http://www.expeditionportal.com/for...ke-a-cheap-isolated-dual-battery-setup-for-50
A diagram is worth 3 thousand words.
Wiring a solenoid in is not overly complex, but can certainly appear to be before one does so.
I just like to stress using thicker copper and having these thicker copper cables properly terminated with quality ring terminals, for minimum voltage drop and maximum performance.
Chronic undercharging is the biggest killer of all batteries, that, and the attitude that the all mighty alternator is an instant and totally effective charging source.
The alternator can be very effective bring a depleted battery to 80% charged, but only if thick copper mates alternator to solenoid to house battery. This thicker copper also allows the vehicles voltage regulator to "see" a depleted battery, and allow higher voltages to continue for longer, charging the battery faster, as voltage is electrical pressure. The higher the pressure, the more amps can flow.
The more amps can flow, the faster the battery recharges.
The faster the battery recharges, the closer it returns to full charge
The closer it returns to full charge each drive, the longer the battery will last, and the better it will perform during that lifespan.
On the above link there is a prominent poster. DWH, who knows his stuff, but we disagree on thick copper being necessary.
He is wrong, and obstinately so. You want 4 awg, and you want this 4awg to have good terminations.
properly terminating large cables requires skill and tools. Better the Novice have quality cables professionally made.
Once one figures out the lengths of copper needed:
http://www.genuinedealz.com/custom-cables
These are among the best prices you will find for top quality cabling anywhere. What you spend on these cables, will be saved in terms of buying less batteries in the future.
Finding your battery dead can be stressful, and getting them back to their full remaining capacity is a chore, and rarely occurs, and the battery is replaced shortly after, only to have the next one start suffering the same fate. Batterycide through chronic undercharging.