Li cell charge tests

Yes, that ties in with my much more rudimentary test on a 40 AH cell which indicated a loss of about 2.5% when charged to 3.6v as against 4.2v.
I wonder whether there is a significant increase in life if charged to the lower level?

Well done, TJ. But I note that it's not certain that putting in 0.31 Ah means you get an extra 0.31 Ah from the next cycle... or even 0.2 Ah. I suspect a lot of it goes into heat.

But since the basic conclusion is "it matters bugger all", and remembering that 5/8 of bugger all is still bugger all, I guess the result stands.

Interesting that they have changed their specs:

http://www.thunder-sky.com/pdf/TS-LFP40.pdf

They are now claiming 5000 cycles at 70 % DOD, 20C impulse discharge, and their discharge curves now include 5C (and down to 2.1 V as well). They still state the minimum operating discharge level is 2.5 V.

Nice one TJ.
Just because it takes no effort at all to ask. It would be interesting (but probably not helpful) to extend this down to about the Lose 5% mark.
That is - what voltage are you down to before the cell shows as 95% charged.
I guess the amount of time to do this may be a restricting factor.

It would be nice to repeat these tests with an aged cell. i.e. one that has 500 or 1000 cycles on it. To see if the charge cutoff voltage requirement changes with age.
Remember all the tests are being done on new cells.
I know the old lead acid yellow tops required higher and higher end voltages on charge to reach the full recharge. Yes different chemistry, but I don't have a feel for Li chemistry in aging.

Also, what effect has extremes of temperature on this charging voltage vs Ah ?

They're not using phosphate chemistry cells though, so that could make all the difference.

acmotor, the two cells I've run this test on so far aren't brand new, but probably only have about 50 cycles on them. They're a bit over two years old. I agree an older cell would be a useful test.

Matt, thanks for the offer, but this power supply I have is going away mid - end of the week, so it's probably pushing things a bit to try and fit another test in before it goes.

Hi guys, great thread.

We are building an Electric SAE racer here at RMIT.

We have 40AH Thundersky cells:
Vnom=3.2V
Vcharged=4.25V

We have a battery management system on the cells (dont know the manufacturer) that warns you to stop charging cells at 3.8V (red led turns on).

We have been disconnecting charger as soon as we see the warning light. Given the info in this thread, I'm concerned we undercharging the batteries.

I've looked at the BMS system at EV Works their cells also warn you or activate a shunt at 3.65V. This doesn't seem right since TS cells have Vcharge=4.25V

http://www.evworks.com.au/index.php?product=BMS-CM060-V6

Any thoughts guys? Is anyone familiar with the 40AH TS cells?
Should we be charging the cells to a higher voltage?


Cheers,

Ben

Ben,

The red light on your BMS might be turning on at 3.8, but what voltage is the balance resistor turning on at? If it's also turning on at 3.8, then you probably actually want to charge your pack at a much reduced rate when the first LED turns on, and keep charging until they are ALL on. This will give you a balanced pack. What the 'much reduced rate' is will depend on what balance current your BMS can do.

BMS can mean "Battery Monitoring System" or "Battery Management System". In the former case, it merely monitors the cell voltages and perhaps temperatures and it may give a warning of some sort if a cell gets too high or too low in either of those.
The latter is a bit smarter in that it will have some form of balancing system, usually a load resistor across each cell that can be turned on or off by the controller. When the controller sees a cell at the fully charged voltage, it turns on the shunt resistor and so the charging current bypasses that cell but continues to flow into the others. This happens for the next cell and so on until all cells are at the correct charging voltage and therefore "balanced". At which point the charger shuts off.
Note that the amount of current that can be bypassed is pretty small as it results in a lot of heat in the bypass shunt. For example, if you bypass 1A at say 4.2V, that's 4.2 watts per bypass resistor. If you have 45 cells to manage, there's a nice little heater!
Another method of balancing is to simply shut off the charger when the first cell reaches max volts and then use the bypass resistors to bring each cell down to whatever voltage the lowest one is at. Doing it this way usually means you only need about 200 mA of bypass current per cell, but the cells will not be quite at full charge. The difference would appear to be very small though. Unless you have one cell that is very low compared with the others - then it brings them all down to that low level, but they will be balanced! And get better as they are cycled more - we hope.
If you really want to balance a pack right at the start, then you should stick a 4.2V (or whatever) charger across each cell and make sure they are all at full charge before being put into service.
Is your BMS turning on a red light for each cell? If it doesn't control the charger current, then it is most likely just showing that the cell has reached its charge voltage. It may indicate a shunt being turned on, but that is unlikely unless the charger current is down to an amp or so.
Note also that the recommended charge voltage for Thunder Sky cells is 4.20V and the voltage for Sky Energy cells is 3.60V.

Edited by Nevilleh - 17 March 2010 at 4:26am

welcome to the forum borrows

perhaps you missed a link? I see no figure 1.

Matt