nimh replacement batteries for nicad cordless drill

inetquestion wrote in news:6a27ff3b-b06b-4c1b- snipped-for-privacy@k13g2000hse.googlegroups.com:

NiMH requires a different charging system. Most NiCd chargers use a thermal sensor in the pack to cut off charge when the pack nears full charge,but that doesn't work with NiMH. You may have permanently damaged your NiMH cells.(ruined them)

--
Jim Yanik
jyanik
at
kua.net

"inetquestion"

** So you bought 12 NiMH cells - what size, brand and what mAH capacity ??

Do ( or did) the cells get warm during charging ??

How did you connect them all up ?

Apply a hot soldering iron to them, maybe ??

....... Phil

The nicads were replaced with 12 - 3500mah sub-c nimh cells with solder tabs. Connected in series...yes they were soldered.

They don't seem to get hot during charging that I've noticed.

"inetquestion" "Phil Allison"

** Those tabs are meant for spot welding - but soldering them is OK.

Long as you know how to solder.

** No-one here can give any sensible advice as the problem is unique to you are the charger you own.

Normally what you SAY you have done works.

....... Phil

Hmmm. In my readings of the literature from battery manufacturers, it appears that a thermal-sensing cutoff is actually _preferred_ for NiMH (as long as it's properly designed and calibrated).

NiCd chargers sometimes use a thermal cutoff, and often use "negative delta V" detection circuit (the battery terminal voltage rises during charge, and then drops slightly after full charge is achieved and the battery warms up).

The negative-delta-V system isn't very good for NiMH batteries, though, as the voltage doesn't drop as much after full-charge is achieved, and the battery can suffer a significant overcharge before it occurs. I've read in several places that *this* is the reason that chargers originally designed for NiCd batteries are often ungood for NiMH charging.

The cutoff regime that manufacturers seem to recommend, is a primary cutoff based on rate-of-temperature-rise (absolute-temperature-rise is also usable but not as sensitive), with a secondary cutoff based on a zero-delta-V detection (i.e. peak voltage), and a tertiary cutoff based on a timer.

--
Dave Platt                                    AE6EO
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SNIP

I agree, it should work fine for your purposes... Try charging them twice...leave a few hours between charges. Try measuring the terminal voltage after a charge. You should see about

1.4 - 1.5 V per cell

Sounds like your charger is on the blink, and there was nothing wrong with your NiCd pack...

IIRC nickel hydride cells don't have the peak capacity of a similar NiCd, so they're not as well suited to things like drills, where you need lots and lots of torque for brief periods.

Cheers,

Phil Hobbs

"Phil Hobbs"

** Very far from true, nowadays.

The OP has used 3500mAH sub-C size NiMH cells.

These have extraordinary performance compared to any Ni-Cd ever made of the same case size.

Ask any RC modeller.

....... Phil

I'm not sure about that, they were designed as a less toxic replacement for the NiCd market, they certainly come in large capacities and battery packs for tools. The only notable downside to the NiMH chemistry that I'm aware of is the relatively high self-discharge rate, but this is now overcome, see the Rayovac "hybrid" NiMH technology for example, right now available only in AA and AAA size. This seems like a fairly comprehensive comparative survey and summary of NiMH power tool performance:

See Annex 2 Behnke, German EPA, 2001, Summary of discussion with power tool suppliers (Makita, Panasonic, Bosch, Black & Decker, Metabo) additions by S. Karcher, 2004, says "high current approximately the same."

Good to know, thanks all. One of the many things I've been out of date on, obviously.

Cheers,

Phil Hobbs