15 May 2013

First experiences with Li-ion cells

I have been curious for a while about Lithium based rechargable batteries, and how I could get to play with them without investing too much money. Yesterday I was given three identical exhausted HP laptop batteries, marked to be 14.4V 4400 mAh, Li-ion. Not having a way to try a recharge, a disassemble was strictly necessary.

Those batteries even have 5 LEDs that show the charge level, and of course they were reported as dead. Once open I was presented a series of two elements in parallel, 4 each: 4s2p, and a lot of electronics. 
Meanwhile I had read something about these batteries at batteryuniversity dot com and I learned that:
  • to protect cells from overdischarge, then internal circuitry disables the cell, resulting in 0V across the poles; the cell can be reactivated with a charge current
  • Li-ion cells can be recharged with a constant voltage not higher than 4.20V, with a high current, even equal to C [cell's capacity], for the right amount of time
Each of my eight cells, left uncharged for an indefinite amount of time, measured 0V. So far so good. I threw together a 4.04V 2A voltage source and, while monitoring current (DVM in the picture) and voltage (analog voltmeter, not in the picture), I started charging each cell one-by-one for 30-45 minutes: in my opinion this is a safe time that does not pose the risk of (literally) blowing the cell. YMMV.

First cell went fine. After the initial 1.5A spike, it charged at 400 mA (and decreasing, 310mA on the DVM at picture time). After 45 minutes it had reached 3.8V and held it for hours with a slight decrease to 3.7V. Looks good.

Second cell was a surprise, since it initially behaved as the first but then went short circuit! Since it was unattended, I found a pretty warm regulator and cell when I checked in 10 minutes. Current was 3.5A. Maybe this cell was the faulty one in this battery pack?

Lesson learned: add a (resettable) fuse in line so that cells can be left unattended and, if they go short circuit, nothing blows or melts.

Third cell was better too. I will continue with the 45 minutes cycle to see if I can revive these cells. Having 3x8 = 24 potential Li-ion cells for free is interesting, but most important I can learn something new.