Yes, a battery disconnect will stop all outside battery drain. Any battery will lose charge over time even not hooked up, this is much higher in summer temps than winter temps (per JD manual). Many older machines will not leak current and can be left over winter and start fine in the spring. On older stuff, the alternator is the biggest potential leak, I think it's six diodes make up the rectifier in every alternator, any one of those that starts to leak can drain a battery over a week or overnight even, bad leaks could start a fire. So a battery disconnect or removing the ground cable is a good policy, as is charging before starting in the spring, or every couple months over winter, or even keeping a maintainer on every battery.