What does it take to kill a headway cell?
Here is the story.
A couple of us got a bunch of abused headway packs. Some dropped, crushed, shorted, water damaged, generally over-discahrged and left in a pile to rot. Our plan was to rescue the "good" cells, those that showed above 2.0V, and recycle the rest.
But I thought I would try charging up some of the "bad" cells that had <2.0V and see what happens. Some were at 0.0V, a lot at 0.4-0.5V, and various voltage up from there.
The thing is that both "good" and "bad" cells seem to be performing well.
What we have done is:
1) individually charge each cell to 3.6V
- charger is a
Turnigy Accucel-8 150W 7A Balancer/Charger
2) Perform a load test to guage voltage sag.
- a couple of big fat resistors on a heat sink to provide a Fluke clamp ampmeter verfied ~24.8 Amp load
- charge up the cell
- let it rest
- apply 24.8 Amp load, measure voltage at 1min mark
- remove load
- measure resting voltage after 2 mins
- infer a DC internal resistance from this.
3) Self Discharge Test
- fully charge the cell
- let it rest for a week or two
- then charge it again, and measure the mAh needed to top it up.
4) Capacity Test
- use the
Turnigy Accucel-8 150W 7A Balancer/Charger to discharge at 5Amps down to 2.0V
- record the mAh
The interesting things is that we have found a few duds with high Ri, but most of the "good" and "bad" cells are looking fine.
- DC internal resistance of between 8 and 11 mOhm (based on the voltage sag test above, not the AC 1kHz standard measure)
- No significant self discharge, still requires ~120mAh to top it up after a day, or a couple of weeks.
- All still have greater than 10Ah capacity at 0.5C
There doesn't appear to be any correlation to the DOD we found the cells at, and how good the are now.
We obviously haven't done any long term life cycle testing, but from the "quick" testing so far, it looks like these cells don't instantly die when you take them down to 0 volts, it may hurt the cells, but they still look
very useable to us.
Would love to hear the experts opinions on what tests we should do to confirm the health of the batteries, and what it takes to kill them.
- Adrian
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