Zumo 590 won't boot - Battery Issue?

Hi

Suddenly, my Zumo 590 refused to initialize.
Screen shows a battery icon with "? "



I know that my battery is a bit exhausted because before this my Zumo didn't remain many minutes ON without external supply, but at now, my unit does not start still connecting external supply and it always shows battery icon with "? "

Can someone tell me how to solve this?
(The purchase of a new battery looks like the easiest way, but I preffer to be 90% sure that with a new battery the problem will be solved)

Thanks in advance