Battery amperage life, the number of ampere-hours that a battery can supply, is dependent on a number of factors. One of the most important factors is the average electrical current level your battery must supply. The higher the average electrical current, the lower the actual number of ampere-hours your battery will be able to supply. Battery manufacturers list the battery amperage life in units of ampere-hours. However, to calculate the actual ampere-hours, you need to make a discharge calculation and use the manufacturer's ampere-hour derating curves.
Determine the ampere-hour rating of your battery. Look for the ampere-hour rating listed on the battery or battery package. Consider that the rating could read ampere-hours, amp-hours, AHRS or any other type of abbreviation for ampere-hours. Also, consider that smaller batteries often have their ampere-hour rating listed in terms of milliampere-hours. Remember that one thousand milliamperes equals one ampere.
Consider that the ampere-hour rating listed on your battery is for the most part an ideal rating. Also consider that the ampere-hour rating listed can only be obtained if the average current level drawn from the battery is very low.
Calculate the life of your battery for ideal conditions. Estimate the current that the electronic devices connected to your battery will require. Consider that the current a battery must supply is often called the load current. For this example, assume that your battery will have to power a 50 ampere device and that your battery has a listed ampere-hour rating of 200 amperes. Divide the listed battery ampere-hour rating by the load current. For this example, the life of the battery would be four hours, since 200 divided by 50 is 4.
Calculate the discharge period. Divide the battery's ampere-hour rating by the current that your load requires. For this example, use a battery ampere-hour rating of 200 ampere-hours and a 50 ampere load. Check to see if you obtained four hours, since 200 divided by 50 is 4.
Determine the battery-ampere rating for the specific discharge period. Obtain from the manufacturer a plot of the battery ampere-hour rating for the specific battery. Notice on the plot that the horizontal axis is labeled discharge period "in hours," and the vertical axis is labeled "percentage of total battery ampere-hour rating" in percent. On the discharge period axis, find the discharge period calculated in step 2 (4 hours). For this hour value find the corresponding rating percentage on the vertical axis. Note the value. For this example, the corresponding percentage is 70 percent.
Multiply the rating percentage, 70 percent, by the battery's ampere-hour rating, 200 ampere-hours. Since the ampere-hour rating for this example is 200 ampere-hours and the rating percentage is 70 percent (0.7), the battery-ampere rating is 140 ampere-hours, since 200 times 0.7 is 140. Consider that the actual ampere-hours will be significantly reduced if the battery must power a 50 ampere load continually. Consider that the life of the battery, that is the actual ampere-hours obtained, would be even lower for a 100 ampere-hour load. Also consider, that for very low loads, 0.01 or even 0.1 amperes, the actual ampere-hour rating would be very close to the rated battery ampere-hour rating.