Anyone played around with flat GM battery cells?

[Cornelius]

I've just opened and started disassemly of my brothers old 36V16Ah GM battery, which has been beneath a pile of snow a whole winter, and just lying around for a whole another year...

The BMS are ofcourse badly corroded, but the cells aren't that bad - visually... Measuring each bank of 4 cells gives 1 bank of 2.6V, 2 banks of 3.5V - and 7 banks of 0.0V   The 3 banks with actual voltage should be ok.
Having a lab. power supply delivering up to 24V@2.5A, I started charging 5 banks in series for 20 minutes and let the batteries rest for 1/2 hour. after that, 3 of the banks had 2-3 cells still hot to the touch. I'm guessing those cells was being drained by a shorted cell in the same bank. Those 3 banks measured around 0.8V at that time. The other 2 banks fell rapidly down to 0.0V...

Now I've disassembled each bank, and started charging single cells at 4.22V@4A.
So far, I've got 5 cells that keeps 4.00V at rest 2 hours after charging. (I'm keeping a manual eye on the voltageduring charge.) As soon the voltage reaches 4.220V (high precision Fluke meter), I stop the charge.
I haven't started charging those 3 banks that had 2.5V++ volts initially yet; believing those are ok.

Anyone have any experience regarding life reduction of these cells being totally flat for a period of time?

[DirtyGinge]

I have noticed that really bad cells will start holding voltage, but in a short space of time will drop voltage, then it will drag the voltage of the entire pack down....

you could charge them up and try some load testing....

[Cornelius]

You're probably right...

I've reduced the charging Amp to 2A because of the heat the cells developed @4A charge. Some of the cells i've charged so far, reached 4.20V in just 1 hour @ 2A, and that tells us the capacity are significantly reduced.

I'm thinking of using an PicAxe 08M to build a single cell charger that monitors voltage and temperature, and controls the charging output on/off; with logging to pc... That way, I have better control over time used to charge, and since charging current are rather constant until around 4.1V, capacity gained are easily calculated.

[Cornelius]

I just browsed around the net and found some useful info at batteryuniversity.com :

Do not recharge lithium-ion if a cell has stayed at or below 1.5V for more than a week. Copper shunts may have formed inside the cells that can lead to a partial or total electrical short. If recharged, the cells might become unstable, causing excessive heat or showing other anomalies. Li-ion packs that have been under stress are more sensitive to mechanical abuse, such as vibration, dropping and exposure to heat.

Another one:

Charge efficiency is 97 to 99 percent and the cell remains cool during charge.

In other words; when i'm playing at charging these 0.0V cells, and see that they're getting hot rather quick, I should really just throw them away...

And finally, some guidelines:

•A portable device should be turned off while charging. This allows the battery to reach the threshold voltage unhindered and reflects the correct saturation current responsible to terminate the charge. A parasitic load confuses the charger.
 
•Charge at a moderate temperature. Do not charge below freezing.
 
•Lithium-ion does not need to be fully charged; a partial charge is better.
 
•Chargers use different methods for “ready” indication. The light signal may not always indicate a full charge.
 
•Discontinue using charger and/or battery if the battery gets excessively warm.
 
•Before prolonged storage, apply some charge to bring the pack to about half charge.
 
•Over-discharged batteries can be “boosted” to life again. Discard pack if the voltage does not rise to a normal level within a minute while on boost.

Now, we're a bit wiser...