graphene batteries solid-state batteries energy costs maintenance costs battery testers torque wrenches commuter ebikes thermal management Electric Dirt Bike CE certification SUV e-bikes high-torque battery systems mid-drive motors budget folding e-bikes lithium battery care high-torque motors removable batteries heavy-duty suspension high-speed motorcycles long-range motorcycles adult commuting e-bikes foldable e-bike LiFePO4 batteries regenerative braking speed limiters youth models