Batteries are rated according to the tasks they are expected to perform. Batteries rated with cranking amps are designed to deliver high currents for short periods of time, in duties such as starting cars and trucks. Batteries rated in ampere-hours (AH) are designed to deliver low currents for an extended period of time. High discharge rate batteries have an electrode design that exposes large areas of electrode to the electrolyte, allowing for high discharge currents. For low discharge or deep discharge batteries, the electrodes are thicker and less of an electrode is exposed to the electrolyte, making for slower discharges but for longer periods. Batteries designed to work for one kind of duty will not last long if used for another.
Make sure the battery is fully charged. New batteries are usually sold fully charged. Other batteries should be charged with a battery charger. Leave a newly charged battery to sit without load or charging for several hours to eliminate surface charge, which will give a false reading. Measure the voltage across the battery terminals. A fully charged 12-volt lead-acid battery should have a voltage of at least 12.6 V across the terminals.
Connect a resistor of about 1 ohm, 200 W across the battery terminals. The multimeter should show a current of around 12 A. The exact values are not important since the actual values will be used to calculate the AH rating of the battery and the AH rating of the battery is only a rough indication of its capacity. The AH rating is normally given for a full discharge over 20 hours, but the rating can be estimated after a discharge to about 50 percent.
Monitor the voltage once per hour over the next few hours. The voltage should be decreasing about 0.1 volt every two hours. If the decrease is faster, the resistance is too small and the current too high to give a good estimate of the ampere hours. In this case the estimate of AH will be too low. If the voltage decrease is slower, the estimate of AH will be too high. Ideally the voltage will decrease to about 12 V after about 10 hours.
Calculate the AH of the battery. Multiply the current measured through the resistor by the time taken for the voltage to decrease to 12 V. This is the capacity to a 50 percent discharge. Multiply by 2 to get the full battery capacity. If the current was 12 A and the voltage reached 12 V after 10 hours, then the battery capacity is 12 x 10 x 2 = 240 AH.
Determine the useful battery capacity. While the battery capacity of the example above is 240 AH, the battery's life will be reduced if it is discharged to less than 50 percent. The useful battery capacity is therefore only 120 AH or 10 A for 12 hours. The battery capacity is also usually given for operation at 77 degrees F, and the available capacity is sharply reduced for substantially colder temperatures. At 50 degrees F the battery will supply about 90 percent of capacity, and at 30 degrees F it will supply about 80 percent. If the above tests were done at temperatures greatly different from the planned operating temperatures, the calculated capacity must be adjusted accordingly.