"Negative delta V" chargers for NiMH batteries

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I like the higher capacity NiMH cells.  But these high capacity
cells need so much more charge that a fast charger (or much more
charge time) is needed.  

Many fast chargers nowadays decide that full charge is approaching
by sensing the moment when there is a slight drop in cell voltage.  
(Negative delta V.)

However my own -dV fast charger will charge two cells as a single
"unit" and I wonder what happens when one cell will hit its -dV
point a long time before the other cell does.  Does the -dV point
get masked and overlooked?

In fact if I put four AA cells in my charger then it seems to
treat all four cells as a single "unit" and aims to detect -dV
from all four as a whole.  

Surely this arrangement must often lead to overlooking the -dV
point?  Then the large currents used for the fast charging would
now make the cells go well into overcharge until maybe a
temperature sensor detected the situation.  Ugh!  Overcharging
NiMHs is a good way to spoil them.

Am I overlooking any important factors which mean it is not as
gloomy as it may seem?

Or maybe these "2 cell" and "4 cell" -dV fast chargers are just
too crude to consider using if you want your NiMH cells to last?

Re: "Negative delta V" chargers for NiMH batteries

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One of the reasons most Negative Delta-V chips are for small cell quantities is
that the effect is masked over a number of cells. For seriously large
series/parallel Lithium-Ion cells we use individual cell/row sensing, and you
could probably go that route but it makes for a complex charger.

For long strings of NiCd or NiMH (25 or so) we use two thermal switches in the
pack in series with the charger input, discharge down to 1V per cell first and
then charge at accelerated rate for a timed period.

Peter A Forbes
Prepair Ltd, Luton, UK

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