# How do I combine 3 batteries in series and parallel with different outputs?

I'm looking to combine the 2 circuits shown in the picture below using a switch.(ON-OFF-ON) I would like to keep the 12V output seperate from the 24V output. This is to maximize efficiency when using 12V with the added possibility of using 24V and 12V at the same time.

Switching 2 batteries from series to parallel with one output using a DPDT switch seems logical to me, but combining 3 batteries in the desired way is a bit overwhelming because in my attempts there's always a short.

edit: my problem: I have 3 battery mounts on a camera system with interchangeable batteries (sometimes I only use 2 batteries, sometimes 3 because weight distribution is important, all are charged seperately). I need a 12V out all the time and sometimes 24V. I would like to discharge all batteries as evenly as possible.

• You need a 3PDT switch. Commented Jan 27, 2019 at 13:52
• Connecting batteries in parallel like this is not a good idea. The batteries will be slightly different and connecting them in parallel can cause very large currents to flow. Even if you carefully equalize their voltages when charging them they will not behave exactly the same. Commented Jan 27, 2019 at 13:54
• @Darius: Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. Commented Jan 27, 2019 at 14:12
• there's nothing more efficient about only using a subset of your batteries. Anyways, bad idea, as Transistor and Elliot point out Commented Jan 27, 2019 at 16:59
• Thx for the answers, they really helped me. My scenario: 3 battery mounts on a camera system with interchangeable batteries (sometimes I only use 2, sometimes 3 because weight distribution is important, all are charged seperately). I need a 12V out all the time and sometimes 24V. What would be a better solution? ps: Will probably use (ideal) diodes on every battery. Commented Jan 27, 2019 at 18:53

## 1 Answer

Here's how to do it (but it's still a bad idea).

simulate this circuit – Schematic created using CircuitLab

Figure 1. 12 - 24 V switching.

As others have pointed out this is not a good engineering solution to whatever problem you are trying to solve.

• In 12 V mode all batteries will discharge at the same rate. Since they're 12 V I'll assume that they're lead-acid and so you'll be recharging them too. This is fine in parallel mode.
• In 24 V mode BAT1 will discharge at a current I but BAT2 and BAT3 will discharge at I/2 so they will be unevenly discharged and this will cause problems when switched back in parallel as their voltages will be different and high currents may flow.
• BAT3 can be removed from the above schematic. Commented Jan 27, 2019 at 16:20
• So could BAT1 and BAT2 but the original question is about a three battery setup. Commented Jan 27, 2019 at 16:22
• But there are only 2 output voltages. I did understand the question. Commented Jan 28, 2019 at 16:56