Why does temperature have such a dramatic effect on batteries?

Temperature is a major 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 increased as temperature goes up. This is why your car battery has reduced performance on a cold winter morning, and why capacity needs to be taken into account 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 is reduced by 20%. Capacity is increased at higher temperatures. At 122˚F a batteries 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 introduces more things to think about. Because some of these batteries have so much mass, they will change 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˚ degrees. In these circumstances external thermo couples — 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 battery capacity at high temperatures is higher, battery life is shortened. Battery capacity is reduced by 50% at -22˚F — but battery life will be increased by as much as 60%. Battery life is reduced in half for every 15˚F over 77˚. This is true for any type of Lead-Acid battery. In places that experience extreme temperatures, batteries may be sold with different electrolyte specific gravities. These will be higher in colder areas and lower in hotter environments. Much of the information we have given here needs to be reinterpreted in these cases.