Battery weight.Weight is a major issue for batteries. Batteries are fine for small “city cars” or vehicles that travel only 30 to 50 miles each day. But for every extra mile of range, additional batteries are required. Need more range? Add extra batteries. Need to add space for another passenger or room for the golf clubs? Add more batteries.
Actually, the relationship between weight (technically mass) and range is worse than one-to-one. For each extra kilogram of battery mass, the vehicle mass grows due to:
- Added support structure for the larger battery
- Slightly larger motor capacity to accelerate the extra mass
- Slightly larger brakes to stop the heavier vehicle
- Slightly larger suspension system to provide a smooth ride
- And still more batteries to propel all the above incremental mass in a nonlinear feedback loop, a process called “weight compounding” or “mass compounding” Each extra kilogram of battery requires the addition of much more than one kilogram of total car mass.
Fuel cell weight. A fuel cell power train, on the other hand, requires only more hydrogen to go an extra mile. The fuel cell size doesn’t change. Hydrogen, the lightest element in the universe, was used as the fuel for the Space Shuttle main engines because it has more specific energy (basically energy per unit mass) than any other fuel. Adding more hydrogen to a FCEV does not appreciably increase vehicle mass as shown in a graph below (the hydrogen tank to hold more hydrogen does become larger, but the extra mass for a large tank is far less than the extra mass for a battery to go the same distance.)
Specific energy. The useful specific energy of various batteries and two fuel cell systems are shown in this graph:
By “useful” we mean only the energy stored in either the battery or the hydrogen tank that can be used to power the electric motor. For example, a battery might store 200 Wh of energy per kilogram of mass, but if that battery can only be discharged over a 70% state of charge range without degrading battery lifetime, then the useful specific energy would be only 140 Wh/kg. In addition, the mass in the denominator of the specific energy calculation must include the entire battery system, such as all support structure, control electronics and the battery cooling system.
Similarly for the fuel cell system, the useful specific energy includes only the hydrogen that can be extracted from the hydrogen tank, and the mass must cover the entire fuel cell system including the hydrogen tank, the peak power battery and the fuel cell system itself plus all its sub-components such as electronic controls, cooling, support structure and plumbing hardware.
As shown in the graph above, an advanced lithium ion battery is projected to have a useful specific energy of 150 Wh/kg, far better than the lead acid or even nickel metal hydride batteries used in some gasoline hybrids. But a fuel cell system with 35 MPa (350 bar or 5,000 psi) hydrogen tanks would have over 3.6 times greater specific energy than an advanced Li-ion battery system. This means that FCEVs will weigh much less than BEVs for a given travel range
Vehicle test mass. This graph shows the dramatic impact of mass compounding for battery EVs if car makers attempted to build longer range BEVs. These BEVs are all based on a lightweight Mercury Sable mid-size passenger vehicle (the mass of this special vehicle was reduced using aluminum in place of steel to provide the best possible range for a BEV). As shown, the vehicle mass grows dramatically for lead acid battery EVs with ranges greater than 50 to 80 miles. EVs with nickel metal hydride (NiMH) batteries might travel 120 to 180 miles with potentially manageable weight penalties (still twice the weight of a FCEV), while advanced lithium-ion batteries might travel up to 300 miles, but with much greater weight than a FCEV. This figure includes the test mass for the Nissan Leaf midsize sedan, which is based on a steel body Nissan Versa glider that is heavier than the aluminum body used for the other vehicles on this chart. (The Nissan Leaf battery specific energy is only 80 Wh/kg [660 lb /300kg battery for 24 kWh storage], vs. the 150 Wh/kg used for the other vehicles based on expected future Li-Ion battery improvements.) [Note: the calculation behind the above graph assumes that the BEV is designed according to modern automotive engineering standards. Thus a startup company or hobbyist might cram enough batteries in a sedan to provide larger ranges than shown in the chart, but if they properly sized the vehicle components to match engineering practice for a heavy vehicle, then that properly designed vehicle would weigh approximately the values shown above; for example, one could add more batteries to the Nissan Leaf to increase its range, without beefing up the brakes or the suspension system to provide safe and comfortable vehicle ride performance.
Note that the fuel cell EV mass is nearly constant with range. The hydrogen tank has to be slightly larger and heavier as the range increases. Otherwise the vehicle mass is unchanged. At the 300 to 350 miles range expected by most US drivers, even an advanced Li-ion battery EV might weigh twice as much as a FCEV. This extra mass would then require extra stored electrical energy for the BEV to travel a given distance, cutting down its efficiency advantage relative to the FCEV. The BEV by itself would still be slightly more efficient than the FCEV, but the total well-to-wheels energy utilization including the efficiencies of electricity and hydrogen production strongly favor the FCEV.