I've looked at the other answers here regarding building a low voltage UPS and have come up with the following design.

Requirements: 1) At least a couple of hours of up time; 2) USB power 5V @500mA, DECT phone 9V @300mA, DSL Modem + Router both 12V @250mA; 3) Always connected/auto switch over.

Components BoM: 1) 12V Lead Acid battery + commercial intelligent charger; 2) Use 3 SEPIC converters to supply the 3 different voltages.

Simply connect together and leave permanently plugged in. I need a charger with enough power to keep the battery charged, as well as the devices operational. When the mains power fails, the SEPIC converters go from dropping the 13.8-14.5V charging voltage (typically what I have seen from car chargers) to their set voltage and then maintaining that voltage as the power drains from the battery.

Having tested a decent sized car battery, I know that will power the DSL modem + router for nearly 2 days (just connected straight to the battery, no converter). Maybe overkill but I do have at least one going spare. Those devices didn't like being connected to battery + charger though. Hopefully the SEPIC controllers will provide a steady voltage.

My questions are:

1) What size battery and charger should I go for? Assuming the SEPIC converters are 90% efficient I would need less than 1.5A to power them all full time. One of my car batteries specifies a bench charge rate of 4A, I assume that once fully charged the trickle requirement would be a lot less - it seems to measure less, about 150mA DC.

2) Do I need to add a mains detecting changeover relay to switch from the charger supplying power to the devices to the battery?

Thanks for getting this far.


2 Answers 2


The best system is to simply have the charger constantly powering the battery. And the devices constantly drawing power from the battery at the same time. This is equivalent to a "full online" UPS. As there is zero transfer time. (cheap mains UPSes have a small drop in the mains output when they transfer the load from mains to battery during a power failure).

Check the output from your charger as mentioned above. As lots of 12V battery chargers have poor output voltage regulation. And are typically designed to charge a battery, and then be disconnected. They will often overcharge the battery if left connected to a battery 24/7. Get a regulated power supply that you can adjust the output to 13.5V or whatever the battery manufacturer specifies as the ideal "float" voltage for that battery.

Also consider how reliable your SEPIC power supplies are. Especially in relation to over voltage protection on their outputs. You don't want a faulty SEPIC inverter to destroy your connected devices from over voltage. Do a search for "shunt regulator" and "crowbar circuit" for examples of over voltage protection circuits.

And make sure that you put some fuses on the connections to your battery, as close to the positive terminal as practical. As lead acid batteries can provide very large currents if a short circuit occurs. The main purpose of the fuses in this case, is to stop a fire from occouring due to overloaded wires, if one of the SEPIC inverters or your battery charger fails short circuit.


What you need is a power inverter and a battery charger. You can connect the charger to the wall outlet and then of coarse to the car battery. The power inverter will connect to the 12VDC of the battery and supply 120VAC to the computer and router power supplies. You will take a small hit in efficiency, converting to AC and then back to DC, but that is what a traditional UPS does any ways. Both the charger and inverter are pretty inexpensive and can be purchased at Wal-mart, Amazon, and your local RadioShack (although ours closed).

Alternatively, you could just use a laptop. It already has a battery and a charger. And you could power the other items from USB with Buck, Boost, or SEPIC converters.

  • \$\begingroup\$ No, you've missed the whole point of this question. I want a low voltage (5V-12V) DC UPS. I don't want to involve mains AC at all. \$\endgroup\$
    – Mr Banana
    Feb 5, 2017 at 21:24
  • \$\begingroup\$ First off, @JeffBanana you should probably start a discussion first instead of just down voting an answer that doesn't exactly match your question. \$\endgroup\$
    – py_man
    Feb 13, 2017 at 23:48
  • \$\begingroup\$ @JeffBanana especially when nobody else seems interested in helping you out. But why would they if you are just going to vote them down right away. BTW, if you look online you will find that devices known as UPS systems are generally "AC to battery back to AC ". What you are really asking for is how to make them battery powered and where to stick the charger. This is essentially what a laptop does. \$\endgroup\$
    – py_man
    Feb 14, 2017 at 0:13
  • \$\begingroup\$ Second, I don't think the current draw you calculated is correct. I think you just did 500mA+300mA+250mA+250mA and this is wrong. Assuming the numbers you mentioned are correct, you should first calculate the power draw. 5V*500mA+9V*300mA+12V*250mA+12V*250mA = 11W. So now how much current must a 12V supply provide? 11W/12V = 900mA assuming 100% efficiency or 900mA/0.9 =1A assuming 90% conversion efficiency. \$\endgroup\$
    – py_man
    Feb 14, 2017 at 0:14
  • \$\begingroup\$ Third, from your post, it sounds like you just hooked your switching supplies (SEPIC) right to the charger directly. If so, this will probably not work because the charger is really noisy and would mess up your SEPICs. Try first putting the battery into the circuit too. The battery will act to stabilize the voltage and you should then be able to tap into the battery with the SEPICs. \$\endgroup\$
    – py_man
    Feb 14, 2017 at 0:19

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