Uninterruptible power supply

[jheatac]

Hello,

I have a small 500va uninterruptible power supply labelled IBM OP500. I am wayyy over my head but if I let the smoke out it is not a big deal.

I've had this thing 20 years or so. It's on its 4th battery, I believe. The battery light started flashing a few months back so I ordered a replacement.

I put it in and gave it a couple days and the battery light still flashes bad battery. It does not carry a load when line voltage is removed.

I threw it on the bench yesterday and determined that it is charging - which I assumed because it got acceptably warm when I changed the battery.

My question is if anyone can give me a direction to a chip to look around.

I am certainly charging. How do these charge and sense battery voltage at the same time?

It does switch to battery when line voltage is removed but it isn't enough to carry a load. I had a 200w 12v power supply on the bench (idle) I used for a makeshift load and it was getting 24vac, I believe. So it is switching and trying, but not getting any current. I did not test output without a load.

My thinking is the same pathway that senses the battery voltage has a considerable voltage drop.

I don't have the board out to get pictures yet.

[indeterminate]

You need to get yourself a multimeter without one you are blind to whats going on
https://eevblog.store/collections/test-equipment
once you have a meter you can see if the battery is being charged at the wright voltage
and go hunting for the fault if it is not

[jheatac]

I'm going to file this under "Even a blind squirrel finds a nut every once in a while"

I separated the case and inspected the board. Everything looked perfect. Nothing hot. Solder connections all good. I checked the battery while it was charging and it was 13.6v to 14v

Being overwhelmed at where to start, I checked ohms on the power switch - SPDT slide. Really not on/off... More charge/and active UPS. It has a green LED regardless. When off, the charge LED blinks.

One way was 0.12ohm, roughly the test leads. When I checked the other, it was in the 100 ohms then kohms then if I wiggled the switch I got it down to 22 ohm. I took the switch apart and cleaned the contacts and the unit works again.

If only they were all so easy.