Battery Equality Monitor Circuit Schematic


Almost all 24V power systems in trucks, 4WDs, RVs, boats, etc, employ two series-connected 12V lead-acid batteries. The charging system can only maintain the sum of the individual battery voltages. If one battery is failing, this circuit will light a LED. Hence impending battery problems can be forecast. The circuit works by detecting a voltage difference between the two series connected 12V batteries. Idle current is low enough to allow the unit to be permanently left across the batteries.

Circuit diagram:

Circuit diagram


  • R1 = 2.K
  • R2 = 4.7K
  • R3 = 39K
  • R4 = 39K
  • R5 = 1.5K
  • R6 = 1.5K
  • Q1 = BC547
  • Q2 = BC547
  • Q3 = BC557
  • D1 = 3mm Red LED
  • D2 = 3mm GreenLED
  • B1 = DC 12 Volt
  • B2 = DC 12 Volt