Energy density. Everyone knows that batteries are heavy.  Many do not realize that batteries also take up lots of space...they have low “energy density” or stored energy per unit volume. 

Conversely, many commentators are concerned about the space required for compressed hydrogen storage tanks, the preferred method of storing hydrogen on motor vehicles.    But the useful energy density of hydrogen tanks at 35 MPa (350 bar or 5,000 psi) including the entire fuel cell system plus a peak power battery bank is greater than that of the older batteries, and slightly greater than that of the advanced Li-ion battery; several car companies are now using 70 MPa hydrogen tanks, which have an even greater advantage over advanced Li-Ion batteries.

Vehicle storage volume. While the energy density of the advanced Li-ion battery system is almost as high as that of the fuel cell system at 35 MPa, the battery system weighs much more as discussed on the EV Weight page. Therefore, for a given range requirement, the BEV will require more stored energy on board to accelerate the heavier vehicle. As a result, even a BEV with advanced Li-ion batteries will require more space on the vehicle than the combination of hydrogen tanks, the fuel cell system and the peak power battery bank for the FCEV ===>

These curves are all for a lightweight Mercury Sable (aluminum body) vehicle. For 100 miles range, the battery bank would take up about the same space as the hydrogen storage tanks and fuel cell system.  At 300 to 350 miles range, however, even the advanced Li-ion battery system would require up to twice the space on the vehicle as the hydrogen tanks, peak power battery bank and fuel cell system.