I've got an 18v battery drill whose pack is just about completely dead. I've been thinking about repacking it, but I'd really like to use high-performance NiMH batteries in place of the current NiCads. The question is, will the existing temperature sensor still work for terminating the charge, or is the temp change for NiMHs too different?
I can't answer you question, but I've opened several brands of recharger for 14.4 and 18 volt drill batteries, and none of them had anything connected to the temperature sensor.
I think I might have seen that one - is that where they came up with a ridiculously complicated charger to add temp sensing to cheapo drill packs? The whole article seemed to be motivated by Jaycar stocking (overpriced) Sub-C NiCads and needing something to help shift the buggers.
Some of the one's I've opened had encapsulated bi-metal strip cut-outs.
It would be better for the OP to stick with NiCd if that's what's used originally, NiMh are not as good for high discharge current and easier to damage by overcharging.
That was true maybe 10 years ago, but these days you can get NiMHs that out-perform NiCads in both capacity and current delivery. I am concerned about overcharging though.
At the risk of being a pain... if you are going to replace the batteries completely, Lithium Polymers are the way to go. Varta (as one supplier) will help out with charger application circuits too
I've actually had a lot of experience with Lithium Polymer cells in electric R/C planes, but there are a number of catches - I have a charger that will do Lipos, but not 18V worth, but that aside, the big issue is that lipos need careful management when in use - if you exceed their maxiumum discharge rate, they'll die very quickly, and in the worst case, swell up and explode - without some clever electronics, this would be a hard thing to take care of in a drill where it's not uncommon to stall the motor. Similarly, if they are allowed to discharge to too low a voltage, they'll also fail prematurely.
With R/C electric planes, you can carefully choose the combination of motor, gearbox and propellor such that you know the maxiumum current draw and match it to the lipos you are using (or vice versa), and the speed controllers are quite smart - particularly the brushless ones (brushless motors are pretty much standard now) and can be programmed to sense the appropriate low voltage point and either wind back the power, shut off, or pulse the power to indicate to the pilot that it's time to land.
Given the huge advances in brushless motor, controller and battery technology in the R/C field, I kind of wonder why you don't see more of it flowing over into power tools, although it is kind of happening slowly with the introduction of Lithium Ion-powered drills (still with brushed motors), but they cost a bloody fortune.
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