Solar Battery Regulator/Controller

Posted on Mar 29, 2013

In the actual device the transistors are bolted to the aluminium case. The schematic diagram shown here represents how the circuit would be built if all components were on-board. Separate paths for load current and voltage sensing allow the battery voltage to be measured accurately even under loads of several amps. The LM4041 provides an accurate low-power voltage reference for the sensing circuit. This 1.225V reference is used directly for the conventional lead-acid setting and via two alternative dividers for the sealed lead acid and lithium voltages. Using a 1% version for the voltage reference and 1% resistors in these dividers keeps us from going too far above the magic 4.1V limit on standard lithium cells without having a pesky trimmer, or worse, a set of trimmers.

Solar Battery Regulator/Controller
Click here to download the full size of the above Circuit.

As the voltage across the battery rises under charge, the main load output will be switched on when a voltage some way above the fully discharged level is reached. If the load current exceeds the available solar charge current, the batteries will drain back down to the fully discharged state and the load will be disconnected again. Some hysteresis avoids the load switching on and off too frequently, but this all depends on the available charge current, battery capacity and load current. If the charge current exceeds the load, the battery voltage will continue rising until the full charge voltage is reached. At this point the secondary load is turned on to prevent overcharging. If no secondary load is naturally available, one must be provided in the form of a resistor. If the standard load current exceeds the maximum output of the solar array this is not needed. IRF350LC MOSFETS are used for load switching which allows loads of more than 10 amps to be switched. A dual CMOS rail-to-rail output op-amp is used which simplifies the calculation of the switching voltages. LED indicators drawing about 2mA each show which loads are turned on. There is space inside the regulator enclosure for about 7Amp-Hours worth of surplus mobile phone lithium batteries. Three 3.6V nominal voltage cells are wired in series which produces a battery of 10.8V, then multiple banks of three are wired in parallel. The voltage varies over the charge...

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