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.

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.

Active Battery Balancing

A while back, I had a problem with the AGM battery bank getting out of balance. This would have damaged it by having one 12V battery under charge all the time, while the other one over charged and gassed (eventually drying up and being killed).

Luckily, I found a fairly cheap (£29) battery balancer that has fixed this.

This one is from Rapid Electronics, made by a British company called Camden Boss.
You connect the balancer to the 24V + terminal and the 0V - terminal and the yellow wire to the 12V middle point. It then works when each battery is over 12.8V (when being charged) and shunts up to 1A of current across either battery, to keep the mid-point at exactly half the full terminal Voltage, ensuring that the batteries charge evenly and fully.

I actually ordered two of these but one hasn't turned up yet, on back order from the maker.

It gets a bit warm when it's working but as the batteries get to be equal in charge, it gradually stops taking power and cools down. So it wastes a bit of solar power by dumping it as heat, but that's better than a fried battery :/

When you've discharging the batteries, they are below 12.8V each and so the balancer does not work and draws no power.

In order for the balancer to work across all the batteries in the AGM bank, I installed the equalisation network that I said I was going to install months ago (but was too lazy to actually do :D ).

It has fuses in the links so that if there is a problem with one string of batteries or the balancing current is too big, the fuse will prevent a melt-down. It doesn't matter so much that the wires are not going to a star point as the current across the links should be zero (or close to it over time). The battery balancing module only moves 1A so that doesn't cause any real Voltage drop either.

The whole thing seems to be behaving itself perfectly and the six batteries now charge much more evenly and fully than before.

More on Balancing Batteries

Googling around the last couple of days I discovered that Elecsol (well known maker of leisure batteries) has released a new range of sealed VRLA AGM batteries primarily aimed at solar storage.

They make some impressive claims like that these batteries can deliver 1,100 100% discharges and 1,400 80% discharges.  They offer a 7 year "unlimited" warranty on them.

You can read the blurb on them here:  http://www.elecsolbatteries.com/literature/

The tech book / brochure did have an interesting addition to the traditional star pattern series-parallel wiring scheme.  It had not occurred to me to put balancer cables between the mid-points of the series strings to allow internal equalisation of the bank.  I'd had a problem with one of the six new batteries in my bank being low (out of balance with the other one in series) but I cured it by taking that pair out of circuit and slow charging the weak one to bring it up and then put them back in circuit.  The whole bank appears to be behaving ok now but I periodically measure the volt differences on each block. 

I might put a star equaliser network in (connecting the mid points of all three pairs to a common point so they will equalise er... equally).

The red/black lines are the power lines I have on my bank and the green ones would be the equalisation network that would allow all the weak batteries in pairs to charge up more without over charging the stronger ones.  In my "bad" pair they showed 28.2V across the pair but one was 13.8 and the other was 14.4.  With the equaliser network added, the weak "bottom side" battery might continue to charge by "finding" another weak "top side" battery to pass current through (13.8 + 13.8 is only 27.6 so that pair would continue to charge without over charging the others sitting at 14.1V).  It wouldn't help in every scenario though - if all the bottom side batteries were weak and all the top side batteries were strong then the equaliser network wouldn't help. 

Maybe it's just as well to rotate batteries in strings (like tyres on a car)?

Meanwhile, it's been a great couple of days harvesting.  In those partial cloud surges that I mentioned before, I've seen surges up to 1.98kW from my 1.8kW array (over 109% of rated power). The Sharp ND170 340Wp string stole the show though, putting out 409W (120% of rated power).  They might have managed more but the charge controller capped the output at 15 Amps!  The other charge controller was also close to capping its output to the 60 Amp limit, as it surged to over 57 Amps - the pair pumping an eye-watering 72 Amps into the battery bank.

Balancing Batteries

When connecting batteries in parallel to get more capacity, you have to be careful to balance the individual strings of batteries so that they all do the same amount of work.  Otherwise, some will become more discharged than others and suffer damage from hard lead sulphate forming on the plates that cannot be dissolved by charging.

Having batteries of different sizes is also not a good idea as the small ones will discharge to a greater extent than the bigger ones and so suffer again.

So what did I do?  I bought a bunch of different sizes and types of battery and connected them all up together... Go figure.

So, how do we bodge this so that the batteries all stand a chance of surviving?

 

The main block of batteries are gel types and 180Ah in size.  They have a rating of 1,000 cycles at 50% depth of discharge (the deeper you discharge a battery, the fewer times you can do it).  The Marathon ones I've just bought are AGM types and 104Ah in size.  They are intended for computer uninterruptible power supplies (UPS) and as such are not expected to be discharged every day (if ever).  They probably have a rating of 250 cycles to 50% discharge.  But if discharged by only say 25%, they might last nearly 1,000 cycles.

So, to balance the different batteries lifespans, you have to try to balance the amount of work each does. To do this your main tools are size of battery and the wiring that connects them.  Thinner wires resist the flow of current and so large loads will drain batteries connected by fat wires faster than ones connected by thin ones.

In the diagram above you can see how I've wired the batteries together.  The big gel ones are directly connected to the solar chargers and the AC inverter by very heavy gauge 35mmsq cables.  This lets the gel bank do the bulk of the work when under high load. The weaker Marathon batteries are connected in 3 groups that use a star wiring pattern.  This means that each group is connected by wires that are individually quite thin (6mmsq).  But the wires are deliberately a bit long and are all the exact same length (hard to draw so take my word for it).  The other important fact is that they are all connected together at one point (I soldered them together after weaving the ends into a sort of knot. This means that, as far as possible, the wire resistance for each branch is the same and so each group of batteries will do the same work.

The whole group of six batteries is then connected to the main bank by a single pair of 6mmsq cables with a 30A fuse.  The fuse is important as 6mmsq cable can only carry about 50A and a fault could cause a few hundred Amps to flow.  If the main battery dried up or got a short circuit somewhere inside it, a very large current could flow and start a fire.  Actually, each of the groups should have a fuse but adding fuses in every leg makes it difficult to balance the resistances as the fuses and connectors introduce variances.

So, by having the Marathon battery bank 1.7 times the size of the gel one and using thin wires to connect it, I hope to keep the workload low enough on the Marathon bank to make it live for as many cycles as the gel ones.

When running loads, I've noticed that the gel bank does do more work (delivering about 2/3rds of the current).  But as the gel bank starts to run down the Marathons start to assist more.  This is because gel batteries are designed to resist deep discharge by being acid starved.  They don't have enough acid to fully convert all the lead in the plates.  As the acid runs out, the gel battery increases its internal resistance and so does less work until the output voltage falls to a point where the protection in inverters causes them to turn off.  

Because the resistance of the gel battery increases as it gets discharged, the balance of resistance between the gel bank and the Marathons will change.  At some point, the Marathons will have the lower resistance path and so assist the gel bank by doing more work.  So it's more important that before for me to stop discharging the bank before it gets much below 50% as beyond that point, the Marathons will start to be drained quite quickly.  But even then, that shouldn't be a big problem as they are 1.7 times bigger than the gel bank.

Tonight we're at 50% as the last couple of days has been very gloomy (it rained all day today).  Hopefully tomorrow will bring some sun.

I'll have to watch the charge in the morning as the reverse is true for charge balancing.  Because the gel bank is low and has high resistance, the Marathons will accept more charge current than the gel battery (at first).  The charger can deliver up to 73A and could cause the 30A fuse on the link wire to blow...