This External Battery Charger Control applies line power to the motor of a 1kW motor-generator set via a high current relay contact. Upper and lower voltage limit (UL & LL) potentiometers provide full control of the On /Off voltage thresholds thus preventing both battery overcharging and over-discharging. Note that this can also control a static battery charger instead of the M-G set, or even an actively generating solar panel.
Schematic of the battery charger control circuit
On the bench, I simulated a motor-generator set with a simple current limited power supply that is switched on at the lower limit and off at the upper limit. Inverter load is simulated with a power resistor.
The LM317LZ may be applied as a very inexpensive 1.25V reference. R5 provides a fixed load to bring its current up to 5mA, the specified minimum operating current. The lower limit (LL) and upper limit (UL) pots are adjusted for the required threshold voltages.
U1C & U1D compare the feedback voltage from the battery voltage attenuator (R6 & R7) with the voltages from the two potentiometers. C2 provides a 0.47sec time constant (delay) in order to prevent noise disturbances—the delay is a non-issue in real life as the battery voltage changes very slowly.
R-S Flip Flop
U1A & U1B are wired as an R-S flip flop. One unique feature of the LM339 comparator is the use of open collector outputs. The open collector feature enables the use of the wired /OR function. When either of the comparator outputs goes low, it either sets or resets the R-S flip flop. It is about as simple as possible.
Q1 is the relay driver—it gets its gate drive signal directly from U1-C. To protect the MOSFET gate from excessive voltage, zener diode D2 clamps the voltage to 10V.
The Omron G8P series relay provides both high current (30A) and low cost ($4.57). The relay in my bench setup is an open version of the same relay. Why a relay? While this can be done with a solid state switch, relays have numerous advantages as follows: low cost, inherent isolation, high power, low losses and robustness.
It is easiest to set up the control using a variable voltage power supply. Simply set the UL pot to max and the LL pot to min. Adjust the power supply voltage to the desired lower threshold voltage, then rotate the LL pot slowly CW until the LED comes on. Then adjust the power supply voltage to the desired UL threshold and rotate the UL pot CCW until the LED extinguishes. Done!
Photos of the external battery charger control