But can anyone tell me if NiMH batteries can be overcharged? I imagine they are probably no different from any other kind of battery, but I thought I would ask just in case. I'm building something that will take a 9V NiMH (which will actually be 8.4V), and am wondering if I need to give it a regulated power supply to keep it charged, or will any old wall-wart do to keep it topped off.
Thanks,
Dave
Hope your device doesn't need much power - 9 volt alkaline batteries have very small AH capaility and "9 volt" NiMH have even less.
For maximum battery life, you need a charger that recognizes the charge voltage curve changes and knows when to shut off. Neither a regulated supply nor a wallwart will work well (as in resonable battery life). I've seen a number of power tool batteries damaged by the cheap chargers provided with the tool (Craftsman, Ryobi, etc).
lithium and lead-acid batteries can be charged to a voltage.
NiMH is harder to handle as the voltage profile as it charges is very flat and reverses at the end.
So unless you can switch to a different battery chemistry charging correctly will be tricky.
-- ?? 100% natural
Early NiMH batteries were very sensitive to overcharge. Nowadays, they are much more tolerant, but still not as robust NiCd.
With a gentle trickle charge, expect the battery to last a few months to a year.
Different battery chemistries are actually very different. A charger made for one chemistry will generally not work with another. The exception is that a NiMH charger will usually work with a NiCd battery.
Float charging a NiMh (or NiCd) battery is essentially not possible, certainly not practical. The voltage is affected by numerous variables, and the voltage would have to be extremely accurate.
Depending on your application, a different solution might be better.
A primary (non-rechargeable) battery might be applicable. If you get a lithium based one, they have excellent shelf life.
For a more dynamic application, perhaps a supercap? Here's an idea:
-- RoRo
THANKS all, for the posts in answer to my question. Will look at Ni-cds as a possibility, but more than likely will simply use standard alkaline batteries and take them out of the circkuit while the item is not in use, sitting in its cradle. Do appreciate the help.
Dave
AFAIK NiCd is exempt from EU RoHS legislation, but looking through various catalogues for a couple of AA cells for my shaver - they seem to have virtually disapeared.
Voltage sensing chargers were starting to come onto the market just as NiCd cells were starting to get scarce, the NiMh cells have a similar terminal voltage dip at full charge, but its more subtle than with NiCd cells and took the manufacturers a while to catch up with reliable chargers.
The correct (dip sensing) chargers for 9V block batteries are more complex, more expensive & unreliable compared to chargers with a row of single-cell compartments.
A holder for 6x AAA isn't that much bigger than a PP3 and you get a huge increase in Ah capacity, and reliability with rechargeables.
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