Roll Your Own Battery Backup System

[Chris] and [Dom] wanted to build their own battery backup system on the cheap. They were very creative in sourcing the parts, and ended up putting together a battery-backed CCTV system for about eighty bucks.

Since short power outages are fairly common in the area this battery backup makes sense. We’ve seen some pretty gnarly whole-house systems but this is more of a novelty. That’s a good thing, because the hacking duo decided to reuse batteries which were headed for the scrap yard. They’re connected to a trickle charger which makes sure that they’re continually topped off when mains power is energized. But when there’s a blackout a relay switches an outlet box over to the inverter (also a used part).

The system is outlined in the entertaining video after the break. You’ll see they guys show off the completed build, followed by a walk through of the circuit they designed and how it functions.

16 thoughts on “Roll Your Own Battery Backup System

  1. Be careful if using batteries in higher voltage configurations. There is no shortage of lead acid battery fire pictures on google. From my experience, the maintenance free batteries can go dry allowing the weakest cells to burn completely away. Leaking acid from charging non-maintenace free batteries can build up a layer of conducting scum and burn the covers off. Keep the batteries clean. Fun stuff. Possible fire hazard. Use a smoke detector.

    1. In this case the batteries are connected in parallel for a 12v high current output, we have had some fun with them connected up in series before (take a look at our YouTube channel). Thanks for the tip we will be sure to take care and I will buy an extra smoke alarm next time i am at the hardware store.

      Thanks for the interest

    1. Not quite, it is a 12v power supply from an old hard drive. Not exactly ideal as we are still waiting for a cheap float charger to pop up!

      Thanks for the interest

      1. A float charger is nothing but a fixed-voltage power supply. For a 12v lead-acid battery, set it to about 13.5v. Most inexpensive regulated supplies will have a trimpot to adjust the output to this value.

        13.2v is the lowest practical value. It maximizes battery life, but takes a week or more to fully recharge. 13.8v is the highest practical value. It recharges in a day or less, but the batteries only last 2-3 years. Ideally, the voltage should be temperature compensated.

  2. I’ve got a 1kW APC UPS going spare. Works fine but needs batteries. I’d happily swap it for a few of those 50p ones if they’re still any good. Pure sinewave output too, and a charger.

    1. Pure sine wave inverter would be amazing if we had the money (Myself and Dom are students)! Still waiting for a cheap float charger to pop up.

      If you want some cheap batteries ask a friend with an alarm system to get in touch with the installer. They should be happy to give/sell you a few batteries.

      Thanks for the interest

  3. What’s the switchover time? Or how much of a power glitch or brownout during the switching from mains to battery/inverter? If it’s more than 11ms, then most cisco gear will reboot, causing network disconnects.


    1. I was thinking the same thing, looks like there will be a brief power cut as the relay flips when the power source is switched. I asked my boss who is an EE and he says that typical relays aren’t fast enough to allow a smooth power transition. It also looks like this won’t handle brownouts properly unless the relay is very sensitive to voltage dips, and it doesn’t sync the AC sine waves between the wall power and the inverter either. It’s a cheap and crude system that could work well as long as a brief power cut is allowable – for example backing a laptop or maybe a small high-quality UPS (although daisy-chaining UPSes can cause problems).

    2. No idea exactly how long switchover time is, might set up a little Arduino based experiment to find out! I expect it is around the 10ms range. My PC and broadband router hand the switchover just fine

  4. Interesting: Certainly a true “hacker” project!

    The inverter is probably 12vdc input, so my guess is that they simply connected all their batteries in parallel; not series.

    It is NOT a sinewave inverter; it’s the cheap “modified sinewave” type, which is marketing doublespeak for a squarewave inverter with some dead time. But probably good enough for switchmode power supplies in computers.

    Note that cheap inverters often don’t last long if used continuously. They are only built for brief, occasional use. I hope yours is better than usual.

    Another thought: Since they had 8 12v batteries, an even simpler setup would be to wire them all in series for 96vdc. Most universal-input switchmode supplies run directly on 90-264v, and so would work directly on DC from the batteries. Then you would just need a float voltage charger to keep them topped off. About 108v for all 8, or separate 13.5v chargers (one per battery).

    On holdup time: All but the worst power supplies have enough filter capacitance to “ride through” the 10-20msec time it take a relay to switch between AC and DC modes.

    1. You are correct, our batteries are wired in parallel producing 12v to feed the modified sine wave (square wave) inverter. Relay switchover time is somewhere in the range of 10ms i expect. The PC and broadband router we are powering handle the switchover fine without restarting. No idea how well it will handle brown outs, just have to wait and see on that one!

      Thanks for the interest

  5. if your so concerened about switchover time then why dont you add bigger capacitors inside of your powersupply???

    … the 200v(or 250v) volt capacitors are usually 220-470uF. put a 470(250v) or 680uF(250v) in parallel …

    just make sure to use same ESR/frequency and temperature capacitors!
    …and mind the polarity ;)

    1. by inside powersupply i meant the computer being protected.

      and after you do this, unplug it and see for yourself, you’d be suprised what a measley 3$ extra will do!

      1. Switchover time is just under 8ms which all our equipment seems to cope fine with, but yes upgrading the caps in equipment that did drop out would be a great improvment!

        Thanks for your interest

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.