APC Smart-UPS displays problem with charging

Hi there all.
My 1st post since I started work as an IT apprentice, so would be thankful for an advice.
I am just investigating my company`s APC Smart-UPS SMX2200RMLV2U which displays me warning state: charger low volt.
I assume it does refer to lack of connection to the batteries or lack of connection at all.
I have checked all connection of it, and teoretically it seems to be all correct.
I am also famiar with troubleshooting manual, but there is no solution (or at least explanation) for that matter.
Can anybody told me why is this fault displayed?
Could it be caused by the need of batteries replacement?

Many thanks