A Chevy Volt does the same thing by design whenever you use it.
When the gas engine runs it powers the electric motor, but does not charge the battery, at least not the gen-1 volts.
It all depends on driving style when and if the gas engine fires up. I have a gen-1 volt that the engine fires on only when the battery is at zero.
A 10 hour charge typically gives me 35 miles of electric travel which is about 11kwh which costs about $1.49 to get 35 miles for another days use. I live 6 miles from any town and usually use all of the electric as close to zero as I can.
I've only had it 3 1/2 months and put 2,500 miles on it with total fuel burned at 0.03 gallons. Charging for the 2,500 miles is approx 833 kwh which in my area costs about $108.00 plugging it into the house at night.
My other vehicle is a Ram pickup that gets about 16mpg which 2,500 miles at $4 a gallon would be a $625 cost. Even if gas went back to $2.75 (doubtful under brandon) cost would still be $430 vs $108
On a fixed SS income I can surely put up with the $500.00 savings @ $4 a gallon. I am not a green fanatic and the move to a plug in hybrid is for purely financial reasons.