# 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

 Temperature Add or Subtract to Hydrometer Reading Add or Subtract to Voltmeter Reading 0°F / -17.8°C -.032 -.192 10°F / -12.2°C -.028 -.168 20°F / -6.7°C -.024 -.144 30°F / -1.1°C -.020 -.120 40°F / 4.4°C -.016 -.096 50°F / 10°C -.012 -.072 60°F / 15.6°C -.008 -.048 70°F / 21.1°C -.004 -.024 80°F / 26.7°C 0 0 90°F / 32.2°C +.004 +.024 100°F / 37.8°C +.008 +.048 110°F / 43.3°C +.012 +.072 120°F / 48.9°C +.016 +.096 130°F / 54.4°C +.020 +.120 140°F / 60.0°C +.024 +.144 150°F / 65.6°C +.028 +.168 160°F / 71.1°C +.032 +.192

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

 Temperature Add or Subtract to Hydrometer Reading Add or Subtract to Voltmeter Reading 0°F / -17.8°C -.032 -.192 10°F / -12.2°C -.028 -.168 20°F / -6.7°C -.024 -.144 30°F / -1.1°C -.020 -.120 40°F / 4.4°C -.016 -.096 50°F / 10°C -.012 -.072 60°F / 15.6°C -.008 -.048 70°F / 21.1°C -.004 -.024 80°F / 26.7°C 0 0 90°F / 32.2°C +.004 +.024 100°F / 37.8°C +.008 +.048 110°F / 43.3°C +.012 +.072 120°F / 48.9°C +.016 +.096 130°F / 54.4°C +.020 +.120 140°F / 60.0°C +.024 +.144 150°F / 65.6°C +.028 +.168 160°F / 71.1°C +.032 +.192