One of the advantages of a hybrid powertrain is its ability to recapture energy from the wheels as the vehicle slows down—and to store and reuse the energy when the vehicle needs to accelerate again. There are many ways to accomplish the recovery of energy, each with its own advantages and disadvantages. Depending on the type of system, energy can be recovered in electrical form, electro-chemical form, electro-mechanical form, or compressed media form such as pneumatics and hydraulics.

The storage of energy in electrical form can be accomplished using a technology called capacitors, where the energy is stored as a charge of electrons on metal plates separated by a nonconductive space such as air or a dielectric. These plates hold the electrons until power is needed by the electric motors in the car.

Quick Burst of Power

Special capacitors called ultracapacitors—or ultracaps—quickly store and release large amounts of power. This feature makes them ideal for recovering large power spikes that can occur from heavy braking. They also offer quick high bursts of power to accelerate the vehicle.

Ultracaps have disadvantages too. They are unable to store large amounts of energy. Even though they can absorb and release lots of power, they cannot sustain that output for very long. Like a small balloon that is filled quickly when you blow into it, and goes flying everywhere when you let it go, it doesn’t take long before the small balloon—and an ultracapacitor—is empty.

Batteries Can Store More Energy

Ultra Caps are very different from batteries. A battery is the storage of energy in electrical-chemical form. Batteries use a chemical reaction to absorb energy and store the electrical charge in a chemical form. Storing electrons in a chemical media adds resistance, delay, and heat generation during its process—which, in turn, slows down the rate at which a battery can absorb energy. Thus, a battery is not able to quickly store power like capacitor. But since the charge is stored in a chemical media instead of charged particles on plates, a battery can store larger amounts of energy.

If you push a battery too far, it can overheat. Therefore, there are limits to how much power a battery can handle when charging. Think of a battery as a huge tank with a thin hose connected to it. The tank can hold a lot of water, but filling and draining it takes a long time compared to that small air balloon, which can fill and drain quickly.

Hybrid Within a Hybrid

Both systems of storage have their advantages and disadvantages. Marrying the two could provide a robust system that can recapture energy quickly—and store large amounts of energy for use latter. Of course, doubling up on the systems adds cost. Batteries and capacitors have both proven to be reliable technologies, and both are scaleable to meet the size and capacity requirements of hybrid cars and trucks.