INTP:
Answer is, don't. Just buy a premade battery monitor. Your use should have been monitored this whole time. How far were the batteries discharged? Even 'deep cycle' batteries should stick to about 50% discharge, even though they can withstand 80%, depending on battery and system. I can't even make sense of how you have your batteries hooked up. It sounds like you have series pairs of batteries that are then paralleled. What's your charging voltage? Is your system 24V?Please explain what it is you want the Arduino to do. Because if the bank is discharging unevenly and they're connected in parallel, then there is nothing for you to do short of disconnecting the faulty bank when it gets lower than the other, and basically just run on half of your batteries and making them last half as long. And if that was your intention, you need a relay that can handle that wattage. 'Pull a load on the blah blah blah' You're going to need to spell that out because I have no idea what that bit means.
The short of it is, you are trying to fix something before you have even bothered to understand the problem. That often leads to unnecessary effort and money, which I don't think you're too eager to spend much of either.
All it may take, as a sensible first step, is to stop by an AutoZone or some such shop that offers free battery health checks. Since you have so many, tip the poor guy. Find out which battery isn't up to snuff. Then come back here and stop ignoring the advice given.
Why not peruse the whole post. Say to #8 where the diagram is?
And #1 where he says what he wants the Arduino to do!
So in short, poster asks for how to hook up an Arduino to monitor the batteries at the mid and end point and how they go about programming it and you berate him for not reading (which you dont seem to have done) and not understanding the problem which was the reason this started.
Does not seem helpfull.
Daz