Energy density batteries

One of the main reasons is the low power density of fuel cells. Power density, defined as power produced per unit weight or unit volume of the power source, is one of the most important factors for a power source for transportation as well as energy density, defined as energy produced per unit weight or volume. As for fuel cells, unlike batteries, energy density would not be such a serious problem as power density because energy density can be increased when more fuel is loaded. 

Peters K, Automotive battery energy density - past, present and fatore, Journal of Power Sources, 33, 1991. 

Other performance requirements and specifications such as efficiency must be stated. The battery energy density and size determine how long a patient can go between battery charges while meeting the requirement. 

The introduction of the li-thionyl chloride primary battery represented a major advance in battery technology. The voltage of a single cell was more than twice that of the Ledanche cell and other primary batteries, energy density was increase by an even higher factor and shelf life was improved by a factor of 5 at least. It became immediately obvious that the next breakthrough would be achieved with the introduction of a rechargeable li battery. This happened about two decades later, with the introduction of the li-ion battery. 

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