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Electronics Forum / Electronics / January 2008Repacking drill battery
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? "Russ" > 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? ** Probably will be OK - but check the rating on that temp sensor. NiMH cells can be damaged by overcharging and overheating. This link ,with seems pretty comprehensive, suggests 45 degrees C is the max temp before charging at 1C ( ie 1.5 hours to full charge) must terminate. http://www.powerstream.com/NiMH.htm ....... Phil > "Russ" >> [quoted text clipped - 13 lines] > terminate. > http://www.powerstream.com/NiMH.htm Cheers Phil - I'll open up the pack and have a look at the sensor. > 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? Silicon Chip recently had an article about this sort of thing. Worth a read, if only to discourage you. >> 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 [quoted text clipped - 4 lines] > Silicon Chip recently had an article about this sort of thing. > Worth a read, if only to discourage you. 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. > 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. Just a possibly-relevant data point :-) Clifford Heath. >> will the existing temperature sensor still work for terminating the >> charge, or is the temp change for NiMHs too different? [quoted text clipped - 6 lines] > > Clifford Heath. 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. >>> will the existing temperature sensor still work for terminating the >>> charge, or is the temp change for NiMHs too different? [quoted text clipped - 12 lines] > 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. "Russ" ian field wrote: >> 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. ** The shortest safe discharge time for NiCds is about two minutes - as used by model boat and model plane hobbyists. Good quality sub-C cells will deliver 50 amps at about 1.1 volts this way. Modern NiMH sub-C cells can also be discharged in two minutes, while delivering up to 100 amps, since they have about twice the Ah capacity. An electric drill normally draws far less current and so is no problem. ....... Phil 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 "TonyR" > At the risk of being a pain.. ** No risk involved, mate - YOU are a 100% pain. > if you are going to replace the batteries > completely, Lithium Polymers are the way to go. ** Straight down the pathway to hell................ http://en.wikipedia.org/wiki/Lithium-polymer There are enormous problems with overcharging and over discharging. Plus a 2 to 3 year limit on life, whether used or not. Bad news for the OP's drill. ...... Phil > 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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