How to find the right Rechargeable NiMH Battery

Thinking about using a Rechargeable NiMH Battery? How do you select the right battery? Which brand battery is a good quality and what does mAh mean? Is the voltage difference in NiMH batteries important? Do rechargeable batteries suffer from a "Memory Effect"?

Brands: Brands are important when choosing a rechargeable nimh battery. Not all brands are equal. Some brands (particularly 'no-name' brands) over-rate their battery capacity. The best way to find the right brand is find out what people are saying about that brand battery. If you're going to use the nimh batteries in your camera visit a camera forum and find out what battery people recommend. If you're putting the batteries in your kids toys then buy value. Reserve the best batteries for your digital camera and MP3 players, and the like.

mAh: This stands for Milli-Amp Hour. It's a rating of the capacity of a battery. This is where the biggest variance comes in between batteries. Generally speaking the higher the capacity (mAh) the better. However, this rating depends on how the batteries were tested. Again, this depends largely on the discharge rate. Most nimh batteries are tested at a 20 hour rate. The capacity listed should be it's rating at 20 hours. If your device is high drain it may perform differently than at the slower 20 hour rate. Again, the best thing to do is to be sure you're buying quality by checking out the recommended nimh battery brands by publications and user forums.

Voltage: Alkaline batteries (non-rechargeable) are rated at 1.5V. Rechargeable NiMH batteries are rated at 1.2V. This may sound like it poses a problem but in most applications it doesn't. The reason is that Alkaline batteries have what we refer to as a sloping discharge curve. It would look something like this:

A Rechargeable NiMH battery has more of a 'cliff' discharge curve. The voltage doesn't drop as quickly and it holds the voltage stiff for longer. The discharge curve looks more like this:

Notice that the alkaline battery only kept the voltage up to 1.2V for about 625 minutes while the NiMH battery kept the voltage up above 1.2V for over 950 minutes. Again, this is based on 1 specific discharge rate, but the general rule holds true regardless of the discharge rate.

Memory Effect: What is commonly referred to as "Memory Effect" is a very misunderstood concept. You can read more about this in detail here. In essence what is referred to as "Memory Effect" does not affect NiMH batteries.

Where do you get the best NiMH Batteries?
The PowerEx NiMH battery was rated by Editor's Choice by PC Photo Magazine. These are usually on the cutting edge and a great choice for photographers. Lenmar also makes a good quality cell and they are usually on the upper end of the capacities available.

Lenmar is also the maker of the much talked-about Mach1 one hour speed charger that has won innovation awards several years in a row at the Consumer Electronic's Show. They currently make the highest capacity Rechargeable NiMH battery. They're cutting edge and their quality is tested and proven.