Originally Posted by
Mundy
The reason is due to current draw. For example, a Warn XD 9000 draws between 70A (no load) and 460A (full load). This current draw is way in excess of the alternator capacity and hence draws down on battery capacity. So, firstly, you have to have thick cables located as close to both the alternator and the winch source battery. The main battery is usually more closely wired to the alternator with big cables. To wire up to your auxilliary battery you have to have a starter battery, not a deep cycle battery, and the cables need to be big. Secondly, assuming you have a charging controller between the alternator and the second battery, it would have to have a capacity of over 500A (for the above winch) to enable the current from the alternator to reach the battery. As soon as the controller opened, you'd be drawing from the main battery, too, and the voltage drop would close the controller again, resulting in open and closing of controller. Don't think it would last long.
To do it, you'd have to have an extra wiring circuit and switch enabling you to connect the auxilliary battery to the alternator and isolating both from the main battery. Too complicated, too risky.