With the engine off put your tester on the battery,now turn everything on,in the car.Lights,heater,wipers demister etc.Then watch to see how far it drops and how fast.
If it starts dropping pretty rapidly to the point where it goes under 10v then it's likely that its your battery.
I tend to drive down to the local tyre place(not kwik sh!t etc) and ask them to do a drop test on your battery,its a similar test,where they put a load on the battery and see how far it drops.
It's always free for the check in 99% of the places too.
