# Switching between series and parallel connection of batteries using relays

I attached a sketch about my project. There are two battery banks which are connect in series and parallel each. The fully charged bank consists of two 12 V batteries should always connected in series. That means that the parallel connection should be disconnected. The battery bank which is discharged should be in parallel (both 12 V batteries in parallel). That mean that the serial connection should be connected. The load (which is the same) should be connected always to the fully charged battery bank. I hope it is clear. :) Which type of relays would you engage?

To detect the SOC of both banks I want to use an Arudino controller. But the first question is how to solve this problem?

• Get a 24V charger and the problem disappears May 20, 2023 at 19:07
• No. I don't want that! it has to stay that way. May 20, 2023 at 21:19
• Your rough schematic seems to show the batteries in bank 1 and bank 2 in both series and parallel connection. Perhaps you mean the blue wires are for series and the black wires for parallel? You will need to consider the current that will be drawn, to specify a switch or relay. May 21, 2023 at 1:49
• Yes, thats correct. May 21, 2023 at 7:04
• @ipving - Hi, FYI I have reversed (rolled-back) the deletion of the image and the mention of relays. Those details have already been used in answers, so it is now too late to change the original question. Thanks. May 22, 2023 at 20:20

The problem is, you need interlocks to assure you don't get a Big Bang Boom when series and parallel contactors (big relays) are closed at the same time. Let me see if I can recall the diagrams.

Here's the relay/contactor contact diagram.

simulate this circuit – Schematic created using CircuitLab

You see there's a huge flaming problem. If either SP1 or SP2 is closed at the same time SS is closed, you have a dead short across the battery. This is not "greasy kid stuff".

So, we need some interlocks (auxiliary contacts) on the big relays/contactors to assure that it is open (at rest) before the opposing one can close. So here's an interlock diagram that might fit - remember, I'm just some voice on the Internet here.

simulate this circuit

Note these are normally closed and open when the contactor energizes.

Of course, if one of those interlocks sticks, big bang boom...

I suppose SP1 and SS could be replaced by one "pole" of a DPDT switch, and SP2 replaced by the other pole. But I'm assuming you're dealing with pretty big DC power here, and you seem price sensitive - a DC-rated transfer switch rated for interrupting big power can be costly. A rinky-dink little switch is a bad idea; arcing within the switch would be unstoppable.

• Thank you so much for your efforts. I see the problem. It's also possible that we start with a fully charged bank (two 12 volt in series) and no wires for parallel and a load on it and the other bank (two 12 volt in parallel) and no wires for series. Then the controller checks always which bank is empty and switch the jobs. Now the two batteries of the other bank which is fully charged, must be wired in series and not in parallel, then attach the load from the discharged bank to the full bank, disconnect the wires in series and wired them in parallel. Is that a better way? May 21, 2023 at 7:24

A DPDT relay and a fuse should suffice.

The fuse prevents the damage that could be caused by relay contact welding.

The probability of contact welding may be reduced by using a 'double-break' relay or contactor.

The output fuse is not shown.

Final edit

The schematic has been edited, further to your clarification that it's a 48 V inverter driven by four 12 V batteries.

It's to be noted that two relays and two 12 V chargers would be required.

However, it would be much simpler to charge the four batteries in series using a 48 V charger.

• This looks very very good. May 21, 2023 at 14:39
• @ipving It does, except when the contacts get stuck together and the batteries are shorted out. There is no safety built-in, such as fuse or interlocking mechanism, so you need to add them. May 22, 2023 at 9:29
• Thank you, ipving. May 22, 2023 at 17:46
• @Justme - Hi, Thank you for your response. My answer has been edited. May 23, 2023 at 2:22
• @vu2nan Is there a DPDT relay which can handle 48 V and max. 60 amps? May 27, 2023 at 20:43