How can I measure my batteries “State of Charge”?
If the battery's temperature — or electrolyte in flooded types — is above 110° F (43.3° C), allow it to cool. To determine the battery's state-of-charge with the battery's temperature at 80° F (26.7° C), use the following table. The table assumes that a 12.65 voltage reading or a 1.265 specific gravity reading represents a fully charged battery. For other battery or electrolyte temperatures, use the Temperature Compensation table below to adjust the Open Circuit Voltage for VRLA batteries or Specific Gravity readings for flooded types.
State of Charge
| Digital Voltmeter Open Circuit Voltage | Approximate State-of-Charge | Hydrometer Average Cell Specific Gravity | Electrolyte Freeze Point |
|---|---|---|---|
| > 12.65 | 100% | > 1.265 | -75˚ F (-59.4˚ C) |
| > 12.45 | 75% | > 1.225 | -55˚ F (-48.3˚ C) |
| > 12.24 | 50% | > 1.190 | -34˚ F (-36.7˚ C) |
| > 12.06 | 25% | > 1.155 | -16˚ F (-26.7˚ C) |
| > 11.89 | Discharged | > 1.120 | -10˚ F (-23.3˚ C) |
Temperature Compensation
| Electrolyte Temperature ˚F/˚C |
Add or Subtract to Hydrometers / Digital Voltmeter reading |
|---|---|
| 160˚/71.1˚ | +.032 / +.192 |
| 150˚/65.6˚ | +.028 / +.168 |
| 140˚/60.0˚ | +.024 / +.144 |
| 130˚/54.4˚ | +.020 / +.120 |
| 120˚/48.9˚ | +.016 / +.096 |
| 110˚/43.3˚ | +.012 / +.072 |
| 100˚/37.8˚ | +.008 / +.048 |
| 90˚/32.2˚ | +.004 / +.024 |
| 80˚/26.7˚ | 0 / 0 |
| 70˚/21.1˚ | -.004 / -.024 |
| 60˚/15.6˚ | -.008 / -.048 |
| 50˚/10˚ | -.012 / -.072 |
| 40˚/4.4˚ | -.016 / -.096 |
| 30˚/-1.1˚ | -.020 / -.120 |
| 20˚/-6.7˚ | -.024 / -.144 |
| 10˚/-12.2˚ | -.028 / -.168 |
| 0˚/-17.8˚ | -.032 / -.192 |