Say I have 2 of 400AH 12V DC battery banks, "A" and "B", and bankA is mobile, opportunistically charged by a HO alternator, the bankB is stationary, charged by a genset and topped up by a large solar array.

Sunny weather and alternator usage are both highly variable, there is usually no source of mains power, and a primary goal here is to minimize use of the fossil fuel engines.

When one bank is getting much more "free power" input than the other, they can be connected by a 180A "battery to battery" charger, but this unit is expensive, and is designed to charge in one direction only, from bankA to bankB.

I would like to install a switching arrangement to be able to swap the input & output wiring on the converter/charger, so I can direct current in the other direction, charging bankA from bankB when appropriate.

Ideally with a single throw, but if such a switch does not exist, I at least want a sequence simple enough to be able to train newbie users. The setup MUST protect from inadvertent direct "combining" of the two banks other than through the B2B charger, or at the very least, make doing so difficult given clear posted instructions.

The HD marine switches (from e.g. Blue Sea ​http://assets.bluesea.com/files/resources/sales_sheets/6823.pdf) are ideal electrically and IMO cost-effective, but the choices there are limited.

Perhaps something could be constructed from these three available designs?


3-position selector: OFF/1/2

Dual Circuit, isolated from each other, both ON or both OFF

I'm pretty sure the remaining two options are useless here:

Dual Circuit plus COMBINE

4-position selector: OFF/1/2/COMBINE

  • \$\begingroup\$ Circuit diagram? \$\endgroup\$ – user95482301 Jan 22 '17 at 11:20
  • \$\begingroup\$ Maybe include some links to the specific switches you're talking about? Am I correct that your question really is just about how to design a high current DPDT switch? My opinion is that switches are usually extremely expensive for high current setups. What's your budget? You could consider building a rotary bus bar setup maybe. If you want high spec for a low price. And then put a fancy panel in front to make it look nice. \$\endgroup\$ – user95482301 Jan 22 '17 at 11:31
  • \$\begingroup\$ It often helps to draw it out. It simplifies the problem and makes it easier to understand what your solution would have to entail. Perhaps the easiest is to have hot-swap-able cables so that input/output is easily switched. A sketch would be useful. I have a battery bank that I want to charge with 100A from an alternator, and for that I've built my own switching using MOSFETs. But that's real complicated. \$\endgroup\$ – user95482301 Jan 22 '17 at 13:58
  • \$\begingroup\$ Example of simple diagram: cdn.shopify.com/s/files/1/1073/9770/files/24v_wiring.png \$\endgroup\$ – user95482301 Jan 22 '17 at 13:59
  • \$\begingroup\$ Here's an example of a custom made, low-resistance/high current switch. You could devise some mechanism that would swap the wires like a DPDT switch. You can buy copper bus bars relatively cheap and make it yourself for ~$10-20: youtube.com/watch?v=Zez2r1RPpWY \$\endgroup\$ – user95482301 Jan 22 '17 at 14:02

You can make your own DPDT switch:


simulate this circuit – Schematic created using CircuitLab

Or just use jumper-style cables, e.g.: enter image description here

Cut it on the one side to allow quick swapping. Fixate with a bolt. Takes a minute or so for each swapping, with little wear. High current capability, super low cost, full control over variables...

Have fun! :)

  • \$\begingroup\$ That might work. It looks like two seperate SPDT though. checco76.files.wordpress.com/2010/11/spdt.png \$\endgroup\$ – user95482301 Jan 22 '17 at 22:58
  • \$\begingroup\$ I'm a bit worried that the switching won't happen simultaneously and that it could cause a short. You could ask the manufacturer or connect an additional solenoid in series which you turn off first before doing any switching. \$\endgroup\$ – user95482301 Jan 22 '17 at 22:59
  • \$\begingroup\$ Also, the solenoid should have a "normally closed" connection which is the default when no power is directed to the solenoid. Since it consumes 15w, you'd want to only enable it when you have surplus power (but at 180A, 15w is definitely acceptable) \$\endgroup\$ – user95482301 Jan 22 '17 at 23:00
  • \$\begingroup\$ Let us continue this discussion in chat. \$\endgroup\$ – HansBKK Jan 23 '17 at 13:58
  • \$\begingroup\$ For the simplest cable swap idea, thinking this could be outside the batt box, found these welding quick connects: fastenal.com/products/details/0812164 \$\endgroup\$ – HansBKK Jan 23 '17 at 14:24

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