Older Smart UPS 1500 giving voltage out but not charging

[Mastiff]

I have a Smart UPS 1500 of the rack mounted type, probably 15 years old or more. I have had it for around five years, with two 65 amp deep cycle batteries and very heavy wires to avoid voltage loss, so it can keep the server and Internet stuff at my cabin  in the south of Norway running for four hours or so if there's a blackout, which happens in the fall and winter, with storms. Usually they are shorter, maybe a minute or two, but the server has 8 VM's and something may happen if it goes down hard. And nothing in the cabin works for five minutes since I have a lot of "cabin automation".

Some weeks ago the UPS started beeping, and the fan spun up. There was nothing external that could cause it. No lightning or anything like that. And the voltage started to drop. I have a voltmeter on it, since it's so old it doesn't have a display, and it only has RS232, not USB. I have done a few measurements, and it  does put out around 27V when the batteries are not plugged in, but when I plug in the batteries, the battery voltage just drops, it doesn't raise. So it seems like it has the voltage but not the amps. I have not taken it out of the rack yet, , so I haven't looked inside it. I'm not really qualified to do that either, I have no idea what to look for. I did the "braindead" cycle thing, by unplugging batteries and power (the load is already taken out, to run without an UPS) and then holding in the power button for five seconds, connecting the batteries again and finally the power. But that did not help.

Does anybody have an idea if this is fixable, or if it's time to look for another UPS, preferably with dead batteries?

[JonPro]

Hello @Mastiff 

I am sorry to hear about the issue that you've experienced with the UPS.  Based on the description that you have provided, it is more likely that the issue lies within the charging mechanism of the UPS.  Considering the age of the unit, it is more likely an internal component failure.

[Mastiff]

Thanks for answering! Do you think that if I can fix it with a working motherboard from eBay, so I won't have to buy the full unit? Or is the charging mechanism a separate board? And if I can fix it with a motherboard would I have to find the model number on the motherboard itself, or is it enough to check the model number on the label on the rear of the unit?

Edit: I have found this motherboard on eBay, it seems to be to the same model. So the main thing is that I know the charging is on the motherboard, but from the cables going in and out I would guess they are:

https://www.ebay.co.uk/itm/195264791280