Discharging batteries at high and low temperatures

Operating a battery at elevated temperatures momentarily improves performance by lowering the internal resistance and speeding up the chemical metabolism, but such a condition shortens service life if allowed to continue for a long period of time. Some manufacturers of lead acid batteries make use of the improved performance at warmer temperatures and specify the batteries at a toasty 27°C.

Cold temperature increases the internal resistance and diminishes the capacity. Batteries that would provide 100 percent capacity at 27°C will typically deliver only 50% at -18°C. The capacity decrease is linear with temperature.

Li-ion also performs better at high temperatures than at low ones. Heat lowers the internal resistance but this stresses the battery. Warming a dying flashlight or cellular phone battery in your jean pocket might provide additional runtime in the winter. As all drivers in cold countries know, a warm battery cranks the car engine easier than a cold one.

The dry solid polymer battery uses heat to promote ion flow in what is called a “true plastic battery.” The battery requires a core temperature of 60° to 100°C to become conductive.

The dry solid polymer has found a niche market for stationary power applications in warm climates where heat serves as a catalyst rather than a disadvantage. Built-in heating elements keep the battery operational at all times.

High battery cost and safety concerns have limited the application of this technology. The more common Li-polymer uses moist electrolyte to enhance conductivity, as discussed earlier.

Batteries achieve optimum service life if used at 20°C or slightly below, and nickel-based chemistries degrade rapidly when cycled at high ambient temperatures. If, for example, a battery operates at 30°C instead of a more moderate room temperature, the cycle life is reduced by 20 percent. At 40°C, the loss jumps to a whopping 40%, and if charged and discharged at 45°C, the cycle life is only half of what can be expected if used at 20°C.

The performance of all battery chemistries drops drastically at low temperatures. At -20°C most nickel-, lead- and lithium-based batteries stop functioning. Although NiCd can go down to -40°C, the permissible discharge is only 0.2C (5-hour rate). Specially built Li-ion brings the operating temperature down to -40°C, but only on discharge and at a reduced discharge.

With lead acid we have the danger of the electrolyte freezing, which can crack the enclosure. Lead acid freezes more easily with a low charge when the specific gravity of the electrolyte is more like water.

Cell matching by using cells of similar capacity plays an important role when discharging at low temperature under heavy load. Since the cells in a battery pack can never be perfectly matched, a negative voltage potential can occur across a weaker cell on a multi-cell pack if the discharge is allowed to continue beyond a safe cut-off point.

Known as cell reversal, the weak cell suffers damage to the point of developing a permanent electrical short. The larger the cell-count, the greater the likelihood that a cell might reverse under load. Over-discharge at a heavy load and low temperature is a large contributor to battery failure of cordless power tools, especially nickel-based packs; Li-ion packs come with protection circuits and the failure rate is lower.

Users of electric vehicles need to understand that the driving distance specified per charge is given under normal temperature; frigid cold will sharply reduce the available mileage. Using electricity for cabin heating is not the only cause for the shorter driving distance between charging; the battery performance is reduced when cold.

Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For more information on batteries, visit www.batteryuniversity.com; product information is on www.cadex.com.