What is Energy Density?

 It means the amount of energy that can be stored in a given Battery / substance / system/ a region of space.

For eg:

A simple analogy is the amount of water a water Bottle can hold is the capacity of that water bottle and the amount of density can change depends on the type of water (Hard or soft).

Same way, Energy Density of a battery Pack can be calculated as  V*Ah/Kg = Wh/Kg.

V = Nominal Battery Voltage
AH = Rated Battery Capacity
Kg = Battery Weigh