Battery capacity is measured in amp hours (Ah) or milliamp hours (mAh), depending on the type of battery. Small batteries, such as AA batteries, are measured in mAh, while deep-cycle lead-acid batteries, fitted in items such as golf carts and wheelchairs, are measured in Ah. Both refer to the time a battery can last, when it's fully charged, but as the battery discharges, so the mAh or Ah decreases. A good method to determine the charge retained in your battery is to check the amperage output using a multimeter.
Look for the label on the side or top of your battery to find out the Ah your battery delivers, when it's fully charged and in good condition. For example, a deep cycle battery may have 12V 50Ah on the label, meaning it produces 12 volts and 50 amp hours.
Turn on the multimeter. Check that the jacks on the ends of the two wires to the meter are inserted in the Ah jack sockets on the meter, if your meter has several sockets, or set it to measure Ah by turning the dial to the appropriate setting or by pressing the Ah button. Refine the Ah setting to a range that fits the Ah on the battery's label. For example, if the label says 50Ah, then set the range between 0 and 60Ah.
Connect the metal alligator clip on the end of the black wire from the meter onto the negative terminal of the battery; it's likely to be labelled "-" or "Neg." If the meter doesn't have clips then you need to hold the sensor onto the terminal.
Connect the other alligator clip on the end of the red wire from the meter onto the positive terminal of the battery, or hold the metal sensor on the terminal; it's labelled "+" or "Pos."
Look at the reading on the meter display. The reading matches the battery label, if it's fully charged. You can work out the percentage charge in your battery by using a calculator to divide the meter reading by the figure on the battery's label and then multiplying the result by 100. For example, if the meter reading is 20, then 20 divided by 50 equals 0.2, multiplied by 100 equals 20, meaning you have 20 per cent capacity remaining.
Use a calculator to work out how long your battery will power your electrical device by checking the amperes on the label on the electric motor or device that the battery powers. For example, if the device consumes 5Ah and the reading on the meter is 20Ah, divide 5 into 20 to get 4, meaning your battery will power your device for 4 hours.