Kash is right on the voltages. Do not go below 50% if you want to make them last. 20% is a joke. Digital meter is a must. There are wires behind the dash near the useless orange light already for this. However, they are hot all the time and will drain the batteries slowly. You can put a tap on the key switch and use the ground wire in the pair for the other lead. They make taps that slip over the post so you do not have to cut any wires.
BTW, FLA batteries perform better if they are always kept charged. When you park the cart, plug it in. Always.