Tuesday, March 17, 2015

Dodged a Bullet ... Maybe?

Quick update ... I set up the battery lab on the workbench again and hoisted the damaged battery module onto the workmate, this time with nylon strap. I bought a small variable DC power supply to bring each cell up gently to around 3 volts so the PowerLab8 would recognize them, then did a charge to 4 volts. After a rest period to make sure it would hold the charge, discharge to 2.75 volts, the bottom balance target.

The first cell seems to have recovered ok, the second cell has been charged and is now in the discharge phase. Looks ok so far, with four more to do after that.

One disconcerting sign is that the capacity seems low at around 35 amp hours. The cells are supposed to be rated for ~60aH. I ran a good cell through the process and got the same roughly 35 aH, so at least it's consistent. It could be a measurement issue, or maybe a result of charging to only 4 volts. We'll tackle that after we're sure that we have recovered from the damage. Stay tuned ...

UPDATE: I went back and watched Jack Rickard's video from March 14, 2014 (yes, that's a whole year ago) and it looks like I'm actually doing pretty well on the amp hour counts. He recorded about 27 aH at 4.0 volts and noted that most of the capacity increase occurred between 4.0 volts and 4.2 volts with about a quarter of the capacity coming in the constant voltage stage of the charge. He got a full 60 aH with that technique and recommends 4.2 volts as the termination point with an extended constant voltage stage down to about 3 amps. Now the question is: will my 4kW charger with it's nominal 389v (4.05v/cell) upper limit stretch to the 403v needed for 4.2v times 96 cells?

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