Rechargeable Battery Storage Life Test

The Rechargeable Battery Storage Life Test for rechargeable NiMH and Ni-Cd batteries is a simple test of the capacity of the battery after it has been stored for a fixed period of time.

All rechargeable batteries lose their stored capacity over time, even when they are not used. Some technologies are better than others (they lose less over time). We suspect that some brands, within the same technology, are better or worse than others.

We will do a longer head to head showdown later, but only with the top batteries. Some manufacturers are claiming their newer technology NiMH batteries will not lose more than 1 - 2% of their energy per month ( instead of 1 - 2 % per day ).

Check out the Rechargeable Battery FAQs for more information on each technology’s storage capabilities or self discharge rates.

Why a Rechargeable Battery Storage Life Test?

We consider this an important test. There are many cases when you will charge a battery for a device that is not used constantly. You'll install the battery, say your LED flashligh, and then put it away. We expect the batteries and the flashlight to be useful after 1 month of not being used.

Well, wouldn't you rather have a battery that maintains a reasonable charge after a few weeks or 1 month. Theoretically, after 1 month, a reasonably good NiMH rechargeable battery should still have at least approximately 60% of its charge. ( 100% - 30 days x 1.5 % ).

Here are some examples where you would want your batteries to maintain a reasonable charge duraing storage.

• A low use device such as a flashlight
• A very low power device - A very low power device may still run out of power due to self discharge. A good example of this is timer based products.
• Kids toys that are not used everday - Some toys are sporadically.
• Going away on a trip for an extended period - You might bring your charger, but you shouldn't need to worry about charging your stored batteries.
• Camping or Hunting - Your devices are ready to go, and you may be out for a week or two. There's no electricity, so whether you have extra batteries in you pack or the device is not being used regularly, you have a right to expect them to work when you need them.

The Storage Life Test

Before a rechargeable battery is put into a battery holder for storage, it is fully charged using our reference charger.

We have decided on a simple test of 1, 2 and 4 weeks. We believe that these are reasonable periods of non-use between charges. Once the battery is ready for testing, we use the fixed load test.

The load usesd depends on the battery capacity and category. For example, all AA and AAA rechargeable batteries are tested using a fixed load of 10 Ohms. Larger higher capacity batteries, like C and D sizes, will use smaller loads. Smaller capacity batteries like the 9v size uses a larger load.