How to fully charge batteries?

It's normal. Battery should stay charged to 6 bars during hwy driving.

7th and 8th bar is for energy recovered from braking and computer will get rid of this charge as soon as it will be possible.

Battery also never (when car is running) go below 2 bars. When you reach 2 bars petrol engine will start and recharge it to 3 or 4 bars then turn off if not needed.

This way your car have plenty of storage for regenerative braking and (on the bottom part) enough charge to leave car not used for longer period of time without damage for hybrid battery (aux battery is another story).
It also gives battery a real chance to stay in usable state for about 10 years.

Rarely (especially in winter, when engine works to generate heat) you may see 7th bar in traffic jam.
8th bar is mostly visible on long downhills. I've never seen in in the city and hwy driving, even when regen braking from Prius top speed to 50 mph.

 

And will quickly add that if you do get 8/8 bars such as going down a long hill, the car may start revving the engine like mad, but it's just burning off the battery charge - no petrol is used at that time. You may also get no regen braking either. The Prius doesn't like being fully fully charged as it can affect the long term life of the battery so if you get 8/8 bars you'll find the car does weird things like revving the engine at about 3,500 rpm and will also accelerate quickly on electric only and/or allow you to drive almost like a pure electric car upto about 45 mph, though as the HV battery charge reduces, the car then quickly goes back to normal.

Oh and don't leave the car in N neutral as it can drop below 2 bars and if allowed to remain like this may not be able to charge itself up again. Remember some putting the car in N with the a/c blowing and having 'issues' with the HV battery.

Any other questions about the Prius?

 

And when it does say fully charged, say at the bottom of a long downhill run, it's still not fully charged. The car will show fully charged, but behind the scene it maintains a buffer.

In my experience, if you drive "normally" it will stay pretty well charged. If you tend to glide/coast, try to keep the car in EV mode with gentle gas pedal, you'll run down the state-of-charge some. I try to keep the charge at least at the middle, avoid purposely extending EV glides if the state of charge is below half.

 

On long downhills, if the charge goes to 8/8, then go to "B" on the joystik = engine brake, which is like a "Jake Brake" on a tractor trailer rig. Valves are shut down, and braking is assisted by compression & vacuum = saves you caliper brakes. You'll hear a gentle roar, similar to a big rig. Jake Brake automatically engages if you're running downhill with cruise.

BlackBerry 9930 ?

 

And to supplement B mode which I experimented a bit in Italy last year, in initial part of B mode driving engine will be barerly audible. As SoC will go up the engine will work harder to dissipate the energy and when fully charged (from the user point of view) it will revv like a mad.

On the other hand... when in mountains... switch to B, select PWR mode and Prius can be fun to drive (it is fun but this is other kind of fun )