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:
- Be sure all electrical loads are turned off.
- Determine whether the battery has been used in a vehicle or charged within the past 12 hours.
- Estimate the battery temperature by determining the average temperature to which the battery has been exposed for the past 12 hours.
- 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:
| • | 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. | Turn the carbon pile tester load completely OFF. |
| 2. | Connect the carbon pile tester across the battery terminals. |
| 3. | Discharge the battery at 300 amperes for 15 seconds. |
| 4. | Turn the carbon pile tester load completely OFF. |
| 5. | Disconnect the carbon pile tester from the battery terminals. |
| 6. | Wait for at least 15 seconds for the battery to recover. |
AGM Battery Voltage | % Charge at 0°C (32°F) | % Charge at 25°C (75°F) |
|---|---|---|
12.8 V | 100% | 100% |
12.6 V | 100% | 75% |
12.4 V | 75% | 65% |
12.3 V | 65% | 45% |
12.1 V | 40% | 20% |
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. |
Charging Time Required
The time required to charge a battery will vary depending upon the following factors:
- The battery charger capacity. The higher the charger's amperage, the less time it will take to charge the battery.
- The state-of-charge of the battery. A completely discharged battery requires more than twice as much charging time as a half charged battery. In a discharged battery with a voltage below 11 volts, the battery has a very high internal resistance and may only accept a very low current at first. Later, as the charging current causes the acid content to increase in the electrolyte, the charging current will increase. Extremely discharged batteries may not activate the reversed voltage protection in some chargers. Refer to the manufacturers instructions for operating this circuitry.
- The temperature of the battery. The colder the battery is, the more time it takes to recharge the battery. The charging current accepted by a cold battery is very low at first; then, as the battery warms, the charging current will increase.
Charging Procedure
Notice: Turn OFF the ignition when connecting or disconnecting the battery cables, the battery charger or the jumper cables. Failure to do so may damage the PCM or other electronic components.
When charging side-terminal batteries with the battery cables connected, connect the charger to the positive cable bolt and to a ground located away from the battery. When charging side-terminal batteries with the battery cables disconnected, install the battery side terminal adapters and connect the charger to the adapters.
Tighten
Tighten the battery side terminal adapters to 15 N·m (11 lb ft).
Use the following procedure to charge the battery:
- Turn OFF the charger.
- Ensure that all of the battery terminal connections are clean and tight.
- Connect the charger positive lead to the battery positive terminal.
- Connect the negative charger lead to a solid engine ground or to a ground stud in the engine compartment that is connected directly to the battery negative terminal, but away from the battery. If the negative battery cable is disconnected and a terminal adapter is being used, connect directly to the adapter.
- Turn ON the charger and set to the highest setting for normal charging.
- After charging, test the battery. Refer to Battery Inspection/Test .
