If this article from the UK-based AutoExpress is to be believed, GM will opt for a smaller battery for the second generation Vauxhall/Opel Ampera and presumably the near identical +Chevrolet Volt. The story quotes Thomas Sedran, GM’s Vice President of Strategy and Operations as saying 18-30 miles of electric range is sufficient compared to the 25-50 miles the current model is capable of. 

Going with a 40% smaller battery would allow the engineers to drop about 150 pounds and probably several thousand dollars of cost.  Re-configuring the current T-shaped battery pack would also potentially allow the designers to add accommodations for a third passenger in the back seat addressing one of the biggest complaints against the Volt. 

The weight reduction and a new more efficient range extender engine would lead to improved fuel efficiency in charge sustaining mode. GM is also no doubt learning lessons from both current Volt customers and its competitors.  

The current electric range of approximately 40 miles was selected based on data that said 75% of drivers go less than that on a daily basis. The 30,000+ Volts and Amperas on the road already are feeding back a lot of usage data to the OnStar databases. GM has probably found that the shorter range would still meet the requirements of the way drivers are actually using these vehicles. 

More importantly the shorter range would still allow GM to complete the EPA driving cycles without running the engine, in the same way that Ford is able to do with the Fusion Energi and C-MAX Energi. The reduced weight would help the hybrid mode mileage allowing the new Volt to get much better label numbers than the current model.