Charging 750ma Ladda's at 1000mah

Batteries and Chargers Rechargeable Batteries
1

So my xtar vc4sl that everyone hates actually cuts off NiMH charging pretty darn well most of the time with every cell I’ve tried EXCEPT these Ladda 750mah AAAs. Cuts of all other Ladda’s perfectly, but with these 750 AAA’s, at 250mah or 500mah it just doesn’t recognize dv/dt at all and will put 1200mah in them and let them get to 120°F if I let it.

Then I noticed on that Lygte review site on his review of the eneloop 750ma AAAs he uses a 1000mah current through the whole charge. Not sure if that’s just a random coincidence or whatever but I gave it a shot just now and it worked perfectly. Terminated charge at almost exactly 750mah, dead on. Ambient temp was 73°F, batteries stayed below 80°F for the first half of charging, hovered around 85°F in the later stage, topped out at like 90-91°F.

Should I just charge them like this all the time? Is this a known thing?

2

I think NiMH has a minimum charging current or else your charger can miss the cutoff.

3

Yes this is a known “problem”. I don’t know enough to explain the physics/chemistry.

I charge my AAA/AA NiMh at an amp (mostly because it’s max my chargers can do).

4

Thanks for posting Jeffg, that is some good test data to understand why chargers are missing the cutoff and how to correct this problem.

5

I havent had any termination issue charging at los currents, nor with my laddas nor with my eneloops… Maybe I have been lucky…

Thanks for the tip.

6

Correct. But typically 500mah is more enough for the charger to recognize the cutoff with every other AAA I’ve tried. 500mah is pretty standard for fast charging AAA’s. I’m wondering about these 750ma AAAs specifically, they’re the only ones the charger doesn’t recognize cutoff at 500mah.

They are the lowest capacity aaa’s I’ve used, I wonder if that has something to do with it. I thought itd be the opposite because 500mah is a higher C relative to the others, I thought thatd correlate to an even more noticeable drop to the charger but maybe not. I’ve read below 0.5C is typically when the difference might be small enough for the charger to miss, but over 0.5C is typically noticable. So I figured 500mah for a 750mah cell would be plenty. It has no problem cutting off chinese amazon or energizer or w/e AAAs at 500mah, all of which have higher capacity than these. And it has no problem cutting off the 2450mah ladda AA’s at 1000mah. Maybe capacity is just as big a factor in providing a noticable voltage drop as relative charging current is. Hm.

7

Thanks. Also, although this was probably already obvious, this does really highlight the risk and issues using this charger’s grading function on NiMH cells. The current it uses to charge when grading is even lower and can’t be adjusted, it’s like 300mah I believe. It’ll miss the cutoff and overcharge them everytime. Needless to say, do not use the grading function for NiMH with this charger. For lithium ion though it works just fine.