The battery meter should normally fluctuate between 3 and 7 bars
It depends much by the context you drive in.
In urban plain context, it fluctuates from 3 to 4, it may just reach 5 if you have a good brake when it's 4.
If you drive at 70 km/h (45 mph), in plain, it fluctuates from 3 to 6.
But if you climb a hill, engine is always on and battery may get up to 8 bars, even without braking.
In downhill, when braking, battery can get up to 10 bars, obviously.