Charging AAA batteries

I bought a 4 pack of AAA Duracell white top NiMH at Walmart on Saturday. Today I thought I would charge them on my Maha C9000 charger to make sure they were fully charged. The batteries were labelled as 800 mah by the way. This is my first experience with AAA on this charger. I first did a discharge cycle to see how much capacity they had right out of the store and got values of 566, 573, 564, and 570 mah. Within 2% of each other, pretty good. I had a little trouble getting the charger to 'take' all of the batteries at first, some of them I had to reinstall a few times before they began the discharge. I discharged them at 0.25C or 200 ma. I waited a couple hours and did a normal charge cycle at 0.5C or 400 ma. One of the batteries i had to reinstall a couple times before it would charge The charges that the batteries accepted were 700, 731, 742, and 708 mah.
I was expecting the batteries to accept a charge more like 1000 mah before termination. Have any of you had any experience like this or any explanation/suggestion? The charger worked normally after a little finickiness to begin with. The batteries got slightly warm, about as expected.

Thanks, Phil

You might try doing a couple charge-discharge cycles to see if those numbers change at all.

Maybe modern NiMh cells don't need it anymore, but in the past, NiMh cells needed a couple of cycles before they reached their max capacity.

--flatline

That's somewhat good news to me, Phil.

I just picked up an MH-C9000 this last week and a bunch more AA and AAA Eneloops (they're comparable to the AAA cells you picked up).

From what I understand, the charge termination on the C9000 is on the conservative side to help prevent damaging overcharging because of missed peak detection. To compensate for this, the newer firmware of the C9000 also applies a top off charge of 100mA for 2 hours after the normal charging process is "done." So, leave your cells on the charger for a couple hours to top them off after they're "done" if you want (the mAh displayed won't actually change to reflect this additional charge).

I was afraid the C9000 would overcharge my AAAs and reduce their cycle life because of the 100mA 2 hour top off charge, but it looks like that probably isn't the case.

Also, keep in mind that the applied charge capacity is going to be more than the corresponding discharge capacity because no batteries and chargers are 100% efficient.

To get the most out of my new cells, I do a slow discharge at 1/5 C (or less), then slowly form charge them at 1/10 C. The "break-in" feature of the C9000 is a good way to do this form charge, and lets you know fairly accurately what the capacity of a cell is.

But you're right, 725mAh or so does seem low. I would recommend doing another discharge and then running the break-in on the C9000 to see what the actual available charged capacities of your cells are (it should list the capacity of the discharge portion of the process for this value when it's done).

Also, make sure the AAA cells are pushed down in all the way.

Thank you Wrend (and flatline). I was trying to be careful and conservative with the AAA batteries. The C9000 has good defaults for the AA batteries but I was afraid they would be too much for the smaller ones. I will probably use the break in mode on them tomorrow after discharging them as you recommended, there is no danger of overcharging them that way. Our Walmart occasionally has the white top Duracells and they are cheaper than I can order Eneloops so I sometimes take advantage of that. Thank you for your advice.

If you had trouble getting the charger to 'take' the batteries, it sounds like an electrical contact problem. Occasional causes for this are insulation wrapping around the negative end of the cell, or the positive button failing to make good contact with the charger where the shoulders protrude. The C9000 doesn't have the best possible battery contacts, unfortunately.

As for charging, I wouldn't be too worried about the charge supplied being less than 800 mAh. This is because the cells are only about 90% full when Done appears. As Wrend says, you can leave them on the charger for another two hours to be fully topped off. But unless you plan to use them immediately I would not bother. Within a few days storage that top off charge will just bleed off again.

AAA's in the c9000 can be tricky sometimes. I found that insert the bottom end first, then the top of the battery and pushing down hard on the top gave a "click" as it snapped into place and no problems after that. If I didn't press down the top I'd get problems.

As others have mentioned the c9000 will cutoff the charge when the battery hits 1.48v, which is less than fully charged particularly for eneloops. AA eneloops in particular really need that extra 2 hours after "DONE" appears.

You are correct, the problem was that I didn't get the top of the battery pushed in that last sixteenth of an inch or less. Everything is working normally now. I didn't know about the two hour topping off charge so I will let the charger run a little longer.