An experimental electric vehicle of mass \(m\) uses a regenerative braking system that converts all of its kinetic energy into stored electrical energy while bringing the vehicle to a complete stop with constant acceleration. In Test 1, the vehicle enters the braking zone at speed \(v_0\) and comes to rest over a distance \(d\). In Test 2, the same vehicle enters the braking zone at speed \(2v_0\) and comes to rest over a distance \(2d\). What is the ratio \(P_2 / P_1\) of the average power delivered to the energy storage system during Test 2 compared to Test 1?