I have a solar battery backup system that I use when there is no power to run my transceivers. Currently, the process of switching over from the power supply's 12V power rails to the 12V output of the backup system consists of unscrewing the radio wires from the PSU and then twisting them to the output of the backup. This takes time and is tedious to access.

So, I was considering connecting the backup system's output to the power supply's output, so that when one is off the other can provide power.

My question then is if there are any safety risks for this. Assuming the power supply is totally off, could this damage it by having 12V on its output rails?

I also want to state that, while I don't plan to ever have both on and connected at once, the battery has feedback protection so that the output is output only in case the battery and psu are on at the same time. Just mentioning that in case anyone was concerned.

  • 1
    \$\begingroup\$ Do you have specs for this power supply? Tolerance to backfeeding depends on design and construction. Alternatively, consider making your design robust without relying on power supply assumptions using a "power OR" circuit and/or diodes. \$\endgroup\$
    – nanofarad
    Apr 25, 2022 at 2:48
  • \$\begingroup\$ I would use a switch to switch between the main and backup supply. this would eliminate any risk of damage to either supply if it is not designed to tolerate being back-fed. \$\endgroup\$ Apr 25, 2022 at 3:00

2 Answers 2


Never connect a power supply to a back feed voltage unless the supply datasheet specifically states that it it protected against this fault.

That said, a solar power controller would deal this, integrating shore power together with battery and solar, with the added bonus of keeping the battery topped up while there is sunlight. Some units allow selecting different types of batteries, so you can choose a type that best fits your needs and budget.

As an example, the ZAMP Solar setup on my camper has a 70Ah AGM battery and a 160W panel. This is enough to run a 3 cu. ft fridge all the time, day and night. The battery lasted about 5 years in service before it needed to be replaced.


The simplest solution would be to use an SPDT toggle switch and manually change over when required.

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