How to temperature compensate instrument readings?

If you are using a non-temperature compensated hydrometer, make the adjustments in the table below.

For example, at 30° F (-1.1° C), the specific gravity reading would be 1.245 for a 100% State-of-Charge. At 100° F (37.8° C), the specific gravity would be 1.273 for a 100% State-of-Charge. This is why a temperature-compensated hydrometer is highly recommended and more accurate than other means when testing flooded battery types.

For non-sealed batteries, check the specific gravity in each cell with a hydrometer and average the readings.

Table 2 - Hydrometer Temperature Compensation

How do I temperature compensate for my voltmeter readings?

If you are using a digital voltmeter, make the adjustments in the table below. For example, at 30° F (-1.1° C), the voltage reading would be 12.53 for a 100% State-of-Charge. At 100° F (37.8° C), the voltage would be 12.698 for 100% State-of-Charge. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with an accurate digital voltmeter. This is the only way to determine the State-of-Charge on sealed, non-accessible batteries.

Table 3 - Voltmeter Temperature Compensation