Power Tweak – Solar Diverter

The blue battery bank was falling behind and the tan battery bank was fully charged by noon each day. As a solution a solar diverter was developed and installed to switch that excess solar power from the tan bank to the blue bank. Once state-of-charge falls behind on any of the battery banks it’s hard to catch up without some additional outside help. A solar diverter switches one array of solar panels from one battery to another. A nice feature of solar controllers is multiple controllers can be run in parallel to provide increased charge power to a battery. Hardware is a typical 30A automotive relay controlled by a 12 volt port on a Rigrunner 4005i. The station’s solar arrays each peak out at 18 amps so a 30A relay allows a good safety margin. Here is the white-board sketch and capacitor calculation.

Why the capacitor?

A relay typically takes 3 ms to switch from one state to another. During that 3 ms no battery is connected to the solar charge controller. With no battery a controller will shut down and pass through the 20 volt solar power directly to the load port by default. That would destroy the attached 12 volt equipment ( This has been learned the hard way unfortunately and more than one Rigrunner 4005i has been burned up ). Will a 470 uF capacitor from the junk box provide power long enough to bridge the 3 ms gap and prevent the controller from shutting down? Morningstar’s manual says the controller consumes 22 mA. Using ohm’s law that means the resistance is 614 ohms when battery voltage is 13.5 V. Plugging the known values into the formula for a time constant (T = RC ) produces a result of 300 ms or roughly a 100 times factor of safety. Happy with the results and moving forward with the installation, this is what the ridiculously simple solar diverter looks like.

After going live for two days no problems have appeared and the blue battery bank has increased it’s state-of-charge. It’s closing in on 100% probably in a day or two. Rigrunner 4005i’s have a built-in timer for each port so turning on that feature enabled the solar diverter to easily be on a noon-to-8pm schedule. No spikes have been noticed.

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