# 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.

• What are you trying to accomplish? Commented Mar 29, 2017 at 3:07
• Like I said, connect two batteries in parallel with transistors.. Commented Mar 29, 2017 at 3:08
• Why? Why not a wire? What do you expect the transistors to do for you? Commented Mar 29, 2017 at 3:08
• There is a point where the two batteries need to be disconnected without human intervention so I need the transistors. Commented Mar 29, 2017 at 3:09
• Ah, OK, this wasn't clear from your question. How will the transistors be controlled? Your drawing doesn't seem to allow for any external control, as a relay would. Commented Mar 29, 2017 at 3:10

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

• Will this allow the batteries to equalize each other when the NPN transistor is turned on? I do want the two batteries currents to flow to and from each other most of the time because the motor is off most of the time. Commented Mar 29, 2017 at 3:23
• Yes when turned on they will both conduct in either direction. Commented Mar 29, 2017 at 3:26
• Thanks. Are Q2 and Q3 both P channel FETs? Commented Mar 29, 2017 at 3:27
• Yes, both P-ch, and you would want to make sure they can handle Vgs of at least the max battery voltage. Most will take up to +/-20V so this should be fine. If the batteries are higher voltage you would need to modify this. Commented Mar 29, 2017 at 3:28
• When Q1 is turned off, when the batteries want to equalize with each other, will this current in-rush destroy Q2 or Q3? Commented Mar 29, 2017 at 3:31