Hybrid Car – More Fun with Less Gas

APC Smart-UPS ... battery float voltage adjustment?

register ::  Login Password  :: Lost Password?
Posted by Jordan Hazen on June 30, 2003, 1:20 pm
Does anyone know how to adjust the voltage at which an APC Smart-UPS floats
its batteries?  Their factory default at room temperature seems to be about
2.33V/cell, which exceeds the manufacturer-recommended voltages for every
sealed-lead acid battery I've looked up (equivalent to 14.0V on a 12V string).

We've had to replace batteries these UPSes every 2-3 years, on average,
probably as a result of electrolyte loss from the high charging voltage.

On smaller APCs (Back-UPS line), it's easy enough to tweak a trim pot inside
and turn down the charging voltage.  This decreases the runtime slightly, but
does halp the batteries last longer.  But Smart-UPSes have no such adjustment
pot... their charging regime seems to be under the microprocessor's control.

Can this be controlled from an attached PC through the serial interface?
APC's tools seem to offer no such option, but I've heard rumors about an
undocumented calibration procedure.

Failing that, any suggestions on physically modifying one to drop the float
level a bit?  I've thought about changing the thermistor circuit to fool the
UPS into seeing a higher internal temperature (charging is supposedly
temperature-correlated), but then it would probably run its fan constantly.

Posted by Browntimdc on July 1, 2003, 2:59 pm

You're right. In the past I have put a diode in series with a charger and
readjusted the float to compensate for the drop. This was to prevent the
charger from draining the battery during a power failure.


Posted by stu_e on December 9, 2004, 7:49 pm
 Sealed lead acid batteries recommend a charge voltage from
2.25-2.33volts/cell.  This works out at the top end at 14.0v and should be
ok.  Batteries typically last about 3 years based on the frequency of
discharges, the duration of discharge and the ambient temperature.  The best
thing you can do is make sure the battery/UPS is kept at 77 degrees F.

The diode option will not work because it is directional and the battery
needs to work in both directions (charging when utility is present and
discharge when the UPS is on battery).

Posted by John R. Copeland on December 9, 2004, 8:18 pm
 Read again what Tim said, not what you thought.  Of course it works.

Posted by Dave VanHorn on December 10, 2004, 10:12 pm
 I have an APC like this as well, it seems to eat batteries rather
I don't suppose they publish a schematic anywhere...

This Thread
Bookmark this thread:
  • Subject
  • Author
  • Date
please rate this thread