Is there a "right" charging rate for NiMH batteries?

The recent purchase of a new camera has prompted me to get some new rechargeable batteries. And the new batteries have prompted me to get a more advanced charger to I could fill up my batteries more smartly. I got this La Crosse BC-900 charger, as it came highly recommended from some camera geeks.

Problem is, I don’t know exactly how to charge batteries smartly. Specifically, I don’t know how to pick a charging rate. Typical Google research has yielded no good answers, and sometimes contradictory ones. The manual for the charger says that you can’t go wrong with 200 mA charging rate, which equated to about a 0.1C rate for most AA batteries (in the 2000-2500 mAh range).

However, according to some info I’ve found, rates in the 0.1C range don’t result in a sharp enough voltage drop at the end of charging to trip the charger and cause it to terminate properly, which risks overcharging the batteries. Not to mention that it takes a long time to be so conservative. For those reasons, some people recommend rates from 0.5C to 1C (1000+ mA for AA cells). But then there are other people who say rates that high are too aggressive, and will damage the batteries over time.

So what’s the real story? Will I wear my batteries out if I charge them at a rate as high as 0.5C? Or will I just waste my time by keeping them down at 200 mA? Obviously, there’s a tradeoff between speed and longevity (although really low charge rates are apparently bad news as well). Is there a sweet spot?

The problem has to do with heating. Good battery chargers detect over-temperature conditions and terminate the charge cycle. You can safely charge NiMH at 10C+ if you are careful.

I have that charger, in short here’s what I do:

  • I use the 200 mAh setting most of the time and it is the default, so it’s easy.
  • I have some batteries, a particular brand that has that problem you stated, I have enough that I took them out of service for now.
  • If I’m in a rush I will up that to what is needed.

Just to add to the above post the major benefit to me with this charger is finding out the capacity of older Nimh batteries, find out which ones are on their way out, and grouping them (and using them) by capacities.