Batteries, large or small, produce electricity which is measured in voltage and milliamp hours. Voltage is the force and remains fairly constant until the battery is nearly discharged. Milliamp hours are the current and reduce as the battery discharges. Test your small batteries using a multimeter to determine if the battery is producing the correct voltage and how long you can expect it to last before you need to replace or recharge it.
Look on the label of the small battery to find out the voltage and milliamp hours. Write down the two figures on a sheet of paper.
Turn on your multimeter. Set it to measure voltage.
Place the metal sensors on the end of the red and black wires from the meter onto the positive and negative terminals of the small battery. The sensor on the end of the red wires goes to the positive terminal and the sensor on the end of the black wire goes to the negative terminal.
Look at the meter display. If the voltage is near 20 percent less than the figure you wrote down it means your battery is nearly discharged, so you need to replace it or recharge it. Remove the sensors from the small battery terminals.
Set your multimeter to measure milliamp hours. Place the sensors on the battery terminals as before.
Read the meter display. If it reads the same as the milliamp hours figure you wrote down earlier your battery is fully charged. If the reading is less than the figure you wrote down you can calculate the percentage charge left in the battery by dividing the meter reading by the milliamp hour number you wrote down and multiplying the result by 100. For example, if the milliamp hours on the battery label is 2,000 and the meter reading is 1,500 divide 1,500 by 2,000 to get 0.75 then multiply 0.75 by 100 which equals 75. This means your battery is 75 percent charged.