To what extent are NiCd cells replaceable with NiMH, in charging units?

Factual Questions
1

I bought a really, really old laptop - one of the first PC laptops to boast a colour screen - equipped with an amazing 386SX processor and a 400MB hard drive.

I intend to use it as the brain for a robotics project.

The battery is of course almost completely dead (‘almost’ meaning it provides about ten seconds of independent power when fully charged). I probably won’t be able to find a replacement for it now.

I took the battery apart - unlike modern ones, it wasn’t welded shut - the casing just unclips to reveal a neat row of six standard size C NiCd rechargeable cells in series (7.2V in total).

I could buy some more NiCd cells to put in there, but I’d prefer to use NiMH - do they have the same charging requirements and methods as NiCd (i.e. are they suitable for direct replacment in a machine that was built with only NiCd cells in mind?), or am I courting disaster?

2

NiMH chargers use a slightly different end-of-charge determination (as I recall it’s inflection point vs. delta-slope). Charging NiMH batteries in a NiCd charge is likely to overcharge them.

3

Thanks for that. I guess I’ll have to stick with NiCd then…

I’ve just found that the cells in the battery actually aren’t standard C size - they’re a couple of mm shorter and slightly slimmer. Hmpf.

ETA: they’re apparently a size called Sub-C (still available mostly for power tools and RC, so I might be able to get some).

4

It might be worth taking a little trouble to use NiMHs though - they tend to have nearly double the capacity of NiCds and they’re not that delicate, unlike lithium batteries which will combust if you look at them in a funny way.

5

That would be nice, but since the charging controller is going to be built into the laptop, I don’t think there’s any way I can modify it.

6

If weight / space isn’t an issue, you might try slapping a UPS in there with it and running the show from that.