at the pedals rather than at the wheel, being eventually applied to the wheel via the bicycle's standard drive train. Because the power is applied through the chain and sprocket, power is typically limited to around 250–500 watts to protect against fast wear on the drivetrain. An electric mid-drive combined with an internal gear hub at the back hub may require care due to the lack of a clutch mechanism to soften the shock to the gears at the moment of re-engagement. A continuously variable transmission or a fully automatic internal gear hub may reduce the shocks due to the viscosity of oils used for liquid coupling instead of the mechanical couplings of the conventional internal gear hubs. The main advantage mid-drive motors have over hub motors is that power is applied through the chain (or belt) and thus it utilizes the existing rear gears (either external or internal). This allows for the motor to operate more efficiently at a wider range of vehicle speeds. Without utilizing the bicycle's gears, equivalent hub motors tend to be less effective propelling the ebike slowly up steep hills and also propelling the ebike fast on the flat. Batteries E-bikes use rechargeable batteries in addition to electric motors and some form of control. Battery systems in use include sealed lead–acid (SLA), nickel–cadmium (NiCad), nickel–metal hydride (NiMH) or lithium-ion polymer (Li-ion). Batteries vary according to the voltage, total charge capacity (amp hours), weight, the number of charging cycles before performance degrades, and ability to handle over-voltage charging conditions. The energy costs of operating e-bikes are small, but there can be considerable battery replacement costs. The lifespan of a battery pack varies depending on the type of usage. Shallow discharge/recharge cycles will help extend the overall battery life. Range is a key consideration with e-bikes, and is affected by factors such as motor efficiency, battery capacity, efficiency of the driving electronics, aerodynamics, hills and weight of the bike and rider. Some manufacturers, such as the Canadian BionX or American Vintage Electric Bikes, have the option of using regenerative braking, the motor acts as a generator to slow the bike down prior to the brake pads engaging. This is useful for extending the range and the life of brake pads and wheel rims. There are also experiments using fuel cells. e.g. the PHB. Some experiments have also been undertaken with super capacitors to supplement or replace batteries