In-Car Charger And Switcher Circuit For SLA Battery Circuit

Posted on Feb 6, 2014

This circuit was devised to switch power to a Peltier cooler in a vehicle. Power to the load from the vehicle`s battery is switched by a SPDT relay while the ignition switch is turned on and from the SLA auxiliary battery when the ignition is off. The SLA battery is charged from the vehicle`s battery. When the engine is running, the voltage remain

In-Car Charger And Switcher Circuit For SLA Battery Circuit
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s fairly constant, which greatly simplifies the charging circuit. If the SLA battery is fully charged, any further charging current from the vehicle battery is limited by a 3. 3W 5W resistor (R1). If the SLA battery is deeply discharged, the voltage drop across this resistor will be enough to bias on PNP transistor Q1. This will turn on P-channel Mosfet Q2 and it will provide further charging current via R2, effectively becoming a 2-step charger. Since the paralleled resistors (R1 & R2) have a lower combined voltage drop, Q1 will receive lower base bias, which in turn will cause Mosfet Q2 to fully saturate. This positive feedback creates a clean transition between the two states and prevents Q2 from over-dissipating by being partially on. The current then will ramp down until the battery is only receiving a trickle charge and the voltage drop across the paralleled resistors is only a few dozen millivolts. Schottky diode D1 prevents the SLA battery from discharging into the vehicle`s accessory circuits when the engine is off. Two safety devices are included in the circuit, the first being in-line fuse F1 which will prevent serious damage in case of shorts. In addition, a PTC resettable thermistor (RT1) protects the battery from sustained over-currents during the charging phase. It is a 1. 85A hold, 3. 70A trip device at 23 °C. Since it has a positive temperature coefficient, at 70 °C, these ratings decrease to 1A and 2A for...

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