Rechargeable batteries operate under a reasonably wide temperature range. This, however, does not automatically permit charging under these same temperature extremes. While operating batteries under hot or cold conditions cannot always be avoided, the user has some control over charging. Efforts must be made to charge the batteries at moderate temperatures.

Lead acid Battery  is reasonably forgiving on temperature extremes, as we are familiar with our car batteries. Part of this tolerance is credited to the sluggishness of the lead-acid system. Some battery brands permit freezing and low level charging; others sustain damage and deliver reduced capacity and a short service life.

To improve charge performance of lead-acid batteries at colder temperatures and avoid thermal runaway during heat spells, controlling the voltage limits, to which the battery is charged, is important. Implementing such a measure can prolong battery life by up to 15%. General guidelines suggest a compensation of approximately 3mV per cell per degree Celsius. The voltage adjustment has a negative coefficient, meaning that the voltage threshold drops as the temperature increases.

Heat kills batteries. The warmer the cells, the shorter the life is. Elevated temperatures cannot always be prevented, especially during fast charging, but efforts must be made to keep this time brief. While 45°C (113°F) is acceptable if kept short, at 50°C (122°F) and above, the battery starts to suffer. Note that the cells inside the pack are always a few degrees warmer than the temperature of the housing.

Ultra-fast chargers

Some charger manufacturers claim amazingly short charge times of 30 minutes or less. With well-balanced cells and operating at moderate room temperatures, nickel-cadmium batteries designed for fast charging can indeed be charged in a very short time. This is done by simply dumping in a high charge current during the first 70% of the charge cycle.

In the second phase of the charge cycle, the charge current must be lowered. The efficiency to absorb charge is progressively reduced as the battery moves to a higher state-of-charge. If the charge current remains too high in the later part of the charge cycle, the excess energy turns into heat and high cell pressure. Eventually, venting will occur, releasing oxygen and hydrogen. Not only do the escaping gases deplete the electrolyte, they are highly flammable! A white powdery substance accumulating at the vent area indicates previous venting.

Ultra-fast charging can only be applied to batteries that are designed for fast charging. Applying a high current charge to regular cells will cause the conductive path to heat up. The contacts on portable packs also suffer if the current handling of the spring-loaded plunger contacts is underrated. These contacts may wear out prematurely. Often, a fine and almost invisible crater appears on the tip of the contact, which causes a high resistive path or forms an isolator. The heat generated by a bad contact often melts the plastic. Higher contact tensions improve the current flow.

Aged batteries with high internal resistance and mismatched cells do not lend themselves to ultra-fast charging, even if they are designed for it. Low cell conductivity turns into heat, which further deteriorates the cells. The weak cells holding less capacity are fully charged before the others and begin to heat up rapidly. Some batteries create sufficient heat to soften and distort the plastic housing. Temperature sensing is a prerequisite with fast and ultra-fast charging.

Several manufacturers offer pulse chargers. Interspersing brief discharge pulses between each charge pulse can further enhance charging. This method promotes recombination of oxygen and hydrogen gases, resulting in reduced pressure buildup and lower cell temperature. Pulse chargers are also known to reduce crystalline formation (memory) on nickel-based batteries. Most Cadex chargers for nickel-based batteries apply this feature.

Some advanced chargers regulate the charge current according to the battery's ability to accept charge. An empty battery will initially take a very high charge current. Towards the end of a charge, the current is tapered down. Aged batteries are given their due respect and are automatically charged at rates suitable to their condition.