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Restoring "dead" Lithium-Ion battery pack

I have an old Panasonic Toughbook (CF-47) that I wanted to use as a semi-portable terminal. It was in decent condition, but would not power on, even with the AC adaptor, because of a completely dead battery.* New battery packs are expensive and largely out of stock. What to do?

I figured the charge controller was in some kind of failsafe mode since the batteries were left to discharge to zero volts. It wouldn't know if it was zero because of discharge (okay) or shorting (dangerous).

I opened the pack and mapped the cells. There were 3 strings of 3 cells at nominal 3.6V, for overall voltage of 10.8V. All cells looked in pristine condition. Could I manually charge them, at least part-way?

I looked up the specs. Initial charging was given as about 1100mA at 4.0V. I decided to start with a meager 10mA and see what happens. For unknown reasons, I used a 5V power supply instead of 12V (I think I was mistakenly thinking of voltage across one cell instead of the string of 3). I used a diode and a resistor of 470 ohms, given the initial drop across the battery was 0V.

With my ammeter inline, I switched on. Initial current 10mA and falling. Felt the batteries; they stayed cool. When it dropped to 4-5mA, I switched to a 220 ohm resistor and charged a bit more.

Would the laptop charge it now? I took the clip leads off, closed up the pack and put it in the laptop and plugged it in. Success. Instead of the flashing red battery LED, I had the orange "charging" indicator. I booted into a very old SuSE 9 linux and watched as the battery charged eventually to 100% charge. It worked. I don't know what the runtime is like, but at least the laptop is usuable now.

So if you have a discharged lithium battery pack that won't take a charge, it might just need a short trickle charge by hand to bootstrap the power management unit/charge controller.

It is possible that I was just pressing the wrong power button. There's a sleep button on top I mistook for power; the real power button/switch is on the side.

;Date: 2023-09-28 16:11
;Desc: Trickle charging a dead lithium pack to kickstart
;Desc: the charge controller.
;Tag: Batteries
;Tag: Computers