Author Topic: Dead battery packs? Maybe not.  (Read 1792 times)

Dead battery packs? Maybe not.
« on: April 26, 2012, 02:51:22 pm »
I was recently given a Smart bottle cage fitting 6 volt battery pack: the second generation Nimh type, rather than the much larger & heavier NiCad type. It hadn't been used for several years, & its state was uncertain. If functioning, it'd hold up to 4.5 amp hours.

The pack had a switch, & an LED to indicate if it was switched on. The LED lit up when the switch was pressed, but faintly & flickering. Lights plugged in to it failed to light up. I therefore charged the battery fully, & tried again, with a 10 watt halogen light. It flared briefly, then went out. The light worked when powered by another battery. A multimeter showed no voltage from the battery pack, & it wouldn't light up any other lights.

My first thought was that the cells were dead, but I was puzzled by the brief flare from the light. I tried a bit more charge, & got the same result. Third time, nothing. Odd, I thought. Since the pack was useless as it was, I saw no reason not to dismantle it, so did so. This turned out to be a destructive process, despite the presence of two screws in the base. They were holding internal bits in place, not holding the casing together. It was glued, rather securely.

Inside I found a PCB (broken in the dismantling) & a pack of five 18670 nimh cells. According to my multimeter, the pack was putting out a nice healthy 6 volts (actually slightly over, but it's a cheap & I suspect not very accurate meter), so I thought I'd see what it held, & wired it up to a pair of 6 volt 6 watt lights, which should draw 2 amps. Two hours later, they were still it up, though starting to dim a little, & the voltage tested at a bit under 6. Result! That's at least 4 usable amp hours, from a bit over 300 grams of battery.

I wired a 2.5mm DC socket cut off a dead light to the pack, weatherproofed it with a plastic bag & tape, & bought a Magicshine battery bag to hang it off top tubes (just fits in). Works a treat.

The moral of this story? If you have a battery pack with electronics in it, & it stops working, don't assume you have to bin the whole thing. The cells may still be sound, & recoverable.
"A woman on a bicycle has all the world before her where to choose; she can go where she will, no man hindering." The Type-Writer Girl, 1897