Slow Battery Charging Versus Fast Battery Charging

Rate

• No hard and fast definition exists as to what distinguishes a fast charge from a slow one. A battery̵7;s size plays a part in the rate, as a car battery stores more charge and therefore takes longer to recharge than a AA battery. Keeping this in mind, a slow charge for a vehicle battery is generally an overnight process. A fast charge is less than four hours, and a rapid charge is 30 minutes. Small consumer batteries, such as a AA or C cell, slow-charge in a few hours and fast charge in 15 to 30 minutes.

Current

• Fast charging requires more current from a charger than slow charging. Higher currents require more robust electronic components, which costs more, so a fast charger is more expensive than a slow one. A slow charger produces about one of current; rapid chargers for lead-acid car batteries supply up to 25 amps.

Hydrogen Gas

• When you charge a battery, a chemical reaction releases hydrogen, a combustible gas. A slow charge produces hydrogen at a much slower rate than a fast charge. A hazard of fast charging is the accumulation of substantial amounts of hydrogen, which can ignite, explode or physically burst the battery. Though modern batteries have designs that vent hydrogen and avoid battery damage, a spark can still ignite it, causing an explosion.

Heat

• Any time you charge a battery, whether slowly or quickly, the chemical reaction produces heat, warming the battery. The larger currents used for fast charging produce more heat. If a fast charge is done under hot conditions, such as on a summer day, the excess heat can degrade the battery̵7;s chemicals and possibly some of its plastic parts. A slow charge produces less heat, so overheating becomes much less of a concern.

Capacity

• The speed of battery charging affects its capacity. A lead-acid battery, for example, during a rapid charge, stores its charge in zones near the terminals. After charging, the chemical reaction migrates throughout the battery but at a lower concentration than if the battery were charged slowly. A battery used soon after a rapid charge produces less current than after a slow charge. In a vehicle, this is less of a problem, as after a jump-start or other rapid charge, the vehicle̵7;s own charging system takes over, maintaining the battery at its correct voltage.