Thursday, March 29, 2012

CellLog8s Hardware Mods

In researching the firmware issues with the CellLog8s, I discovered on the RC Groups forum that there are a couple of hardware modifications that can be done to the device to make it work properly when using more than 6 cells.

The OEM, Junsi, posted an item in the thread here (links to the post) describing how the CellLog8s only draws power for the CPU and LCD from the first 6 cells connected.  This will cause a 7 or 8 cell pack attached to have a charge imbalance over time as cells 7 and 8 are not loaded.  It's only a few dozen mAh per day but for a solar pack connected 24 hours a day, it all adds up.

I've copied the photo from the forum, so you can see it here more easily.  Click on it to see it bigger or follow the link I gave above to see the whole article posted by Junsi.

The red line denotes where the link wire has to go and the resistor is circled in pink.
The cure (for users who only want to use more than 6 cells all the time, like me) is to add a wire link and replace one microscopic surface mounted device (SMD) resistor.  The resistor is an optional modification for those with excellent soldering skills (or the right tools) :D. 

Changing the resistor from the "202" (2k2) value to a "103" (10k) value reduces the CellLog8 power consumption when used with more than 6 cells.

Note that the modifications make the CellLog8 unable to function on just 2 cells, so it's not a full fix for the issue.

Of course, applying this "fix" may well invalidate your warranty from the supplier (especially if you muck it up and fry the thing).  But given that the device has a known hardware "bug" out of the box, you could always argue for a refund on the basis that the device was "not fit for purpose" in the first place.  So anything you do to remedy this has no bearing on the initial situation.  Either that, or you could ask the supplier to make the modification for you, again citing that the device was not fit for purpose (if your purpose was to monitor 7 or 8 cells).

On the plus side, at about 28 Euros, they're cheap if you fry one :D

Wednesday, March 28, 2012

Getting CellLog8 Data Out

I've been helping a contact with getting the CellLog8s to transmit data to a PC and thought that the info might be useful to anyone else trying to get the slightly quirky data transfer to the slightly quirky German LogView software to work.

First thing to note about the CellLog8s and PC connections is that the USB port on the logger is not isolated from the PC and is not isolated from the battery pack being monitored.

The CellLog8s should have opto-isolated data lines to USB.  But they also wanted to be able to power the thingy from USB so that means a connection to the PC...  The later product (PowerLog) comes with a cludge of a solution in that it has a USB cable with the power pins not connected!  Rubbish solution, as immediately you'll loose the original "isolated" cable and use a normal one and BOOM!

I know about the issue and have from my own experience blown up a hand-held oscilloscope by running it from the DC-DC converter on the solar system instead of the internal batteries only.

I get away with plugging the CellLog8s USB into my little laptop because I make damn sure that I only run the laptop on batteries and not plugged into ANYTHING else (even use wi-fi for network connection).  It's all good if the PC is running on internal power and is not connected to anything where the solar battery power can escape back to itself in a loop and... BOOM!

That's why all the data interfaces on the Morningstar charge controllers are opto-isolated.

Next is that you need to install the Junsi serial port driver that came with the device on a mini CD.  Do not just plug the CellLog8s into the PC as Windows 7 will find the device and install a built-in USB serial port driver that does not work.  Install the Junsi one first and then plug the device into the USB port and check that the Windows USB notification says it has found and successfully configured the Junsi driver.

Also note that you must be using a non-64 bit version of Windows.  Windows 7 comes in 32 and 64 bit flavours and the Junsi drivers do not work with 64 bit Windows.  This is not because they don't work but because Windows 7 64 bit enforces a "no install" security policy for unsigned 3rd party drivers.  The 32 bit editions do not enforce this.  They'll complain that the driver is unsigned but allow you to install it anyway.

I've managed to get the LogView thing to work without problem.  I don't log continuously but have managed to download the CellLog8 on-board log file with no trouble.  I'm using v2.7.3.481 (which came on the CD with the CellLog8).  You don't need to mess about with the device file.  Go to the Device> Choose Devices and Ports menu.  In there, use the drop down lists to choose Junsi and CellLog8.  It even shows a picture of the device when you select it.

Then all I did was tick the "Automatic start recording" option and select the COM port to talk to it over in the "RS232 Seriel" drop down list.  The CellLog8 driver you installed earlier (you did install the supplied driver, not rely on the Windows one?) will be listed as "Junsi something or other COM x".  You have to have the CellLog8 connected to the PC via the USB cable before starting LogView for it to find the Com port that gets discovered and enabled by Windows as soon as you plug the CellLog8 in.  If you don't plug the logger into the USB before starting LogView, it won't find the serial port.

The "Automatic start recording" option means that whenever you start LogView, it tries to connect to the port used last time for the CellLog8 and it opens the recorder session in LogView (ready to receive any data).  If CellLog8 is in logging mode it will transmit individual data every 2 seconds (or whatever you set in CellLog8).  If you are not logging, it will still open the channel but have nothing to receive.  When you then go to the logfiles menu on the CellLog8 and select "transmit", the file will be broadcast on the serial port and LogView will be listening for it and will save it in the PC memory (which you can then save to disk).

If you do not put the "Automatic start recording" option on then you have to press the record button on the LogView panel BEFORE hitting "Transmit" on the CellLog8 (otherwise LogView won't be listening for the broadcast).

Transmission of a big log file (say 30,000 entries) can take a few minutes!  You will see NOTHING on LogView while this is happening (apart from the green communication lights on LogView flashing).  You WILL see CellLog8 counting though the log entries to show progress of the BROADCAST.

When the transmission is completed, the CellLog8s will beep and then LogView will draw the graph of the data received.  At this point the data is only in memory and you should save the data from the File > Save As menu option in LogView before messing about with the data.

Thursday, March 22, 2012

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.

Sunday, March 18, 2012

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.

Friday, March 16, 2012

Statistically Off Grid

Just looking at the recent generating and usage figures, I realised that for a few days I have been statistically off the grid.

These lithium batteries are 95% charge efficient.  This means that the losses are very small to put energy into the battery and get it out again.  For every 100Wh put in, you can take 95Wh out.  That compares well with lead acid batteries where the best you can get out is about 80Wh.

Another way to look at it is that by changing to lithium batteries, the effective size of your PV array increases by about 15%.  Or you effectively have 15% more charging hours in the day (as it still takes time to charge the wasted Watt.hours of energy lost in an inefficient battery).

As I had more power available with the new batteries, I moved a couple more loads to solar from being on the grid.  These were the garage CCTV camera and the central heating system.

The central heating system runs on gas but it also uses electricity for the boiler controller (valves, fans, thermostat controls), the heating timer programmer, as well as the zone valves and circulating pump upstairs.  But it was powered from one place in the airing cupboard.

With these things moved to run on solar power, that leaves very little in the house that runs on the grid:

The main cooker and oven in the kitchen;
The instant water heating shower (2 minutes a day);
A hair dryer (2 minutes a day);
The microwave in the kitchen (a few minutes a week);
A clock radio in the bedroom (24x7 load);
A radio and DECT phone in the kitchen (18x7 on a timer);
Two bedside lights (a few minutes a day);

On a number of days, the import meter did not increment at all (it only counts whole kWhs).  So on those days, the house statistically consumed no grid power and so the house appeared to be entirely powered from the solar batteries.

But it's not just a "rounding error".  The wireless energy meter spends most of the day now reading "zero" Watts and the import meter also stops.  This is because they cannot measure very small power loads of less than about 10-20 Watts.  Below that threshold, the import meter enters an "anti creep" mode where it stops counting and the LED that normally blinks to show the passing of Watt.hours lights continuously to show that it has entered the non-counting mode. 

This prevents the meter erroneously counting units that were just a calibration error (for example it might read 5W when the consumer unit is actually turned off).
The electricity company does not usually care about this small error, and in any case, who could run a house on just 10W of energy?

So it seems I don't have to bother with running wires to the bedroom for the clock radio or the kitchen for the radio and DECT phone, as they don't consume enough power to cross the threshold at which the import meter will start to count :D

The side effect is that the energy company doesn't believe my self-entered meter readings on their web site now.  They asked for a reading for the latest bill and I gave it to them.  Then a few days later they sent someone to re-read my meters, as they didn't believe my readings!

Unlike folk who have grid tied solar, it's entirely possible for my import meter to stop counting up.  People with grid tied solar still consume grid electricity in the evening that has to be paid for.  They just earn it back on the FIT for the energy they export (usually measured on a separate generating and export meter or "deemed" exports from the generating meter only).


Wednesday, March 14, 2012

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.

Thursday, March 1, 2012

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.