butter-fry avatar image
butter-fry asked

Power source switching | Simple 75/15 + Battery setup


I've got a small, single purpose solar setup. Here's the relevant connected components:

  1. 4 x 100w panels wired as 24v
  2. Victron 75/15
  3. 24v LifePO4 (30a total)

The Scenario:

It's a small setup as you can see. I have built a LPG Heated Hot Tub with custom IoT components and software I've written. With a constant draw of around 20w of power, and a peak of around 220w when the water pump is active, everything works great. I built the setup over the summer, when the sun was shining well. My solar / battery to draw calculations were great on paper, but now that winter is setting in I'm seeing some power gaps from lower solar production on cloudy days than I anticipated. After ~3 days of rainy/overcast days my battery is getting depleted. Sure, I can throw more battery at it (expensive), or More solar (expensive and no space).

The Ask:

The Manual way: I'm seeing a need to monitor and intervene before the battery shuts the system down. My proposed solution is to put a 24v source inline with the solar input connected to a transfer switch. It's pretty trivial to monitor battery voltage and perform a source switch on a low-voltage trigger setpoint. I worry about obtaining a fast enough relay with a non-solar specific IoT switch. There's no point in putting this in place if I get a power burp that resets all the equipment on switchover.

The better way?: Is there a specific Victron product that can do this for me? I know some of the higher end full-fledged devices have Transfer Switches built in, but this small setup is way to simple for that. I'd like a stand alone product recomendation if there exists one.

Any input or ideas would be appreciated. I see other transfer switches out there, but I'm here because of the data integration Victron products provide.



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1 Answer
Murray van Graan avatar image
Murray van Graan answered ·

If your 24V supply is stable, who not just switch it straight onto the battery DC bus? That way you won’t have a switchover from one supply to another, and the 24V supply would prevent the battery from draining any further..

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butter-fry avatar image butter-fry commented ·

hmm, sure. I suppose I could monitor solar and trigger the DC 24v supply switch to disconnect when the panels are active and producing too, not just battery recovery voltage.

I was hoping for an integrated solution from Victron, but this'll work.



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