Why does temperature affect a battery's available capacity?

A battery's available capacity varies depending on the temperature. As the ambient temperature rises, a battery's ability to deliver current increases. As the temperature falls, so does the battery's ability to deliver current.

Temperature is a significant factor in battery performance, shelf life, charging and voltage control. At higher temperatures, there is dramatically more chemical activity inside a battery than at lower temperatures. Battery capacity is reduced as temperature goes down and increases as temperature goes up. This is why your car battery has reduced performance on a cold winter morning and why capacity needs to be considered when sizing your battery for use in different environments. The standard rating for batteries is at room temperature (25°C/77°F). At approximately -22°F (-27°C), battery capacity drops by 50%. At freezing capacity, it is reduced by 20%. Capacity is increased at higher temperatures. At 122°F, a battery's capacity will be increased by about 10-15%. As mentioned earlier, battery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell at -40°C to 2.3 volts per cell at 50°C. This is why temperature sensing and compensating chargers are so important.

The Thermal Mass of larger batteries and battery banks leads to more discussion. Because some of these batteries have so much mass, they will change the internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10° over 24 hours internally, even though the air temperature varies from 20° to 70°. In these circumstances, external thermocouples attached and insulated to one of the positive terminals are a good idea. The sensor will then read very close to the actual internal battery temperature.

Even though the battery capacity at high temperatures is higher, battery life is shortened. High temperatures affect the battery's service life according to a common “rule of thumb” or the law of “Arrhenius,” which states that the corrosion rate will be doubled at 10° C. Therefore, the lifetime will be halved per 10° C increase in temperature.

Example:

• Fifteen years at 20° C becomes reduced to 7.5 years at 30°C

NOTE: Even though a battery's ability to deliver current increases as temperature rises, prolonged operation at extreme temperatures may shorten the battery's life.

To calculate the approximate capacity correlation due to temperature, add or subtract the % adjustments shown in the following table:

Typical self-discharge of VRLA DRY CELL batteries at different temperatures:

A fully charged battery's shelf life will discharge at approximately:

• 2% per month when stored at 8°C/46°F
• 3% per month when stored at 20°C/68°F
• 5% per month when stored at 30°C/86°F
• 10% per month when stored at 40°C/104°F

Typical self-discharge of quality Deep Cycle Flooded batteries at different temperatures:

A fully charged battery's shelf life may discharge at approx;

• 6% per month when stored at 8°C/46°F
• 9% per month when stored at 20°C/68°F
• 15% per month when stored at 30°C/86°F
• 30% per month when stored at 40°C/104°F

Batteries kept in storage while discharged will not perform as intended when put into service. Battery inventories should be constantly checked and recharged when necessary. A battery in storage should never be allowed to discharge more than 45-50% of its original capacity.