NiMh battery charging.

Nearest that I know is one that I bought after asking on here

Surely that has been incorrectly taken from the standard advice to slow charge a new battery for a long time (say 8 hours) to ensure all cells reach max charge. Hard to see why it's applied to single cells.

That Youshiko YC4000 looks like a clone of the original La Crosse BC-700, which is a nice charger.

NiMH has a faint "negative delta V" voltage drop at low charge rates. If a charger uses that method (and no others) to determine termination then the charger may miss it and overcharge the cell causing vented gas that reduces capacity. For that reason some people prefer to charge at no less than 0.5C (which is 400mA for the AAAs).

There used to be a forum on charging at CandlePower which was quite active. Here's a thread which goes into more detail than I have time to read but seems to discuss this.

Thanks for that. It's a bit like my Nitecore Digicharger D4EU. That seems to have an (automatic) charge current of 375mA.

Looking at the spec of the one you linked it seems to suggest it only goes down to 500mA, bit I'm not sure I trust it all:

"500mAh / 500mAh, means 2000mAh battery can be fully charged in 2 hours ..."?

Cheers, T i m

Oh, I have already one of those (D4EU) Peter (and use it mainly for

Oh, that's interesting (and would be handy if it could). OOI, I currently have an AA and AAA in mine and both slots suggest they are charging at 375mA.

Going on what you said though, I just had a play (what manual? <g>) and found you can indeed toggle between '-dV/dt' and '-dV/dt Low' and on low it drops to 150mA. ;-)

Whilst 150mA is still nearly twice the recommended value for the AAA's, it's a bit better than the 200mA on the other charger so thanks for that. ;-)

Cheers, T i m

There's a decision tree of sorts, here.

Most charger manufacturers, they hate BOM cost, and the odds of them putting thermistors in places they're supposed to, is just about zero.

Paul

Same thing though isn't it Pamela? A battery only (typically) being a combination of single cells wired in series and so the current though the battery = the current though each cell?

Yup ;-)

I had one and it (eventually) died so I replaced it with that one.

Ok, makes sense.

That looks very interesting / informative thanks. ;-)

Cheers, T i m

Yeah, thanks for that Paul.

Yeah, I've read that elsewhere and judging how hot some of the cells

Cheers, T i m

Yes and no. NiMH cells can tolerate a certain amount of overcharge without damage - they just turn it into heat. In a pack, you aren't monitoring the cell voltages (like you would in lithium ion) so you're applying a current and hoping the cells take charge equally (ie that current x their cell voltage is the amount of power they're taking) until you reach charge termination of the whole pack. If the current is low, it doesn't matter if cell A is fully charged and burns it off as heat while cell B is still charging. But if it's higher the risk is cell A overcooks before B has finished.

If you're charging a single cell you know charge termination is for that cell alone and so you can run a higher current and still detect it correctly.

Theo

I'll err on the side of 'Yes', given the context of these 'safe' low current charging questions. ;-)

Sure, as you say, dependant on that level of 'heat' etc.

Agreed, and why when doing such it's often *safer* to do so at a low current so any overcharging *doesn't* created an excess temperature condition.

And what I believed the topic was on at that point in the conversation, low current charging / chargers. [1]

You can have an overcharge *rate* even at the beginning of the charge cycle etc.

I have a few 4.5AH NiMh D cells that have gone to sleep and the terminal voltage is sufficiently depressed that my automatic charger determines they are too low a voltage (below 1V) and so won't charge. I put it on my new bench charger and gave it a couple of mins of 3A (way above the suggested 900mA max) and it got noticeably warm in that short time. (One has since been auto cycled on said charger and is currently on a self discharge test, the others are waiting to see if more 'shocks' might wake them up). ;-)

Understood. When RC car racing we preferred 6 cell 'stick' packs because each cell had (nearly) the same exposed surface area and / so we could monitor the temperature and voltage of each cell easily.

Understood (if that's what you are doing). ;-)

The same (worse?) issue occurs when discharging a non balanced battery of course, where the weakest cell can end up becoming reverse charged, helping to make it even weaker (and why I prefer chargers / dischargers that treat each cell separately).

Cheers, T i m

If one cell in a battery (where the cells are in series not parallel) is weaker than the others then it will overload during charge sooner. A very low charge rate, say C/10, doesn't cause an NIMH cell to vent -- so it can be applied for much longer while all cells reach full charge.

Similarly on discharge to a low state, albeit not at issue here, a weak cell in a battery can be forced into reverse polarity by the other cells discharging normally -- as I have seen.

Anyway, the bottom line seems to be that a single NIMH cell doesn't actually need a long low-current conditioning charge.

I would hope capacitors would supercede cells eventually, but they are expensive at the moment.

Bless! Are you an Art Student?

Must be. Can't even spell supersede...

I'm hoping for little, hand wound perpetual motion machines that just need winding up every day to overcome losses will produce indefinite amounts of electricity to replace batteries. You know, like when you stroke a dry cat on a nylon carpet. I expect it is the insulation that is the biggest technical problem.

I must have been thinking of supercilious.

I note today in my wry scan of the latest media bollocks that the Scots all wanted free public transport.

Well mine's a tireless ridable unicorn with wings. Thats what I want. I think the government should respect my wishes, don't you?

Actually what I want is an an Atomic Aga. With back boiler. Running off the decay heat of high level nuclear waste. When not employed cooking, the heat is diverted to a back boiler for central heating and DHW, or to a Stirling engine to generate electricity....

After 60 years you give the no longer hot waste back to Sellafield, and receive another free brick of hot nuclides...

Can you do Thinking? Wow!

You may yet get your wish; you know the Government has nearly tied up a wonderful post-Brexit trade deal with Fairyland?