Use a voltmeter to measure the open circuit voltage of the battery (see car battery voltage chart). Open circuit voltage is the voltage of the battery with no load or disconnected from any circuit. Don’t charge the battery before you test the open circuit voltage. If the voltmeter shows a voltage reading of 12.2 volts then the battery is approximately 50% charged.
Since the battery is 50% charged, this means that there is approximately half of the 60 amp-hours in the battery or 30 amp-hours. Therefore it is necessary to put about 30 amp hours plus 20% more to compensate for the internal resistance in the battery for a total of 30 amps x 0.2 = 36 amp-hours to fully charge your battery.
Car Battery Voltage Chart
| 12V BATTERY VOLTAGE | VOLTS PER CELL | STATE OF CHARGE |
|---|---|---|
| 12.7 | 2.12 | 100% |
| 12.5 | 2.08 | 90% |
| 12.42 | 2.07 | 80% |
| 12.32 | 2.05 | 70% |
| 12.2 | 2.03 | 60% |
| 12.06 | 2.01 | 50% |
| 11.9 | 1.98 | 40% |
| 11.79 | 1.96 | 30% |
| 11.58 | 1.93 | 20% |
| 11.31 | 1.89 | 10% |
| 10.5 | 1.75 | 0% |
Fully charged car battery voltage: 12.6 volts
How to Maintain a Car Battery
- Check your battery every now and then to make sure its terminal connections are clean, snug and protected from the elements. Signs of corrosion or leakage could mean that your battery is no longer operating as well as it should;
- Always unplug accessories and turn off lights when your car is turned off;
- Keep the battery in cooler places whenever possible. Heat damages batteries;
- Scrub corrosion from the terminals with a solution of water and baking soda.
