Increase running time after a charge.

1. Decrease Chasis's mass

1.1. More expensive material

1.2. Decreases endurance

1.3. More time to research

2. Add more battery pack

2.1. Heavier

2.2. More time to recharge

2.3. Need more space

3. Better engine

3.1. Compatible to the battery

3.2. More expensive

3.3. More time to research