I don't know how to add a battery to my cyber deck

Hello so me and my brother are making a cyber deck and we scored something called a endat 3857 and requires a 9V ~ 24V DC to work

My question is how do we add a battery so we can work with it like a laptop. Also how do we calculate how much we need for all the power and how do we get a big battery that last us max 4-5 hours ?