The battery should read 12-13 Vdc when charged.
I had one of these come in a while back.
The battery had a bad cell (would not charge past 6 Vdc).
It also had weird noises going on.
I resoldered/reflowed the whole amp.
Even the DSP board.
That resolved the noises.
PS: Make a drawing of where the connectors go. It is not hard to mix them up.