Question: I'm shopping for batteries and it's easy to find a 48V lithium ion battery that has like 30A max continuous discharge rating. However, I know that the battery supply has a variable voltage from 54V - 42V depending on how much energy has been drained.

I'm using a 48V to 24V buck converter to power many of my electronics for a stable 24V power supply. However, I still have some 48V motors that takes 10A max for my robotics application. I can't figure out a way to power my 48V motors with a stable power supply.

I found a 36V to 48V step up converter but it only outputs 5A.

Answer: 
The DH method is ambigous. There are multiple solutions which are correct and leat to the same TCP pose.

Try to make a minumum amount of changes in the coordinate systems with the transition. If you can choose the x axis pointing in the same direction as the previous one then do so. If not, you can try to anticipate how the next X axis should be or how the TCP X direction will be and try to match that. In other words, as a rule of thumb, the more 0-s in the DH table the better.
You should use right-handed coordinate system, that means if you curl your palm/fingers of your right hand from +x to +y, then your thumb direction shows the +z direction. 

For DH you have to follow these rules ![Case-1](https://i.stack.imgur.com/Qt0UO.png), ![Case-2](https://i.stack.imgur.com/TH1kF.png) ![Case-3](https://i.stack.imgur.com/rofiP.png)