long leads for batteries, how to minimize negative effects
The issues with long wires is added inductance and added resistance.

First off, resistance: easier to work out the effect of this. The datasheet for the wire will tell you the resistance per metre (or at least the information to work it out). You know the current draw of your system. Applying V=IR will tell you the volt drop seen at your system end, choose a wire where this drop is low enough for you. Don't forget you'll also have the wires getting hotter if it is a big drop, so will need to factor that in.

Secondly, the added inductance: this is only an issue if you have a large switching load drawing gulps of current. In this case, it is probably best for you to simulate your system (using LTSpice or similar) and use that to see how the difference inductances of different wires will effect the power available to your system. You may find this site useful to see the inductive value of your wires.

(All this assumes that you are doing this system at home for fun, and not making a saleable product with all the regulations etc that is required for one of them).