AGM Battery State of Charge

The AGM maintenance free battery's state of charge is estimated by reading the voltage of the battery. Because the voltage is affected by current flow into or out of the battery, the engine must be stopped and all electrical loads turned off when checking the voltage. The voltage can also be affected if the battery has just been charged or discharged, so it is important to consider what has happened to the battery in the time just before testing. Use the following procedure to determine the battery's state of charge:

1. Be sure all electrical loads are turned off.
2. Determine whether the battery has been used in a vehicle or charged within the past 12 hours.
   • If the answer is no, the terminal voltage will be stabilized and no action is necessary before reading the voltage. Skip to Step 3.
   • If the answer is yes, remove any surface charge with a carbon pile tester as follows:
     1. 2.1. Turn the carbon pile tester load completely OFF.
     2. 2.2. Connect the carbon pile tester across the battery terminals.
     3. 2.3. Discharge the battery at 300 amperes for 15 seconds.
     4. 2.4. Turn the carbon pile tester load completely OFF.
     5. 2.5. Disconnect the carbon pile tester from the battery terminals.
     6. 2.6. Wait for at least 15 seconds for the battery to recover.
3. Estimate the battery temperature by determining the average temperature to which the battery has been exposed for the past 12 hours.
4. Measure the battery voltage at the battery terminals. Refer to the following table to determine the state of charge according to the estimated battery temperature:
   Fig 1: Battery Charging Specification Table

   Courtesy of GENERAL MOTORS CORP.

Use the state of charge information as follows:

• A battery with a state of charge that is below 65% must always be recharged before returning it to service or continuing storage.
• A battery with a state of charge that is 65% or greater is generally considered to be charged enough in order to be returned to normal service or in order to continue storage. However, if the battery is being used in slow traffic or with short drive times, or if the temperature is very hot or very cold, the battery should be fully charged (to at least 90%) before returning it to service or continuing storage.
• For best results, use an automatic taper-rate battery charger with a voltage capability of 16 volts.
• The charging area should be well ventilated.
• Do not charge a battery that appears to be frozen; allow the battery to warm to room temperature and test it before charging.