rechargeable battery [technology] swap

[Jules Richardson]

So, what's likely to happen if I put a 600mAh Ni-MH battery pack in a device (cordless phone in this case) that's expecting a 400mAh Ni-Cd pack?

Right now it gives the impression of working - the Ni-MH pack seems to be taking a charge and isn't getting hot (yet). But I've read elsewhere that under charge Ni-Cd cell voltage will peak and then drop off, whereas Ni- MH cell voltage will simply level off - so potentially a charging circuit designed for Ni-Cd might look for that drop and keep on charging a Ni-MH cell until it eventually goes bang. What I don't know is if the charging circuit typically found in small consumer devices (as mentioned above, in this case a phone) is that clever...

I had a spare Ni-MH pack and so cut its plastic sleeve open, along with the dead Ni-Cd pack, and there's no cleverness in the packs themselves, just three cells in each along with a wire (such that the wires to the outside world come out on the same side of the pack, but which also could be designed to work as a fusible link, I suppose).

cheers

Jules

[meow2222]

Nothings going to explode. I've found NiMh to be far more tolerant of mischarging than NiCd were, I wouldnt worry about it. You just get more talk time.

NT

[Gazz]

wot e sez, the cells have vents in the caps, so worst that will happen is they vent a little, but i imagine you'd have noticed the hot electronics smell ages before that, and anyone using the phone would be complaining of the back of it burning their hands.,

Modern chargers use the delta peak method of determining full charge, one of the battery chemistries gives a much smaller and thus harder to detect peak, and i believe it is the ni-mh ones, so the phone charger could miss the fully charged 'signal' but it prolly charges the batteries at such a low current it dosent matter that much, especially if your missus is owt like mine, the batteries will spend 90% of their life discharging.

[Brian Gaff]

Well I've certainly had no issues since the newer batteries that do not self discharge as much became available. It was indeed true that the first generation of the NiCad replacements died in the machines quite fast, but as long as they are the newer kind I've seen no overheating or early death issues. Of course as has been said it really depends on the sophistication of the charging circuit. I'm sure its very easy to design one that is crap on any battery. The classic one is just s series resistor of course as I discoverd when I took a dustbuster apart some time ago. Brian

[Andrew Gabriel]

I suspect a phone old enough to be using NiCds is probably just a current limiting resistor for the charging circuit. There was little to be gained by having anything more expensive in slow charging applications.

[The Natural Philosopher]

NiCc is and always was a fast death technology. the required charging regime is an accurate current limited constant voltage source.

a resistor is suitable, but the constant voltage is mandatory.