# To what extent are NiCd cells replaceable with NiMH, in charging units? **URL:** \ **Category:** Factual Questions\ **Created:** [December 14, 2007, 12:36am UTC](https://boards.straightdope.com/t/to-what-extent-are-nicd-cells-replaceable-with-nimh-in-charging-units/429935 "2007-12-14T00:36:20Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 14, 2007, 12:36am UTC](https://boards.straightdope.com/t/to-what-extent-are-nicd-cells-replaceable-with-nimh-in-charging-units/429935/1 "2007-12-14T00:36:20Z")
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? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 14, 2007, 1:36am UTC](https://boards.straightdope.com/t/to-what-extent-are-nicd-cells-replaceable-with-nimh-in-charging-units/429935/2 "2007-12-14T01:36:08Z")
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. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 14, 2007, 9:14am UTC](https://boards.straightdope.com/t/to-what-extent-are-nicd-cells-replaceable-with-nimh-in-charging-units/429935/3 "2007-12-14T09:14:55Z")
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). ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [December 14, 2007, 9:38am UTC](https://boards.straightdope.com/t/to-what-extent-are-nicd-cells-replaceable-with-nimh-in-charging-units/429935/4 "2007-12-14T09:38:01Z")
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. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 14, 2007, 9:47am UTC](https://boards.straightdope.com/t/to-what-extent-are-nicd-cells-replaceable-with-nimh-in-charging-units/429935/5 "2007-12-14T09:47:57Z")
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. ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [December 15, 2007, 1:52am UTC](https://boards.straightdope.com/t/to-what-extent-are-nicd-cells-replaceable-with-nimh-in-charging-units/429935/6 "2007-12-15T01:52:59Z")
If weight / space isn’t an issue, you might try slapping a UPS in there with it and running the show from that.