*UPS Mod & Fix- APC450*

The Rubicon working for me

By Jaden

17 October 2004


I scored an old APC 450 uninterruptible power supply (UPS). It was going to be thrown away and I asked if I could have it. The owner was glad for me to haul it off.

I figured that the batteries were toast, but they can be replaced. The first thing I did was plug the UPS into the grid power. Then I measured the AC voltage at the UPS outlets. It was fine so I yanked the plug. Voltage dropped to zero.

Plugged in Unplugged


Grid AC passing thru UPS No DC to AC conversion

I checked the battery voltage. Sure enough, deader than a door nail.

2 6-volt batteries in parallel No voltage

I checked the UPSís charge voltage w/ the batteries hooked up and the unit plugged in. It was 13.5 VDC. Then I disconnected the grid power and removed the batteries.


Charge voltage Battery removal

Next I drilled a hole through the back of the UPS for 2 pieces of 12- gauge (stranded) wire to go through. I didnít have a grommet to put in the hole to prevent chaffing so I wrapped some electrical tape around the wires where they go through the metal.

(NOTE: With DC the black is negative)


Wires taped Through the wall

Next I connected the wires to the existing wires on the circuit board. Then I used a zip tie on either side of the wall to keep the wire in place. Then I threw on a couple of ring connectors to attach to a battery.


Wires connected Ring connectors

BTW, there should be a fuse directly after the battery. I didnít happen to have a 12-gauge fuse holder at the time.

I hooked up a 12- volt battery and turned the UPS on (w/o plugging into the grid). It beeped a couple of times and showed about 120 volts AC with no load. I plugged a 100-watt light bulb, which draws about 1.1 amps into it. It worked. Cool!

Dad was rather fascinated with my project so I went and retrieved him to come and have a look. He was amazed that this box was making a light bulb run off a battery. He looked it all over and shut the light off. Then turned it back on again. BEEP BEEP BEEP BEEP BEEP BEEEEPPPPP!!!! The light didnít come back on. Hmm.

I spent the next couple of hours chasing power through the UPS circuit board & transformer with a multimeter. I only had 2 volts AC when the grid was disconnected. I had battery voltage on the primary side of the transformer, but no AC voltage on the secondary. I unplugged the UPS and ohmíd out the transformer, it was working ok. I posted my observations on the alternate energy board and asked for ideas of what might be wrong.

Gogetumnow & Techsar had some really good suggestions. Warlord hit the nail right smack on the head. Hereís the conversation-

War- Some UPS units are designed to ONLY work if a load is on them when the grid power goes out. They don't act as inverters unless there is a load on them already when the grid power goes out.

Me- had the 100- watt bulb turned on w/ the UPS plugged into grid. Yanked the grid plug, light died...UPS went BEEP BEEP x a bazillion times.

War- Something's wrong with the battery then bro.

Well our resident electrical engineer thinks thereís something wrong with the battery. I hooked up a different battery and tested the UPS, it worked flawlessly. The battery voltage was a little low resulting in the AC being a little low (or so *I* thought), but it switched over fine. Then I tried another battery and it worked fine too. This UPS will power up an AC load w/o having to be plugged into the grid first. OUTSTANDING!!

The AC voltage was low. My meter read 97 volts. I thought it was due to the battery voltage being in the low 12 range. I posted on the alternate energy board again with the problem.

Me- A battery voltage of 12.2 produces an AC voltage of 97. I'm guessing that a voltage of 12.8ish would produce about 110 ac. These readings are while under load.

War- It should hold 110 VAC until the battery reaches about 10.7 VDC.. Something's not right with that UPS bud.

Me- Actually, I wonder if there is an AC volt adjustment in there. There are a few pots. I'll have to play w/ them...worst I can do is smoke 'er.

I took a look at the adjustments. They are small potentiometers (pots). With the meter hooked to the AC outlets and the UPS running from DC and with an AC load I started playing with them. Yes, they were the right pots (lucky guess).


Voltage adjustments

I slowly cranked the left pot clockwise. The voltage increased and the buzzing of the transformer started to change. Just for grins I tried the other pot and the voltage increased some more. I cranked it up to 123 volts to match the grid voltage.

I cycled the UPS w/ the load and it worked A-OK! Looks like Iíve got a working unit. This pic was taking w/o a flash. The light is the load on the UPS.


I made another post on the alt power board about what I just did. War advised that I had the voltage set just a little too high.

War- Back it down to 115V.. 123V is too much for a UPS unit to handle long...

So I turned the voltage down to 115V and itís holding fine. Put the cover back on and now itís ready to go!

Hereís an example of the Rubicon working for me. I probably could have figured most of it out on my own and got the thing working, but it would have taken awhile or I might have thrown the thing away after it wouldnít work with the first battery. Most likely I would have canned it. I would have left the voltage at 123 and probably fried the thing. Thanks to the Rubicon I now have a good working 450 watt UPS/inverter and yet another way to make AC power. Hopefully it lasts a long time.



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