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A Battery Charger is an electric device which is plugged into an electrical outlet (eg, 110 volt AC) and connected to the two terminals of the battery to restore the state of charge in the battery. One of leads coming from the charger is red (Positive) and the other is black (Negative). The red lead is clamped on the positive post of the battery while the black is clamped onto the negative post. The purpose of charging a battery is to convert the lead sulfate on the positive and negative plates back to lead oxide (positive plate) and metallic or sponge lead (negative plate). During the conversion or charging process, the water in the electrolyte is converted into sulfuric acid by reacting with the sulfate in the plates. Before this conversion, no usable electric energy exists between the positive and negative plates.

Types of Chargers and Their Controls and Characteristics:

Constant Voltage Charger

This type of charger charges a battery at a designed maximum charging voltage, usually above 13.8 volts for 12 volt batteries. Sometimes this charging voltage can be adjusted. After a discharged battery has been recharged for a period of time, and the designed maximum voltage is reached, the battery charger controls the charging voltage at a constant level of this voltage – Initially at the maximum, but converting to a minimum voltage as the battery becomes charged. Any charge after the battery is fully charged will result in increased battery temperature and gassing.

Constant Current Charger

This type of charger charges a battery at a preset or adjusted rate of constant current. While being charged by a Constant Current Charger, the battery voltage will rise as the state of charge increases. The voltage rise of each battery under charge has a natural limit, usually lower for the antimonial alloy batteries and higher for calcium alloy (maintenance-free) batteries. When the battery the battery is fully charged, unlike a Constant Voltage Charger, the charging current will not decrease but will remain constant. Any charge after the battery is fully charged will result in increased battery temperature and gassing.

Taper Current Charger

Taper Current Chargers have no controlled voltage or current. Their charging voltage and current depend solely upon the battery demanding the charge. The only limit to a taper current charger is its designed power capability. As the voltage increases, the charger typically will decrease or taper off to a minimum. At the minimum current, the battery is fully charged and the current is wasted as heat. When a battery is fully charged by a taper current charger, it will attain a maximum voltage, which is dictated by the chemistry of the battery and a minimum current input.

Automatic Chargers

Automatic Chargers have included within them circuits which sense battery voltage and turn the charger off when the battery reaches or approaches the fully charged state. Automatic chargers are generally recommended because they can provide a safe, full charge with limited operator skill or knowledge.

High Rate or Boost Fast Chargers

High rate fast chargers are able to provide high charging currents regardless if the current is used to convert the battery’s active material or not. A charger that can put out over 40 amperes continuously is considered a high rate fast charger. The reason for a high rate fast charger, sometimes referred to as a Boost Charger, is to quickly charge a discharged, but good battery, in order to start a vehicle. It is not recommended for long term or unattended charging of a battery.

Trickle Chargers

Trickle Chargers are small current chargers. Normally, their charging rate is less than one ampere per battery and sometimes less than 0.03 amperes per battery. A trickle charger cannot charge a completely discharged automotive battery. Its sole purpose is to maintain a battery at its full state of charge over a long period of time by replacing its self discharged capacity. However, a small current over a long period of time for a fully charged battery does promote some grid corrosion.

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