Is it possible the 5.5 kw & 7.5kw ratings might be in reference to the motor power? - which is typically stated as kwH (metric system) or, HP in English system. Motors are general rating in terms of power and max ampere draw. Batteries are measured in voltage, max amp per positive plate & number of plates per cell.
Motors consume the most amperage when the are operated under load - like initial start up while driving (it takes very little power to maintain a travel speed or trying to "chisel" the forks under a load or when lifting or tilting or side shift a capacity load.

All batteries have an amp rating at a 5 hour or in N. America 6 hour rating. This the amp rating per positive plate in each battery cell. Batteries of the same physical outside size can & do come in various amp hour ratings - the greater the rating the more expensive the the battery. Sometimes decisions are based on economics vs actual need. Meaning batteries are very expensive these days due to lead prices and more than doubled in price in the last few years.

If you wish to know what the amperage draw is in your operation there are instruments to record it. They are connected between the battery & the truck & records amp draw over your works shift. The name of the device in the is called a "Power Prover" in the US but you local battery dealer should know what it is in your country.