Cell phones use rechargeable lithium ion batteries. This type of battery can recharge 300 to 500 charge cycles to its full capacity before it will not charge fully. If your cell phone needs charging more often or the charge lasts for several hours less than when the battery was new, you may need a replacement battery. Testing the battery to obtain its capacity will tell you if a new battery is needed.
Plug the cell phone into its charger and plug the other end in to an electrical socket. Allow the battery charge to full capacity.
Press and hold the power button on the cell phone to turn it off. Let the phone power down and remove the back cover to access the battery.
Remove the battery and observe the label for the voltage. Most cell phone batteries are 3.4 to 4.5 volts.
Turn on a multimeter. A multimeter tests current flow in voltage and amps and also resistance in a circuit. Place the black test probe on the negative battery contact and the red test probe on the positive battery contact. The contacts are on the edge or bottom of the battery and should be marked negative (-) and positive (+).
Place one test probe on the left terminal and one on the right, if the battery terminals are not marked. These are usually the correct terminals and the meter will show a positive reading. If the meter shows a negative reading, move the left or right probe to the middle terminal until one pair shows a positive reading.
Read the display on the multimeter. The voltage should be the same or a little higher than the battery label due to the fact that batteries hold an additional charge above the full charge mark when they have just been charged.
Charge a new cell phone battery to its full capacity before using the phone. Multimeters are available at hardware, home improvement and auto parts stores.