Over voltage protector for a battery

Lead acid battery over voltage protector

It allows the charger to charge the battery such that,

  1. Battery never gets charged above 12.9V. A relay will cut off charger from the battery at this voltage.
  2. Battery charging will resume only when the battery voltage goes below 12.4 V.
  3. This hysteresis is inserted to avoid hunting of the circuit and chattering of the relay.

Principle of operation

  1. The circuit uses the principle of Schmidt trigger with
    1. Upper tripping point = 12.9V (6.45V at the non-inverting input)
    2. Lower tripping point = 12.4V(6.2V at the non-inverting input)
  2.  A transistor bypasses bottom resistor of a resistive array and changes the reference value temporarily.
    1. So while the charger charges, and battery voltage goes above 12.9V, the relay cuts off the charger. Up till now the reference voltage remains at 6.45V because the transistor is not bypassing the lowest resistor.
    2. To start charging again, the relay must cut in at a lower level, otherwise hunting will take place. So the transistor bypasses the lowest resistor to make the reference voltage 6.2V.
    3. So the battery voltage goes down till it reaches 12.4 volts and the relay cuts in again.

Circuit diagram