I'm working on a seismic alert network and seismograph station at my school in Puerto Rico. We're building a solar system for the system to continue running during a potential seismic event. The system only consumes about 10 Watts, but with the satellite internet equipment it might go up to about 40 Watts. Based on my calculations, the needed battery bank will be about 200AH (12V System with AGM batteries for safety). This goes a bit over our budget, so I thought of redesigning the system so it would be more of a "backup" instead of 24/7 solar power.


Could I use a relay to change the load to battery power if the mains electricity power goes out? This way, the satellite internet equipment will always be powered by the school's mains electricity, but once it goes out, it automatically changes to battery power from the solar system.

What I had in mind

I thought of using a relay to control the change as follows:

Common: Load Positive (AC)

Normally Disconnected: Mains Electricity Positive (AC)

Normally Connected: Solar System Battery Power Positive (AC)

The relay will be activated with a 5V Power Supply connected to the school's mains electricity, so if the power goes out, it will transfer the load (satellite internet equipment) to the battery backup power.

What I want to accomplish with this

So what I'm thinking is that by maintaining the satellite internet equipment connected the majority of the time to mains electricity, I can downsize a bit the battery capacity needed for the solar system, since it will only consume the 10 Watts from the seismograph. Then in the event of a power outage, it will provide some hours of power to the internet system, even if it goes out before the sun goes up again.


Is this an automatic transfer switch I'm referring to?

Feel free to give suggestions about the system, if it could work, if it would be dangerous, or if there's a better solution to it! I appreciate your feedback!


2 Answers 2


Should this be your schematic, it would be quite okay.

enter image description here

  • \$\begingroup\$ That’s exactly how I imagined it! Thank you very much!! PS. What software you used to make that schematic? I haven’t worked much with schematics (only fritzing) and haven’t found a good program for it! \$\endgroup\$
    – AxelSariel
    Commented Aug 10, 2020 at 2:26
  • \$\begingroup\$ Anytime, AxelSariel! I prepare my schematics using MS Excel. Many thanks. \$\endgroup\$
    – vu2nan
    Commented Aug 10, 2020 at 4:03
  • \$\begingroup\$ That's awesome! I'll check it out, thanks! \$\endgroup\$
    – AxelSariel
    Commented Aug 10, 2020 at 9:31

An alternative to your proposed approach is the "online UPS" strategy:

  • Connect the loads to the battery.
  • Connect a mains-powered battery charger to the battery.
  • Connect a solar battery charger to the battery.

And that's it — no switching devices at all. It's the same way a car battery system works — there's one circuit that the battery, charger (engine-driven alternator), and loads all connect to (except that the ignition switch exists to turns off the loads).

As long as your mains battery charger is capable of supplying enough current to power the loads, it'll work continuously without significant charge-discharge cycling of the battery.

The advantages of this are:

  • There will be no potential interruption of power to the load between the time the main power supply shuts down and the 'detector' power supply lets the relay open (which might be surprisingly long since the relay will be a small load).
  • There are fewer components and circuits involved.


  • The mains-powered battery charger should be one designed for this type of continuous maintenance application — not a one-time fast charging cycle while there's no load attached. I'm afraid I don't have a recommendation for how to identify a suitable charger as I haven't personally built or researched one of these systems.
  • In this arrangement, which power source powers the load will depend on the "float voltage" of the mains charger versus the solar charger. If you want to make the most use of solar power, then the solar charger's float voltage must be higher than the mains charger's float voltage. (They should both be below or equal to the recommended float voltage for the specific type of lead acid battery you are using.)

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