Systems+Engines

Solar Switches

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Jack Fyfe of Westerly, Rhode Island, asks:

"I‘ve really enjoyed reading about Nigel Calder’s solar projects, because I am getting two 24-volt Sanyo 205 watt panels and two Morningstar MPPT controllers. I’m using AWG 2/10 wire for the 25–foot run from the dodger where the panels are located to the battery switch. I wonder whether connecting the charge controllers to the battery selector switch might save me from having to run wires from starboard to port (under an already crowded floor) to the two 24-volt battery banks. I plan to run each solar panel to one bank through the two MPPT controllers—in other words two separate systems. This arrangement, I think, will let me send the panel outputs to either or both banks by means of the battery selector switch."

Nigel Calder replies:

You can wire the positive cables back to the battery switch, but be sure you include the necessary over-current protection (fuse) at the point where you connect to the switch. In principle, you can pick up a ground anywhere as long as the ground cable from that point back to battery negative is big enough to carry all the loads being grounded through that cable. I don’t have your wiring schematic to refer to, but if I understand it correctly, I don’t see a problem wiring two separate controllers to the battery switches and then using the switches to determine which bank is charged.

That having been said, there might be a more cost-effective solution. What about wiring the two 24-volt panels in parallel to a single controller, then wiring this back to the battery side of one of the battery switches? The panels would then be hard-wired to this bank, which you can designate as the house bank. Install a paralleling relay between the two banks and when the first bank is charged the second will be paralleled in.

Tapping 12 volts off half a 24-volt solar array won’t damage the panels, but it will cause imbalances that will negatively impact performance. There’s also a chance that the 12- and 24-volt battery banks will try to balance out through the solar panels circuits, and that could wreck the controllers. So don’t do it. Stay with the 24- to 12-volt DC-to-DC converter!

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