Lead Acid Battery Protector

The circuit described here can be used to  ensure  that  a  12 V  sealed  lead  acid  (SLA)  gel battery isn’t discharged too deeply. The  principal part of the circuit is a bistable relay,  which is driven by the output of an op amp.

Lead Acid Battery Protector Circuit Diagram:

Lead Acid Battery Protector


The battery voltage is first reduced via D1, R1,  P1 and R2, and then continuously compared  with a reference voltage set up by diode D2.  When the battery discharges too much and  its terminal voltage drops below the level  set by P1, the output of the opamp becomes  High, which causes the relay to toggle. This  in turn isolates the load from the battery. The  battery can be reconnected via S1 once the  battery has been replaced or recharged.

The relay used in the prototype is a 5 V bistable type made by Omron (G6AK-234P-ST-US  5 VDC). The two windings of the relay each  have a resistance of 139 Ω (for the RAL-D 5  W-K made by Fujitsu this is 167 Ω). When the  battery voltage starts to become too low and  the relay is being reset the current consumption of the circuit is about 45 mA. Shortly  after the load has been disconnected, when the battery voltage rises above the reference  voltage again, the reset coil will no longer be  powered and the current consumption drops  back to about 2.5 mA.

The range of P1 has intentionally been kept  small. With a reference voltage of 5.6 V (D2)  and a voltage drop of 0.64 V across D1, the circuit reacts within a voltage span of 11.5 V and  11.8 V. This range is obviously dependent on the zener diode used and the tolerance.

For a greater span you can use a larger value  for P1 without any problems. With the potentiometer at its mid setting the circuit switches  at about 11.6 V.

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