Instructables user BrianH replaced the nickel-cadmium (NiCd) factory cells in his cordless drill with nickel-metal-hydride (NiMH) cells. He reports much longer useful lifetime for the drill between charges after the upgrade.
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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.
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DO NOT use a NiCD charger to charge NiMH batteries. A small but important difference in the charging curve means NiMH in a NiCD charger can result in damaged batteries or risk of fire.
The voltage droop at the end of a charge cycle is far greater for NiCD batteries (and therefore easier to detect). NiMH chargers will shut off after a subtle droop, and additionally have temperature sensors to allow them to slow the charging rate to avoid overheating. A NiCD charger will probably keep on chugging well past the cutoff point for NiCD cells until they are overcharged. At that point, the energy the charger is stuffing into them gets converted to heat, and you get either hot and damaged batteries or a house fire.
It’s been several years since I performed this mod (circa 2004)- I was pleasantly surprised to see it on Make’s blog!
Others have made similar comments regarding the difference between NiCad and NiMh. You are welcome to theorize all you want and show graphs of charging curves and manufacturer’s recommendations – I’ve seen them all — but it’s hard to argue with success. To those who insist that using NiMh batteries as a substitute for NiCads is dangerous or will give poor results – here’s my results…
No drill battery lasts forever, and yes the NiMh batteries I’ve used are now showing their age (2009). But I will say that the NiMh batteries I used out-performed the original batteries in terms of run-time and overall life of the cells themselves. I have not recorded test data on the run-time, but I would estimate that the run-time is about twice that of the originally battery pack. Both of the original NiCad packs that came with the drill refused to hold a charge for any reasonable amount of time within a year of purchase (that is what inspired the mod).
I have not been very kind to the NiMh batteries…
I have not been very diligent to unplug the charger after the required charging period. No problems, the battery pack does not even get warm while charging. Even worse, I did the unthinkable – I accidental reversed the polarity while charging the batteries!!! But there was no fire and no explosion as some might predict. Even with that abuse, the same NiMh battery pack still lived on.
As for charging the batteries – everyone has their theory about charging methodology, but I’ve simply used the original NiCad charger without any modification. My reason is two-fold.
(1) The original inspiration for this mod was to SAVE money and get longer run-time on the drill. (My $24 investment has been successful )
(2) The supplied Nicad Charger was designed to charge a LOWER capacity battery, so it’s charge rate is significantly LOWER that the what the higher capacity NiMh batteries require. I estimated that the charge time needs to be increased to about 28 hours due to the lower charge rate.
Sure, I could acquire a NiMh charger and get a more efficient charge, but so far, it has met my needs (if somewhat slow). If you really need a quick charge then I would recommend investing a NiMh charger. The NimH chargers have special circuitry to determine when the charge is complete and shut off the charger.
As for potential danger… I can understand concern here – we’ve all heard stories about laptop batteries causing fires. You certainly don’t want to burn down your house on any DIY project, but since the battery and charger don’t even get warm, I must assume that nothing is being stressed beyond normal use. I would be cautious and monitor the first attempt at charging – and always keep the charger away from flammables – perhaps in the middle of a concrete floor.
Or… If you want… you can pay top dollar and get a new lousy NiCad battery pack (if you can find one) – or discard the whole thing and buy a shiny new one, but you wouldn’t need an instructable for that! :-)
Apologies for issuing too strong a caution above. I checked out the “risk of fire” caution I had read elsewhere with a few people and nobody I know has ever seen a NiMH cell catch fire even from really abusive charging. They’re nowhere near as temperamental as Li-poly. Also, because the capacity of the new battery is more than double the old one, the charge rate will be easy on the cells.
So all you really risk here is the longevity of the new pack. And a cheap timer switch on the outlet would be an easy way to shut off the charger as a backup. Set to something like capacity in mAh / charging rate in mA, plus some fudge factor for inefficiency.
I just want to say there is a company around that does these things for you, if you don’t have the tools named Batteries Plus.
There are a lot of these stores around America, and they can be a reasonably cheap method if you don’t have the tools to do this.