Faulty UPS

[chipwitch]

I purchased a "new" APC 600 from ebay.  I think it may have been a return item that was resold.  I plugged in the battery then plugged into mains with no protected load.  Switched on, the power light flashes and the alarm sounds indicating that the power is out and the unit has switched to battery.  A moment later the light goes out and a continuous alarm sounds indicating the battery isn't charged or it needs a new battery.  The ebay seller issued a full refund without returning it.  So, naturally I took it apart!

The limit of my knowledge is checking line input, battery voltage and looking for anything obvious like fuses, blown caps, transistors etc.  Battery voltage is 13.3v and it holds that voltage day to day.  I tested it briefly under load with a 6A fan motor.  It worked fine and still had roughly the same voltage afterward.  I spotted one visual artifact, pictures attached, but cleaned up with a brush and alcohol easily.  Once dried and again tested with the same results.

Any suggestions would be much appreciated.

[m k]

There seems to be quite a few of those designs.
L5 could be a choke of some sort, based on its location and it shouldn't affect your situation.

Do I understand it correctly that you have mains connected when battery voltage is over 13V?

You can test if voltage changes when mains is on or off but that probably is not your problem eigter but it can indicate something.

Since there seems to be two different problems, no mains and low battery voltage, maybe there is a simple component not connected situation.

[chipwitch]

m k... you understand correctly.  I tested the battery voltage with and without mains... connected and not.  Battery unconnected is about 13.3.  Connected it is charging at about 13.6 so that part of the circuit seems to be in order... There's about a minute window after startup where I can test voltage while being powered.

[SmokedComponent]

The white stuff...looks a lot like that big cap spewed its guts. Not always do electrolytic caps blow their top, they can leak around leads as well.

[perieanuo]

start digging around L5 D77 IC19

[PKTKS]

eks this cap is trashed..

[CJay]

Hopefully you're already fully aware of this and I'm wasting my time telling you but please be careful, even though they have a transformer not all UPS are isolated from the mains on the low voltage side, a slip could put you in contact with line voltage.

If I read it right your UPS is indicating a mains failure and a failed battery, it's possible the two are related.

You've already determined the battery is OK and that it charges (charge voltage seems a little low mind you) so you need to work out how it's detecting mains input.

Can we have a full model number and a larger picture of the board, both sides if possible?

[chipwitch]

@CJay....  Yes, I'm aware of line voltage.  I've done my fair share of live electrical work on mains.  Even cut the wires ahead of mast head once and reconnected a residential service once.  Besides, I'm in the states... low voltage compared to y'all.

I'm sorry it took so long.  Work got in the way as usual.  Since my last post I purchased a new, similar model, same brand so new info to add.  I swapped batteries.  The old ups/new battery behavior is unchanged so it's a circuit problem.  The new ups/old battery works as expected so battery is also good.  It was obvious to me that the old UPS had been tried before I got it.  Someone had plugged something into the units face and their aim was poor.  Light parallel scratches that circled their way to the socket was a good indicator someone had already purchases this unit and returned it.  My guess is that it was sold to a reseller cheap and they didn't bother to test it.

I only had the top face picture.  Let me know if you need the underside.