NiMH and NiCd interchangability.

[sitdwnandhngon]

I used to lurk in this forum several years ago while dabbling in custom lights, so I figured this would be the best place to ask this question.

I was recently given some MAX tools (rebar tools) that use 9.6v NiMH battery packs, each pack is smoked, they either don't take a full charge or won't charge at all (age related for sure).

The smaller tools use 2.0 mah packs, the larger tool uses a 3.3 mah pack.

I have found some refurb units available, but before buying a whole pile of them I was wondering if it would be possible to just replace all the cells with NiCd cells, as they can be salvaged dirt cheap from Harbor Freight and Ryobi battery packs.

I have multiple chargers for each, and could try to find some specs for them, but I figured I would ask here first to see what you guys think. I've replaced cells on old NiCd packs before, but have never had a tool that uses NiMH cells (everything else I own and use for work now is Li-ion)

This is what the larger packs look like. Thanks

 

[DIWdiver]

As I recall from back when NiMH was the hot new tech (yes, I remember that), you can use NiCad cells in a NiMH charger, but not the other way around.

That's because NiCad chargers tend to do trickle charging, which is bad for NiMH cells. A NiMH charger should charge to completion, then stop.

The charge termination characteristics of the two chemistries are different, so it's possible that some NiMH chargers will not shut off after full charge of NiCad cells. This can result in trickle charging, which is fine, or overcharging, which is not. Overcharged batteries can get quite hot, which is bad for both the batteries and their surroundings.

You could experiment with one pack - if it gets warm during charging, then cools off even if left on charge it's fine, even if it stays slightly warm. If it gets hot, not.

 

[sitdwnandhngon]

So I experimented last night and rebuilt on pack with Harbor Freight NiCd cells. And it worked fine, but the chargers were picky. One didn't care and charged it, one refused to charge it, and one was kind of hit or miss if I wiggled it around in there.

I tore apart a second battery and found that it was very well built (glued like crazy) and decided I didn't want to fight with that one. Then I tore apart the bigger batteries and found that they use a totally different cell size anyway, and some type of circuit board mounted to it.

Because I am going to use these professionally, I reasoned I better just pony up and get new packs. I found aftermarket packs on ebay, that use NiMH cells for pretty cheap, so it seemed like the logical thing to do.

It's kind of odd they use NiMH cells though, most other tools of the era were NiCd, and the NiCd cells seemed to work just fine in the tool.