Adjusting charging voltage of APC Smart-UPS 500

[PA3GMP]

Although it's not really ham equipment, I have an APC Smart-UPS 500 in the shack to clean up the terribly dirty power we get from the local power grid in Johannesburg, South Africa (where I currently live) and of course to keep my packet computer going during the numerous black-outs of recent years. :-)

I'm looking for any service info I can get on this, the APC Smart-UPS 500. I've got a circuit diagram without any description, and I've got a description in Russian. This is less than informative. :-)

The problem is, my Smart-UPS charges the 2 sealed lead/acid batteries with a constant total voltage of 28.1V, which is rather high. Other UPSes of the same model generally don't go over 27.1V which is more like the recommended float charge voltage for these cells.

After I recently put in new batteries only to see one having lost electrolyte only 3 weeks later (generally an indication of either overheating, which it does not, or overcharging) I decided to see if I can adjust the charging voltage.

There doesn't seem to be such an option. In fact I can't find anything that seems to have to do with regulation the charging voltage or current.

If anyone can point me into the right direction, or tell me where I could find service information on this UPS, I'd greatly appreciate it.

TNX!

73 de ZS/PA3GMP

[W8AKR]

Quick fix:  Put two 10 amp diodes between the charger and the batteries. This will lower your voltage 1.2 volts without any charge rate problems.

[PA3GMP]

DR OM,
Thanks for your response. However I'm afraid that using two diodes is not really practical...

While two diodes will do solve a problem during charging, they will create one during discharging. I'd have to add another 25A diode (plus heat sink for same, probably) in the direction opposite to the two ones that pass the charge current, and it must be insulated (i.e. so must the heat sink) from anything else. Difficult... not to mention the fact that the diodes have to be added on the right end of the battery fuse.

And then of course there's the fact that the discharge diode will fool the UPS into thinking the battery is flat long before that's actually the case.

So I was hoping to figure out how to adjust the charging voltage where it should be done, i.e. in the UPS regulating electronics itself...

[KB9YUR]

Have you tried contacting APC via their website or
email for additional info? You can also call them
via 1-800-800-4APC and 1-800-555-2725 Tech Line.
It might also be cheaper to replace the unit with
a slightly larger one. Recently I bought a refurbished
APC Smart 700 for $80 (US) from www.fightblackouts.com .

George ...

[PA3GMP]

Hi, George,
Thanks for responding.

Yes, I've e-mailed APC. Their answer to everything is "Send it in for repairs". Their view on customer care seems to know only two options: the user-servicable part (plugging in the power cord and for some models replacing the battery) and the non-user-servicable part (everything else).

As for ordering replacements, I'm currently located in South Africa, not in the US, which makes things more difficult.