Charging NiMh batteries in series

[JackOfDiamonds]

I have a bunch of C-size NiMh that I bought to power a portable Cpap. They worked great once but I need a way to charge them and it seem C-size battery chargers are not common.

I have a good AA charger and since the chemistry is the same, if I could mechanically adapt the C cells to the charger I think it would work. But I can't think of a good way.

My next idea was put the C cells in a battery holder, and hook it up to my bench top power supply set to 1/10 C charge current. I've been told you can charge NiMh at 1/10C indefinitely and just leave them for 12 hours or so they should be fully charged. But that will take a long time to charge 6 batteries that way. If I put them all in a series battery holder, and charge all 6 of them at 1/10c as a pack, is there any balancing issue that will happen doing this?

 

[Old Man Roger]

I don’t recall there being a balance issue when charging Nimh batteries. There are inexpensive rc car chargers designed to work with Nimh batteries, check out Traxxas wall wart type chargers.

 

[BrandonV]

Low current/indefinite charging is not ideal for NiMH cells at all.

This is a very good website that explains the differences in charging NiMH cells.

Battery Charging NiMH

C-sized battery chargers are pretty easy to obtain cheaply, especially ones with microprocessor based charging algorithms.

https://www.amazon.com/EBL-Rechargeable-Batteries-Discharge-Function/dp/B073ZD7XVV?tag=atomicindus08-20

 

[mm08822]

I bought a "Keenstone H4" charger from Amazon a few years ago. Does several sizes and chemistries. Less than $30.

I've used it several times on new nimh. Worked well. Each of the 4 positions are independent.

 

[JackOfDiamonds]

I'm charging three 5000mAh batteries in series. I set my current to 500mAh. And I read up that the voltage of fully charged NiMh cells os 1.4-1.5v, so I set the max current on my power supply to 4.5V. I think this will charge them up to 1.5V per cell then taper off the current exactly like like a normal battery charger, right? The voltage started at 3.9 and after a few hours it's up to 4.2.