Electrical – Connect two batteries in parallel with transistors


I am trying to figure out how I can connect two small 12V lead acid batteries in parallel using transistors, but when testing this circuit on the breadboard, the voltages didn't equalize so that tells me the two batteries weren't connected to each other.

How could I make what I am wanting to accomplish?

The bases in the schematic will be pulled low by another transistor when a certain condition is met in the microcontroller, thus isolating the batteries.

The goal is for the microcontroller and other circuitry to be running off of the BT2 battery when a motor causes a voltage drop from BT1, and when the motor is turned off, the batteries are normally connected together increasing the capacity of the system.

Thank you.

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Best Answer

You are probably better off doing this with MOSFETs. There are two back to back because otherwise the body diode would cause flow from one battery to the other.

When you apply some voltage to R1 from your MCU, Q1 will pull the gates of Q2 and Q3 low, turning them on, so current will be allowed to flow in either direction. When Q1 is off, all transistors will turn off and the two batteries will be isolated.

The R2 pull-up assumes that V1 is the primary source.


simulate this circuit – Schematic created using CircuitLab

You could also try this. It uses a singe PNP BJT instead so it's simpler but the transistor may dissipate more power than the MOSFETs. You may also run into issues if the voltage differences are high, but I think it would work for you.


simulate this circuit