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Home-made UPS? Answered

I recently acquired a very large 12V SLA battery and want to make a UPS for my computer. I think I have the basic idea, but I'm not entirely sure how I would go about this. I'm guessing I could use a relay to switch from the mains to battery, but then I would need a very large capacitor to run the equipment for a few milliseconds whilst it switches. I would need a separate transformer and regulator for the relay, and also an inverter for the battery, which I will probably buy as it looks like it would be too hard to make. Is this the right approach to it?



Best Answer 7 years ago

You might want to look around and see if you can get a used name brand UPS with a dead battery. Not to many people take the effort to change batteries. Instead they just pitch them and buy new ones with warranties. In addition replacement batteries are almost as expensive as a new unit. I have several sitting here with no batteries. Anyway, you could probably rewire them to fit your battery.

The charger thing that Re-design suggests will also work, but it might use more power as chargers are not usually intended to be on all the time. Some of the new electronic chargers can be real finicky too. And don't forget to keep adding water to the battery. We used a setup like this to backup one of our wireless tower repeaters and we kept having to replace the batteries because the water kept boiling out of them.

I actually have an old UPS with a tiny battery, but it outputs 110v and I want 220. I might pull it apart and see if I can modify it to output 220, but I suspect it won't be simple electronics. At least I might get some idea of how to build my own.

I just made a basic schematic, I think I'm works to some extent. I still need to handle the delay when switching, get the output from the inverter working at 240v, it only outputs ~80-100v atm, and is sometimes very erratic, and check that it can output enough current. I think thats down to the two transistors. I will probably need about 1000w out of it, I don't know what this will provide. There may be some problems with my transformer coils/polarity, I haven't actually used them in any circuits before, so there might be problems with them.


I pretty much have the inverter part sorted, I just need to work on the break detection and switching. I thought I could use a relay, but I then realized this would be impractical.I would need a very large capacitor to power the circuit whilst the relay is switching, and I would also need a regulated transformer to power the relay. Does anyone have any simpler ideas?

The UPSs that I have looked at charge the battery full time on a trickle charge if fully charged or full charge if not charged.

The inverter runs full time off the battery charger, so that there is no changeover delay.

When the power cuts the battery takes over powering the inverter.

I was thinking of doing it this way, but I would have thought it would kill the battery after a while. Its non-rechargeable, so I can only trickle-charge it. I also would need a load of electronics to stop it being overcharged, am I right?

I know this might sound a bit boring, but efficient inverters will most likely come with the switching circuitry to do what you want... I would recommend looking out for an inverter that has a basic UPS mode... it will maintain your battery's charge while on AC and switch to act as a UPS... Just think about your power consumption, will you be powering a CRT or LCD... CRTs can draw a lot of current initially as they charge large capacitors in their power supply... make sure your inverter can supply enough power for your PC/monitor... you might pay a little more for an efficient inverter but normally it is worth the money (better for the battery, nice and quiet operation)... all the best with the build...

Thanks, I will try that. All the equipment it will be powering is all new, but there is quite a bit of it. I think In total a max of 12 amps, but it will usually be around 6 or 7. My battery is 30Ah, I told you it was big, so with an inverter it should keep it running for well over an hour. I ideally want to use it as a back up power supply so I can keep my equipment functioning whilst in a power cut, instead of just an emergency power supply.