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After reading through 20-something posts about 'car/12V/power supply/computer' I've still not found a definitive answer to what I'd like to try here. If this turns out to be a dupe after all, my apologies.

I have this little machine running as a media/file/whatever server in my basement. It's currently (hahaha) being powered by a 65W 12V external 'brick' that came with the box it's mounted in. My (cheapo) kill-a-watt says it uses about 19W, most likely a big part of that are the 2 disks attached to it. Anyway, this works fine but I was wondering if it would be possible to create a 'cheap' UPS for it by 'simply' putting an old car battery in parallel with a PSU (and a 6A fuse).

From my little knowledge I realize that I won't be able to use the 'brick' that's currently powering the setup (need more volt for charging to start with), but I guess I could replace it with a (smallish) car-battery-charger. Going by the technical specifications (see 2.2.3.3 (pdf)) the board allows for an input voltage between 8 and 19 Volts, so I wouldn't be too concerned about things not providing a 'proper' and 'stable' 12V any more.

  • Am I simplifying things too much? Should I rather use something else than an (old) car-battery?
  • Assuming the charger is of the 'trickle-type', is it (un)likely to be smart enough to see the battery go down over time and then do a 'full' recharge or would that bring me into the 'profe$$ional' kind of equipment?
  • And what is my kill-a-watt going to say about this? If this brings my average consumption to like 50W I might just as well rather have the thing crash once a year due to a power-failure.

Alternatively I could go for a sightlier 'more basic' approach and apply the solution found here and make sure my battery is charged "manually/off-line" every once in a while for when it is really needed.

Thanks for any input and remarks!

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  • \$\begingroup\$ One additional thing you might want to consider is what happens when the battery drops close to 8V, it might go through a power on/off cycle a few times as the battery recovers. You'd probably want to set the BIOS to not start after power is restored in case mains isn't restored and it's still running on the battery. \$\endgroup\$
    – PeterJ
    Mar 19, 2015 at 11:16
  • \$\begingroup\$ @PeterJ ah, indeed, well spotted, thx! \$\endgroup\$
    – deroby
    Mar 19, 2015 at 13:15

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You should be able to replace your existing brick power supply with a 12V lead-acid battery with full-time charger attached. The problem is finding a suitable charger.

Most car-battery chargers that you can purchase at automotive stores are NOT suitable for long-term use like this. Either they are not a proper 2-stage or 3-stage charger or they don't have sufficient current output when in float mode.

You mention that your unit needs about 19 Watts. That's about 1.5 Amps.

Check your local suppliers to see if they have a suitable battery charger. You want a unit that can be left connected to the battery all the time and be able to supply at least 1.5 Amps continuous while in float mode.

There are certainly suitable chargers available on-line from a huge variety of suppliers.

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  • \$\begingroup\$ I'll have a look around and will pay attention to those specifications. First time I hear the word 'float-charger' but it brought me to batteryfloatchargers.com/how_float_charger_works so I'll have some reading up to do. Thanks. \$\endgroup\$
    – deroby
    Mar 19, 2015 at 13:09
  • \$\begingroup\$ Although I don't want to start a brand X vs brand Y discussion, something like a CTEC MXS 5.0 seems to be what I need then. From the specifications it seems to trickle charge once it notices the battery is full but will go in full-charge mode again as soon as it notices a drop in voltage. \$\endgroup\$
    – deroby
    Mar 19, 2015 at 21:57

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