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. Circuit diagram : Lead Acid Battery Protector Circuit Diagram 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 b...