[Nut-upsuser] How to lower float charge voltage of an APC Smart UPS - corrected

Stepan Novotill snovotill at hotmail.com
Tue Aug 9 07:36:51 UTC 2005

My first posting had typos. Here is the corrected version:

You can lower the float charge voltage that APC Smart UPS units such as
SU1000NET or SU700NET supply to the battery. This is important to prevent
overcharging when when substituting Lead Acid Deep Cycle batteries for
the usual sealed maintenance free batteries provided. Instructions:

1) Only do this if you're fully trained and qualified to work on a Lead Acid
powered UPS. These units contain lethal voltages and currents and can leak
hydrogen gass, explode, or cause fire. Do so at your own risk and peril. The
following procedure is experimental and dangerous, but it does work.

2) Unplug the UPS, Disconnect and remove the batteries, press the ON button,
and wait five minutes for internal voltages to bleed off.

3) Remove the cover from the UPS and remove the plastic sheet which covers
the circuit board.

4) Locate the 100K ohm resistor labelled R118. You have found the right
resistor if one end of it connects to pin 13 of the intergrated circuit
labelled APC2020. This is the sense resistor for the battery float charge
voltage regulator.

5) Solder a 1.3M ohm resistor in parallel with R118, to lower the resistance
by a few percent. Now the float charge voltage will be a little bit lower so
that you can use Lead Acid batteries.

Note that this modification only affects the float charge voltage, and does
not affect the battery condition monitoring circuits or the battery voltage
monitoring A-D converter accuracy.


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