The battery energy density is the proportion of energy that can be included in a particular unit (mass or capacity).
The energy density of a battery is generally expressed in two ways :
The gravimetric energy density of a battery is a measure of how much energy a battery
contains in comparison to its weight, and is typically expressed in Watt-hours/kilogram
The volumetric energy density of a battery is a measure of how much energy a battery
contains in comparison to its volume, and is typically expressed in Watt-hours/liter
Figure 1: ENERGY DENSITY COMPARISON
According to the comparison table in Fig. 1, lithium ion (Li-Ion) has the better energy density from the three most popular chemical compositions currently, so therefore Lithium batteries are still dominated by the application of all portable electronic devices.
Although Lithium batteries has advantage on the battery energy; Ni-Battery (Ni-Cd and Ni-MH) has a better linear discharge characteristics, so they are well suited for use with Linear Regulator.
Figure 2 Cell Discharge Curve