Remanufacturing a 12V power tool battery

Cordless power tools often die not because they’re worn out but because the battery fails, or only holds a charge for a short time. Every tool’s battery pack seems to be different, either from the sheer cussedness of consumerism making you buy a new tool, or because nobody has standardised battery packs, You can fight back, because though the battery packs are all sorts of sizes, the parts inside are usually drawn from standard sized parts, they’re merely packed in different ways.

I had a cordless strimmer, once advertised years ago in the Guardian, from a no-name supplier with two battery packs. Both of which are now duff – a strimmer is a fairly hard life for the battery packs with about 50-100W. Here’s how I remanufactured the battery packs. These are NiCd battery packs – the key issue to to replace like with like. You can replace NiCd with NiMh but don’t cross any other chemistries.

A possible source of raw materials

A possible source of raw materials

The trouble with remaking the battery packs is that these things are presumably made in China, and the cost of the battery pack is a significant part of the cost of the tool. Parts suppliers like CPC seem poor value for replacements – at over £5 a cell for sub-C types remaking a 12V pack is about £50. Going to UK specialists brings things down to £22 for 10. Go to China on ebay and you can get 6000mAh sub-C cells

6000mAh sub-C cells from ebay. Yeah, right

6000mAh sub-C cells from ebay. Yeah, right

Yeah, right. You don’t get 6000mAh cells in the sub-C size – there’s not enough room. I once bought some 2400mAh AA recharegables from Ebay. As soon as I had them out of the package I knew I’d been had. The batteries were light, more 1500mAh or 1800. And they didn’t last like Maplin 2000mAh AAs.

So you can pay £10 to people with a printing machine who make wildly extravagant claims. Or pay a little bit more £12 to what are probably slightly less outrageously ambitious claims for these.

2900 is a bit more believeable

2900mAh is a bit more believeable

It’s a tough call. The good original battery was tested charged at roughly 1C for 1hr at 1.6A, discharged at 20W (1.6A) for 40min down to about 8V. Looks fairly good… The load of the strimmer is clearly higher than that, it’s only good for about 15 minutes runtime. Now that’s probably fine for consumer use, but the trouble is we’re using this to strim between flowers and the beds are long. The basic problem is cordless devices are gutless and more suited to domestic than professional use. However, by adapting the carger to use a Makita drill battery fast charger this can be made more useful – that takes half an hour to charge one of these batteries, so I went the remake of the second battery using the old cells from the Makita battery.

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