I'm using NiMH 2500mAh AA style cells to make a multi-cell battery pack used inside a robot. The cells all sit together in a multi-cell AA battery holder, from which run a pair of wires to a female connector. I have a male connector on the main electronics of the project and when I connect it to the female I can power the robot. For mechanical reasons the battery pack is buried deep inside the machine, getting it out takes a good hour of unscrewing other stuff, but the male and female wire ends (female wired to battery, male wired to PCB) can be arranged to sit outside the robot, or near the surface so you only need to remove one little panel to connect and disconnect them. The robot takes 8 cells in series to supply enough voltage and runs pretty well. But I'm sick of undoing so many screws to get the battery pack out to charge it.
At present charging it means taking all 8 cells out of the holder, putting each of them into a thing like this
and letting them fast and then trickle charge as controlled by a microcontroller in those special wall warts, note that my circuitry in the robot actually cuts off the power draw* before the cells get quite as low as 1V per cell, so I'm always charging from part charged to full, never from fully empty. Using that kind of wall wart charger means separating out each cell of the pack then reassembling them afterwards, slotting into place and spending another good hour rebuilding the robot around it.
I wondered if, given that programmed** chargers exist*** which can handle multiple NiMH cells in series, I could just get one of those, connect it to the batteries inside the robot via a male connector going into the battery pack's female connector and charge the whole pack at once? That way I would only need to lift one panel to charge the whole pack rather than disassembling the robot each time to get at the cells buried in the centre.
I know that LiPo batteries, which I am NOT using here can ignite and explode if charged in series, or even if discharegd in series. NiMH on the other hand is well known to be safe to discharge in series. LiPos tend to need balancing circuits, I've heard that NiMH do not even when charged in groups? I am not looking here to do any parallel charging of NiMH cells, just series, I've heard parallel can be troublesome and I haven't any need for parallel cells anyway. LiPo multi-cell packs come pre-made from a factory in which some sort of clever techniques are supposedly used to balance the cells before use and ensure they are all identical, when using NiMH cells as a battery pack can I put ANY combination of the same model of cell together? Do they all need to be exactly the same age with identical use histories, or is "pretty similar"**** good enough, most of the places I've bought NiMH cells from sell them in boxes of 2 or 4, not of 8 so while I could guarantee that any I used for this would, once in my hands, get the same use histories I couldn't be sure that the cells had been manufactured on the same day and lost equal amounts to self-discharge during shipping and while sitting in warehouses before I bought them?
My other issue is the location, being deep inside a robot any heat generated in the batteries during charging might struggle to escape. The batteries do get particularly hot when the robot runs, the current draw is about half an amp and they stay cool, but I'm not sure if despite staying cool in their rather well insulated location during discharge they would also stay cool during charging. I know heat can be quite bad for NiMH cells lifespans, I'm not sure if it also presents a risk of them igniting or venting.
Providing that the first few goes of charging find that the cells don't get too hot, should charging together like this be pretty feasible? Is it likely to be particularly risky, should I be looking to put a LiPo safe style fireproof bag around the battery pack between it and the robot which surround it?
Thanks
*if I run at lower voltages things don't work so well, even with this shorter than normal cutoff I get several hours use before the batteries drop below it
**for specially designed charging curves over time
***this sort of thing https://cpc.farnell.com/ansmann/ac24/charger-ac24-nicd-nimh/dp/BT05673 does 2 to 4 cells but I've seen a 5 to 10 cell version somewhere, and plenty of options with the same sort of MCU controlled charging curves as the separated cell wall warts follow
****they will all be the same number of mAh and the same model from the same manufacturer regardless