Back in June, I found a set of six NiMH C cells that’d been totally ignored for about 6 years. I used them to rebuild a camcorder battery pack, but then the camcorder failed. Just on a lark, I decided to try and charge them.

That was a challenge in and of itself, because my favorite charger of late is a LaCrosse BC-900 charger/analyzer, which only accomodates AA and AAA sizes. It took a few minutes and some dowel stock, and I made dummy batteries that allowed me to bring out the contacts to external C-size holders. LaCrosse were amused when I sent them a picture of *that* setup. “Umm yeah, you know that’s not a supported configuration, right?”

But of course, self-discharge had flattened the batteries while in storage, so the charger’s reverse-polarity detection logic couldn’t be sure they were in straight, and refused to turn on. This is normally an excellent feature to have, though it’d be nice if an override control were provided. I just grabbed my retired Ray-O-Vac charger, which I call “The Immolator” because of what its brute-force charge logic does to cheap AA’s.

After a few minutes in The Immolator, five of the six archaeo-batteries had accepted enough charge for the LaCrosse to detect them, but the sixth presented a dead short, so the charger couldn’t do anything with it.

Enter the benchtop power supply, a little (Lambda brand?) $30 beater I picked up at Hamvention. I connected its variable output to the dead C cell, and slowly twisted the voltage control. The voltmeter didn’t budge, indicating that the cell was still shorted, but the ammeter said I was passing 2.5 amps of current, the supply’s maximum. I let it sit like that for a few moments, and the cell wasn’t even getting warm. So I reversed the polarity. (Hey, it works on Star Trek!)

After about 5 seconds of high-amperage reverse current, the voltmeter twitched up and the ammeter sighed down. I immediately disconnected the leads, turned the voltage down (it’d been trying to put 18 volts out!), and reconnected everything forwards again. I gently ramped the voltage back up until the ammeter showed 1 1/2 amps, which is about a C/2 rate for a 2800mAh cell. I left it like that for 20 seconds or so, and the amperage started to droop as the cell’s chemistry figured itself out again. I swapped the cell into the LaCrosse and sure enough, it was detected and started charging!

I put the LaCrosse on “refresh” cycle at the 1A rate, the fastest it’ll go. On their first cycle through, each of these “2700mAh” cells actually held between 300 and 1000mAh, a pretty pathetic showing. But that’s why I ran refresh! It took six or seven cycles in some cases, but by the end of the ordeal, all six cells were performing at 2500mAh or above. They exhibit fairly high self-discharge, so they’re not going in the emergency flashlight, but I’ll find a use for them sooner or later.

Footnote: The cells in question are light green Radio Shack Rechargeable Enercells, from 1999/2000. Back then, 2700mAh was respectable for a C cell, though you can now get AA’s with the same capacity and better self-discharge characteristics. Where the C’s really shine is low internal resistance, because their electrodes are thicker. And yes, the charging regimen was a bit complicated because the BC-900 only has four slots. Details omitted for clarity.