State of Charge
The truest measure of a battery's state of charge is the specific gravity of the battery acid. The following shows the approximate state of charge at various specific gravities at 77ºF / 25ºC.
| Charged | Specific Gravity |
| 100% | 1.255-1.275 |
| 75% | 1.215-1.235 |
| 50% | 1.180-1.200 |
| 25% | 1.155-1.165 |
| 0% | 1.110-1.130 |
Hydrometers can be difficult to use and at best accurate to +/-0.005 points. Voltage can be used to estimate state of charge but caution must be taken when interpreting voltage readings.
All Ampere Hour capacities based on specific gravities of 1.280. Reduce capacities 5% for 1.265 specific gravity and 10% for specific gravities of 1.250