Kudos for disconnecting it while not at home
Local MGE distibuter claims that it is a "battery safety feature". The device simply wasn't designed to be disconnected from the power source without a power failure.
Translation - cheap electronics :trust:
You can experiment with that - disconnecting it while your PC is on, turning off the PC and UPS, then reconnecting it... It should show the last % value
(68% for example).
Even when it shows 30%, if your PC and UPS aren't connected, your battery run time will be way more than 30% :trust:
http://www.lextec.com/prod_ups.html
BTW, if your PC isn't working while you're not at home, why on Earth do you need that
el cheapo software?
And you're welcome :type:
EDIT:
Trust me, almost every MGE user on this forum went through this. We've all been there

It's not good for your piece of mind to worry that much.
You're concerned like you were from
Niš