Switching between series and parallel connection of batteries using relays

Question

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?

Answer 1

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.

Answer 2

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.