I'm working on a project that has a high availability requirement. As such, I must maintain a power supply output of 12vdc ~1a (ideally 13.5vdc ~35a). The 1a option will sustain the core equipment, whereas 35a will maintain everything

Currently, we have the ability to use an AC to DC converter, OR a DC supply (Battery/Alternator/etc.). In order to meet the new requirement, I need the ability to transfer from an AC input source to a DC input source, and back.

I've included a simple diagram below showing what I need. Both the 120v AC and 12v DC inputs will be connected at the same time, but with the default power coming from 120v AC. In the event either is unplugged, it will fail-over to the other. If AC is ever connected, it will run off of AC.

In short, an off the shelf battery backup would function just fine for this - however I would need to find one that offers high amperage DC outputs vs the AC outputs.

Simple Diagram

  • 4
    \$\begingroup\$ Sounds like you want to by an UPS \$\endgroup\$
    – PlasmaHH
    Feb 12, 2016 at 16:06
  • \$\begingroup\$ When you buy your UPS be sure to specify "A" (amps) and not "a" (acceleration or absorption coefficient) \$\endgroup\$
    – Andy aka
    Feb 12, 2016 at 16:38

2 Answers 2


Your looking for some kind of inverter, the topology that you have described may not be the best as most inverters have an AC passthrough. Here is a link to some marine inverters to give you an idea of what to look for. RV's, boats and anything else that plugs into AC and has batteries will have what your looking for.

  • \$\begingroup\$ Thanks - I was able to find the Samlex BBM-12100 device to meet my needs. The difficulty was finding something that could handle the required current. I'll be using the DC output of an AC/DC converter and a battery as the two inputs. \$\endgroup\$ Feb 16, 2016 at 21:24

In general, if your AC/DC converter puts out 13.5VDC@req'd amperage, all you would need to do is wire it in parallel with the battery.

That way, the AC/DC converter charges the battery & runs the load when 'live,' and when the AC/DC converter isn't prodicing all of the load-required power, the battery will supply the rest. No logic circuitry required.


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