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.
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.