Probably a 1st year apprentice question, but can you substitute a NiCd battery of the same voltage/capacity in place of a NiMh without creating charging problems?
Different cell chemistries usually have quite different charging requirements (and usually different cell voltages too) - so unless the charger circuit is specifically designed for both, I'd say no.
Although they are both charged with constant current , the requirements are different, and NiMh are more sensitive to overcharge, and should not be trickle charged at the same rate.
Thanks both. Having great difficulty getting a like for like NiMh replacement. It’s the physical size that’s the problem, all replacements are a good bit longer & won’t fit in the luminaire - trying to avoid the cost of replacing the whole fitting, ‘cos there’s a few to be done!
Probably OK in an EM fitting as most designs do just have a mix of constant voltage and then constant current.
If you have the fitting beside you , and can you tell me the cell size or Ah rating, and then put a meter in series and measure the on-charge current, and I can advise better.
The reverse, fitting a NimH cell where a NiCd used to go is quite common, and usually very successful, except in applications that push the cell hard for fast discharge or fast re-charge.
You may come unstuck if there is circuitry to report failing cells by measuring the cell impedance, as niCd are quite a bit 'droopier' on load, even when nothing is wrong, but having drooped, often have a higher short circuit current.
As ever, the replies are much appreciated. I believe the existing stick consist of 3, 1.2v/1.3 Ah Sub C cells. The stick is only 145mm in length. Since posting, I discovered this afternoon that they were manufactured by/for Emergency Lighting Products Ltd(Ref B057). I've checked their website with no luck - I'll call tomorrow and see if they are open for enquiries.
Mike, unfortunately the fittings are on site & I won't be back there 'till early in the new year. Meantime, thanks again for your advice/views.
Sub C cell size varies a bit between makes, especially with welded tags, but 40-42mm end to end and 20-25mm dia is typical, so a 145 length stick of 3 is nothing magical.
I'd be surprised if you could not get cells in the right chemistry and size, and the same or higher capacity, welded up into a suitable stick, even if it is actually not a stock object.
Folk like Strikalite make a living out of putting together battery packs in obsolete sizes, (they have the spot welder and the heatshrink as well as a shed full of cells ) so if you do not fancy a misfit or rolling your own, they may be worth a call. (I have no connection, other than using them to make some rather special ones up for me at short notice a few years ago. They seem to be a modest outfit, and very helpful on the phone )
