Dual Input using 3 Position Battery Switch


Solar Addict
Mar 27, 2021
Reno, NV
'BREAK' BEFORE 'MAKE'. With that rule being the only significant consideration, this idea should work fine, although the choice of THAT kind of switch remains an open question. I have an analogous configuration, in which my MPPT-SCC (an EpEver "Tracer" 3015BN) is very quickly switched from "Genuine Solar Panels" at V(mp) around 80 volts, to instead connect "fake solar" from my tow vehicle, at only 36 Volts. The EpEver model "BN" very quicky adjusts to the new V(mp), with no issues.

In my case, the SCC has a permanent connection to a 12V battery bank (via "bat- +" and "bat -" terminals). The negative ground is completely shared across all connections: the "BN-" shares the "PV -" and "Bat -" input legs on a common internal bus, and I actually have them further interconnected via a 5-way bus (one fused wire to "genuine Solar PV -" from the panels; another fused wire into "PV -" of the controller; an un-fused wire to battery/frame "-" ground, and a smaller wire providing ground to the switching Relay.)

The 5-pin switching Relay is clever, connecting the genuine "PV +" lead from the panels on the "Normally Open" connection, and connecting the 36V "fake Solar" lead from the Tow Vehicle Bargman cable "trailer battery charge" when more than 24 volts is present on that lead. (It is divided, in a 3-way T-connection, to be both the "coil +" lead and the closed-coil power lead). The mechanical relay falls back to "open" position at about 19V, reconnecting the "genuine" solar panels.

The MECHANICAL automotive power relay is brake before make, with a few milliseconds of unconnected time while the "common" contact arm is electromagnetically pulled from the spring-loaded "NO" contact, over to the Closed-Coil contact. During those milliseconds, the SCC sees a totally "dark" Solar connection.
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In the proposed case, it sounds as if the permanently connected battery bank is 24V. The "fake solar" connection" is 48v, the Voltage on the other is unknown but presumably quite high, maybe higher than my own 80v). In my configuration, the only "Sparks" occurs within the mechanical automotive relay, and it has been switched hundreds of times over several years of use. (The controlling switch is actually in the Tow Vehicle, enabled only when the ignition on and defaulting to "off". When cruising under clouds or cruising at night, with my own trailer in tow, I flick the switch to run "under-the-hood 12v" battery voltage and current into a 36.0V DC->DC "boost converter", and connecting the higher Voltage booster-out "36V" wire as the sole input to the Bargeman's "trailer battery charge" wire. TBC is fused at 30A but runs at 12.2A maximum in this mode of operation, the limiting factor being the maximum output current of my "Tracer "3015BN"). My genuine solar, at higher volts, runs at much lower current.

These "dual battery switches" are rated for vastly higher amperage. But the Voltage ratings, typically 50v or so, are probably inadequate. I have never taken one apart, and one could fail if contact distances are "too close" between dissimilar voltages, allowing the "bat-1" and "bat-2" positions to become effectively fused. My automotive relay (with a clear case, through which I can see the substantial distance between the switched "power contacts" (slightly over 1mm) has no such issue. But my Relay has no position to connect "both" simultaneously, and that is a nearly universal feature of boat-oriented dual battery switches. The capability of doing that, on purpose, requires a much more complicated switch design.

You could maybe disassemble the proposed dual battery switch. My tiny automotive Relay handles, rated for 30/40A, handles my "12A" versus "5.5A" current loads with no issues. But my "solar connections" are about 2.6x and 5.9x the Voltage of my battery output, with correspondingly smaller current to provide about 450 maximum watts.

Question 1: What are the Voltage and Current characteristics of your genuine solar array?
Question 2: What is the maximum output power (V*A) which you will be allowing from your MPPT?

If the switched current will be small and the maximum switched Voltage isn't much above the 80v which I switch in my own configuration, then you perhaps could use a 24V automotive relay just like mine. It is basically IMPOSSIBLE to fuse the two input contacts together, all of the failure modes involve it becoming "stuck" on one connection or the other. Your "coil +" contact would be taken from the 24V battery and run through YOUR manual switch before entering the Relay. (A tiny switch, carrying almost no current at all.) Unless your "genuine Solar Voltage" is crazy high, or you want to run more than 30A on a "solar" input lug, this one should do it: https://www.ebay.com/itm/234192194504