Like Stoneymonster was saying, the rate of charge or discharge of a battery is generally expressed as a function of the nominal capacity - a "1C" rate of charge or discharge is fully filling or draining the battery in 1 hour - so an 85 kW draw on an 85 kWh battery. Lithium batteries are more tolerant than most, but they have limits, too. The P85D's rated 515 kW is a 6C load - very much on the high end for storage chemistries, and a close match for the Volt's 7C 110 kW maximum on its much smaller 16 kWh battery.To sustain 515 kW on 40 kWh would be a 13C load - well up into LiPo only territory (Tesla could build a pack to handle this, but it'd be more expensive cells with lower energy density and worse cycle lives - all about the tradeoffs.)Incidentally, you do know that the current 40 kWh cars are 60 kWh batteries with artificial charge limits, right? Tesla had so few orders for it they didn't think it was worth the money to make a separate pack design. So with the existing packs there's no difference in weight or power output between an S40 or S60, nor your proposed P40D and a P60D. Of course, the existing pack won't give you what you wanted anyway...Walter