Low Voltage 915iS

I have recently noticed that my main buss voltage is only getting to around 12.8V. I have an Earth X 900ETX battery.    At startup the voltage is around 12.6V and rises slightly to around 12.8V - 12.9V once the revs are raised to around 3000rpm.   No Lane warnings.  My avionics and lights draw around 12.5A and the current charging the battery (through the shunt) is only around 1A to 3A at best.   Could this be a battery issue or a weak main generator?