inverter that there was either a low Voltage alarm or that there was a fault in the CellLog8 (open circuit wiring or no power to the device).  In either event, the inverter should be disabled or shut down.

Well, as you can see in the video, it sort of worked.  The alarm was triggered at the appointed Voltage and the LCD display started flashing "Over" to tell me what kind of alarm it was.  The CellLog8 started beeping (as it should) and the LED turned off.  But... It then came back on and randomly turned on and off.

At first I thought it was a hysteresis problem (with the alarm threshold being crossed multiple times as the Voltage crept up) but with the cell well over the limit, the LED continued to randomly turn on and off.  No good.

Time to post a bug report on the RC Groups forum where the manufacturer hangs out.  They've been quite good at listening and producing bug fixes and new features for the device firmware but this is a pretty basic problem that should have been ironed out by now.

Failing that, I could work around the problem with an externally latching switch / relay, but if the software on the logger worked properly in the first place, it would reduce the interface complexity and so the number of points of failure.

Faster Initial Charging and CellLog8

Episode 02 of my video blog:

While charging the first cell, it quickly became clear that the 5A output of the bench power supply wasn't enough to get charging done in a sensible amount of time.  The cells appear to be delivered with 50% charge and so I have to put another 200Ah into them.  At 5A, this would take 40 hours at least.

I'd also had some concern expressed by Jack Rickard about such a slow charge rate being effectively a trickle charge that might not even be able to push the cells to the top and could even overcharge them in some way by trying for too long.

There was some conflict in advice, in that Jack has been using these cells for a couple of years and has never done an initial charge on them and never charges them to 4.00V.  His method is to stop charging at 3.60-3.65V (but often charging the cells in series), safely under charging them, with no active BMS present (or required).  I take his advice seriously, as he's tested lots of cells in his lab and blown up a few when over charging them on the bench!

However, the Winston Battery operator's manual is quite clear (and insistent) that this initial charge to the full 4.00V is done before the first discharge.  GWL echoed this in their user FAQs.  So I contacted the Technical Manager at GWL for advice on whether I should do the initial full charge, whether the 5A bench PSUs are suitable and whether any harm comes of doing the initial charge too slowly.  The Winston Battery manual recommends a charge rate of between 0.1CA and 0.5CA, but this is out of my reach as it means a PSU that can deliver between 40A and 200A!

The response from GWL was go with the full 4.00V CC-CV cycle and it's ok to do it slowly, so long as you terminate the cycle properly when the current has reduced in the CV phase.  They have posted a technical paper (PDF) warning about using shunt BMS systems that hold cells for too long at the terminal Voltage (4.00V). However, this was in the context of holding cells at that Voltage for long (cumulative) periods after the charge acceptance current of that cell has effectively reached nothing.  Not the same case as doing a very slow charge and terminating at the end of charge acceptance of the cell.

So, on balance, I'm going with the advice of the manufacturer and the supplier of the cells on this.  At least that way, if it turns out to be bad advice, they are on the hook for giving it to me - if it comes to a warranty claim.

I'm using these new 80W constant power SMPS PSUs from Maplins.  They can run in parallel with a master/slave link and they also have a remote sense input so that the constant Voltage dialed in is measured at the cell terminals and can compensate for the Voltage drop in the heavy current power leads.  This way you dial in 3.97V and you know the charger will hit the mark.

The unit is small, light and efficient.  Doesn't get hot in use, has no need of a fan and is power factor corrected.  At £100 each, they're not cheap but also checking RS, Farnell and Rapid didn't turn up anything better value.

After the initial charge, the manual advises that it's ok to subsequently charge within any part of the cells safe operating range from 2.50V to 4.00V.  So I'll adopt Jack's strategy of making sure all the cells are bottom balanced (at 2.75V when "empty") and then never fully charging them, to avoid the problems and need for active top balancing and current shunting and all that expensive and failure prone junk.

The CellLog8 should be all I need to prevent over discharge of the pack or individual cells in the pack.  It can monitor each cell individually and I can set an alarm for any cell that goes below 3.00V or the whole pack goes below 24.0V.  It can also alarm if any cell differential Voltage (the difference between individual cells) exceeds a limit.  This will tell you if the pack is out of balance -even before it gets to being near full or empty.  The CellLog8 has a beeper and an open collector transistor output that can be used to provide an "inhibit" signal to my inverter to shut it down until the solar array can recharge the pack.

Of course, the required 9 pin header cable for the CellLog8 didn't come with the thing, so I had to source that from a radio control models shop (Electriflyer).  They also sell the Junsi lithium battery chargers that use the same JST-HX balance lead (model BW-9-11).  Just under £5 delivered.

The cable is actually meant to connect a Junsi iCharger 208b charger to a balance board (hence the 11 way header on the other end) but you can either cut off the big plug or use it.  The CellLog8 doesn't have a proper JST socket so a plug with more than 9 pins will fit;  the spare ones will stick out over the end of the 9 pin header on the device.  I might cut the lead in the middle and then have two usable plugs for the gadget.

Another thing I had to order was some terminal lugs.  These are also a bit harder to track down because they are M14 stud holes.  I found some on eBay from Clarik Engineering Supplies.  They take up to 120mm2 cable (too big for my 35mm2 cable).  I might have to use a blow torch or my pipemaster plumbing soldering iron to attach cable to them.  These alone were another £7 delivered... Everything is expensive when you have such massive terminals on a battery!