Too complex for me, what about you?

Hi all and thanks for reading,

I have an uninterruptible power supply that has problems.

First some background:

I have an OMNIVS1000 from TrippLite. I knew I was leaving for a while so
I unplugged it from the wall and "turned off" the inverter in case of a
lightning strike, etc. Well, I was gone about 4 weeks when I returned
home I found the small amount of draw from the computer and large TV
plugged to it drained the batteries completely! It has 6 battery powered
outlets, and 2 outlets which are just surge protection/line
conditioners. The 2 outlets passed current and worked just fine. I
plugged it back in for a few days to see if they would take a charge,
but to no avail. I assumed it simply needed batteries. I replaced the
batteries and plugged it back in with my computer and other components.

Here's where the problem shows up:

The simple surge/conditioner plugs still work fine, however I get no
current from the battery powered plugins UNTIL I unplug the device from
power. Then I get current from the batteries just fine. What the
heck? Huh

In summary:

I posess a decent multimeter, the ability to solder, BASIC
troubleshooting skills, and the ability to follow direction. I know how
to check for voltage, continuity, obvious signs of shorting, etc. I know
what a diode does, resistors, capacitors, simple components.

I can fix simple stuff, but this is beyond me...any ideas?

Thanks