Li-Po (Lithium Polymer) Battery Repair / Recharge

Well, my 2200 mah 3S (11.1V) 25C battery isn't performing anywhere near where it was when new.  I used to get 20 min of flight time out of a charge, now I'm down to under 7 minutes.  The total charge on the battery after charging is also low, around 12.1 V rather than the 12.3 - 12.4 it used to get after charging.  

So, I decided it was time to do something about it (besides throwing it out and purchasing a new battery).  The battery was obviously not getting a full charge, indicated by the lower 12.1 V after charging.  A quick check with my voltmeter told me the 3 cells, which should each have the same voltage across them, where different.  Cell one was very low at 3.39 V compared to cell 2 and cell 3 which were each around 3.92 V.

 Now, the job of the balancing charger is to charge all three cells to the same voltage of about 4.1 V each.  However, if one cell is low, it will hinder the charger and keep the battery from getting a full charge.  So, I decided to drain the charge out of the other 2 cells to bring their voltage down to match that of cell 1.  To do this I used a 10 ohm resister and plugged it into each cell via the charging connector.  The leads on the resister fit perfectly in the connecter.  

The 10 ohm resister got very hot, and became discoloured from the heat, but it didn't burn out.  A larger resister will generate less heat but will draw a much smaller current and thus will take longer to discharge the battery.  As it was, the 10 ohm resistor took several hours to discharge each cell.  The voltage drops very slowly when the charge is over 3.5 V - it took several hours to go from 3.9 to 3.5 V.  However, once it drops below 3.5 V it starts to drop much quicker; and the closer you get to 3.0 V the faster it drops, so fast in fact that in 1.5 hours it went from 3.6 to 1.9 V!  A bad mistake on my part - you should not discharge below 3.0 V or you risk damaging the battery (or so "they" say).  

However, once I pulled out the resistor, the charge recovered and eventually after about 6 hours settled in at around 2.94 V.  I discharged the other cells to 3.1 V and then plugged the battery into the balancing charger.  After 15 minutes or so, the voltage on all 2 cells was up over 3.3 V, and 3.12 V on the third cell.  So, I used the resister to discharge the cells back to the same level.  It is difficult because after you unplug the resister, the voltage jumps quickly up a few extre tenths of a volt.  But playing with it you can determine what this jump is.  In my case, the voltage would jump 0.3-0.4 V after removing the resistor.  

After getting all three cells to about 3.12 V, I plugged it back into the balancing charger.  A while later the voltage was up to 3.92 on cell 1, but cell 2 and 3 were at 3.80 V.  So, I discharged cell one to 3.77V and after removing the resistor it jumped to 3.80 V - an exact match to cell 2 and cell 3.  I then plugged the battery into the balancing charger and waited to see what would happen.

I have an 1800 mah 3S (11.1 V) 20C battery that seems to be working just fine.  At full charge it reads 12.33V.  I checked the voltage across each cell and here's what I found: cell 1 = 4.08 V; cell 2 = 4.05 V; and cell 3 = 4.20 V.  So, I used a 10 ohm resister to drain the charge from the cells until all three cells ready exactly 4.05 V.  Then I recharged the battery pack.  After charbing, the battery had a total voltage of 12.57 V, and the 3 cells had individual voltages of 4.18, 4.18 and 4.20 V.  I then flew the plane for 14 minutes (flight time cut short by a crash!).  After the flight I checked the battery.  Total charge was 10.92 V, and all three cells read exactly the same 3.64 V!  Amazing.  Balancing the battery seems to have worked wonderfully on this battery pack.