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My camera uses AAA batteries, and I have a few sets of NiMH AAAs. I know that these are only good for a certain number of charge cycles, but how can I tell when it's time to replace them?

I've been charging each AAA by itself, then immediately testing it - if it doesn't show "green", then I assume it's dying. (I have Sanyo smart charger, which shows power as red/yellow/green.)

Is there a better way to test them?

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See also photo.stackexchange.com/questions/570/… - it's not exactly your question, but the answers will help you I think. –  Reid Aug 3 '10 at 15:27

3 Answers 3

up vote 3 down vote accepted

There are battery managers that can be used to detect this and, in many cases, rejuvenate the battery. This website as a good writeup on the topic.

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I agree. The best way is to get a charger that will tell you if the battery is dead. The best I have found is by La Crosse. amazon.com/Crosse-Technology-BC-700-Battery-Charger/dp/… –  Johannes Setiabudi Aug 3 '10 at 16:22
    
I ordered a BC-9009. Seems to take a very long time to test batteries from my "bad" pile, though. –  chris Aug 13 '10 at 0:12
    
I'll second the LaCrosse suggestion. It can test the capacity of each battery and attempt to refresh it. You can use higher power levels to charge/discharge faster, but I prefer a slow charge myself. slickdeals.net is all over these chargers when they go on sale. –  BMitch Mar 30 '11 at 2:29

This is a really simplistic answer, and I assume that the issue is that you don't know which of several batteries is bad, in which case this answer doesn't help, but on the off chance this helps: you need to change the batteries when they no longer hold a charge. In the future, you can deal with this by creating sets of batteries that you keep together (1 set in camera, 1 set in charger), so that when a set stops holding a charge, you get rid of that set (even if not all are quite dead yet, because they've had the same number of charge cycles, they'll at least be pretty much done at the same time).

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Keeping sets together is a problem, two pre-teen kids in the house tend to grab whatever is in the charger when they need something. –  chris Aug 3 '10 at 17:24
    
@chris: 1. Maybe you can label sets with colored electrical tape. 2. If they still mix the sets up, then you can use a Maha MH-C9000 charger or similar to measure the batteries' actual capacity. If you match batteries whose actual capacities are similar — say, within 5% of each other — then you should be fine. 3. Or maybe you can do the following: Teach your kids how to keep matched sets of batteries together. Then buy a few sets for your kids, and a few private sets for yourself to use only. If the kids ruin their own batteries, that's their loss. –  unforgettableid Oct 15 '13 at 17:16

The internal resistance of NiMH batteries increase rapidly as they near the end of their useful life.
See this reference, this reference and this reference

This reference shows you how to measure the internal resistance of a battery.
As a rule of thumb, when the internal resistance has doubled the battery is approaching the end of its life.

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