Hi darren, welcome to MR
I'm using my Opus with the Mobipocket firmware and I don't have any problems at all, not with the battery meter nor with the battery itself.
But the battery meter should not be overestimated, it can only show an approximation.
Is your device new? The battery meter needs two or three charging cycles to adjust.
If your problem is not with the battery meter but with the battery itself: turn off the accelerometer, put it off completely and not shut down into sleep mode.