Battery State of Charge
Using voltage to determine the batteries state of charge is only accurate after the battery has been at rest for 24 hours. This is enough time for the acid in each cell to equalize. If the battery has been charged or discharged in the past 24 hours, the battery state of charge will only be an estimate.
The maintenance-free batteries state of charge is estimated by reading the voltage of the battery across the battery terminals. 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, including parasitic loads, 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 batteries 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.
- 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, terminal voltage will not be stabilized and you should wait 12 hours since the last time the battery was used.
- 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:
The table is accurate to 10 percent only after the battery has been at rest for 12 hours.
| Battery Voltage | % Charge at 0°C (32°F) | % Charge at 25°C (75°F) |
|---|---|---|
| 12.75 V | 100% | 100% |
| 12.7 V | 100% | 90% |
| 12.6 V | 90% | 75% |
| 12.45 V | 75% | 65% |
| 12.2 V | 65% | 45% |
| 12.0 V | 40% | 20% |
Use the state of charge information as follows:
- A battery with a state of charge that is below 65 percent must always be recharged before returning it to service or continuing storage.
- A battery with a state of charge that is 65 percent 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 percent, before returning it to service or continuing storage.