Planar Battery - Balanced or not?

I personally don't feel it is overpowered because in your example. 
Turn one cast it
Turn two tap out to add counters on it.
Turn three have three mana to do something else.
Turn four tap out to add four counters to it
Turn five have five mana and are really only now doing your second "thing" as you are wholly focused on adding counters.

At this point, your opponent has likely done
Turn one cast one drop creature
Turn two cast two drop creature and do 1 or 2 damage.
Turn three cast three drop creature and do 3 to 6 damage
Turn four cast four drop creature and do 10 damage? 
Turn five cast five drop creature and do 20 damage? 

In a 1v1 standard game, you are possibly dead before you can use it.
In a multi player commander game, you are possibly dead because you can't defend yourself and are creating a target with all the counters.

I think it is definitely not overpowered because to maximize it, you need to not play anything else.