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

Stepan Novotill snovotill at hotmail.com
Tue Aug 9 06:46:23 UTC 2005

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

1) Only do this if you are fully trained and qualified to work on 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 all 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 14 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 R110, 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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