On 17 Nov 2020 15:20:42 +0000 (GMT), Theo
wrote:

T i m wrote:
On Tue, 17 Nov 2020 12:13:56 GMT, Pamela
wrote:
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.

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?

Yes and no.

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

NiMH cells can tolerate a certain amount of overcharge without
damage - they just turn it into heat.

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

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.

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.

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.

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). ;-)

But if it's higher the risk is cell A overcooks before B has
finished.

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.

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.

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

[1] If the current is sufficiently low, it doesn't matter if any peak
(charge) detection doesn't work, as long as the charge is terminated
within a reasonable timescale (as you often see with cheap
rechargeable electronics that say 'Charge for at least 12 hours and
then disconnect from the charger').