I checked my leisure battery,( hardly used) and found that it was completely flat 0 volts registering.
I charged it up and it got up to showing 11 volts and it has held this voltage once the charger was removed. Do you think it has been damaged beyond repair by letting it go flat or is 11V ok for use.
It seemed to drive the electric outboard when I tried it for 30 seconds or so.
Leaving them to go flat kills them due to sulfation, a decent battery charger can often revers this, although not very often. However you could try to revive it with a decent charger that has a desulfation mode, my solar charges have this.
My gut feeling is that its time for a new battery