How would you test the Amp Hour rating of a battery?

Question

I have a battery that is supposed to last 40AH, but I don't think its really a 40AH battery because its not lasting as long as it should. Is there a way that I can test the Amp Hour rating of the battery? What kind of equipment would this require?

I wouldn't mind spending a little money on test equipment, because I need a battery that is as small and as lite as possible that puts out 40AH. If I have to spend some money and test a bunch of batteries it would be worth it to me.

Answer 1

We test batteries everyday at work. For a lead-acid battery, the amp-hour capacity of a battery is defined as the constant current that will discharge the battery in 20 hours at 20°C. Generally, fully charged is defined as something like 2.2 V per cell and fully discharged is defined as 1.75 V per cell (depends on the manufacturer).

If you discharge the battery faster than 20 hours, your amp-hour rating will be less than the 20 hour value. If you discharge the battery slower it will be greater than the 20 hour value. Batteries do perform worse with respect to capacity at low temperature. They also have less capacity with time. Note that many batteries (like car batteries) are not designed to be fully discharged repeatedly and this will reduce their lifetime.

As far as equipment, there are a number of ways to measure capacity. We have current sources at work that draw a fixed amperage. We simply monitor the voltage and current while the battery is being discharged. You can then multiply current times time to amp-hours. If you don't have a current source (they are a specialty piece of equipment), I would use a resistor load (battery voltage doesn't vary that much) and a voltage monitor. Knowing battery voltage versus time and resistance, you can determine the current draw versus time. You can then integrate the current over time to get amp-hours.

Answer 2

First, you need to check the specifications for that battery type to see the current drain to which that AH rating applies. For example, 40AH at 0.1c, means it should supply 4A for 10 hours. If you draw more or less current than that, you might not get the full 40AH capacity. If the battery is 12V, the 0.1c load is 48 watts. The easiest and cheapest is to connect just enough light bulbs to consume 48 watts and measure how long they stay lighted.

Normally a battery doesn't retain its full rated capacity for its full rated lifetime. Usage and charging patterns are seldom ideal.