I have 2 Smart UPS 3000 (SU3000RM3U) Units maounted in a rack that will not power on. Both units were working fine when removed from service and placed in storage about a year ago with the batteries removed from the units. I want to try an use them to power a new test/dev environment. I have verified power to the units, verified that the batteries are installed properly, but when I flip the switch on the back of the unit...nothing. No lights or other indications at all. I have had them plugged in for several days and still nothing.
Many APC units will require voltage present at battery terminals before it would even power on. Check voltage on battery, out of the unit. The pack should read around 50v.
1 year is pushing the safe storage time of battery if it wasn't given a top off charge once every 3-6 months, depending on storage conditions, etc. Battery would not even last a month in storage depending on the charge level they were at when you put them away.
The symptoms you're describing seems very much like a case of completely drained battery that does not have enough voltage to activate the unit's interlock.
If the voltage of the battery pack is far below 50v, don't even bother with the pack. Junk it. A SLA battery pack that's been in storage for a year and discharged to this level is damaged beyond use.