This 6/12 Volt lead acid battery charger circuit provides an initial voltage of 2.5 V per cell at 25°C to quickly charge a Lead Acid battery. The charging current reduces as the battery charges, and when the current drops to 180 mA, the charging circuit reduces the output voltage to 2.35 V per cell, leaving the battery in a fully charged state. This lower voltage helps prevent the battery from overcharging, which would shorten its life.
The LM301A compares the voltage drop across R1 with an 18 mV reference set by R2. The comparator’s output controls the voltage regulator, forcing it to produce the lower float voltage when the battery charging current, passing through R1, drops below 180 mA. The 150 mV difference between the charge and float voltages is set by the ratio of R3 to R4. The LED’s indicate the condition of the circuit.
Temperature compensation helps prevent the battery from overcharging, especially when a lead acid battery undergoes wide temperature changes while being charged. The LM334 temperature sensor needs to be placed close or on the battery to reduce the charging voltage by 4 mV/°C per cell. Because batteries require more temperature compensation at lower temperatures, change R5 to 30 for a tc of -5 mV/°C; per cell if application will spot temperatures below – 20°C.
The Lead acid battery charger’s input voltage should be filtered do that is at least 3 V greater than the highest needed output voltage: around 2.5 V per cell. Select a regulator for the highest current needed: LM371 for 2 A, LM350 for 4 A, or LM338 for 8 A. At 25°C with no output load, change R7 for a Vout , of 7.05 V, and change R8 for a Vout of 14.1 V.