I approach this question as I would a boat. Having rewired several boats I have some experience with multiple battery installation and 12V power.You have different "banks" for your power needs. You have a start bank of course and also a "house" bank that you use for your accessory power needs such as winches, inverters, computers, audio, video, whatever. You will isolate those loads from the start load as you want to have the start bank be independent from your other loads so that no matter what you always have a full start battery.
In order to do this, you will want to wire all those items to a separate load system (with common ground of course) In order to charge the house battery, you will need a Yandina Voltage Sensing Relay which will "bleed" excess charging power from the house battery to the start battery. Why primarily charge the house battery? The start battery is only discharged for very short bursts and usually does not get run down appreciably. The house battery on the other hand may become deeply discharged as you use accessories without the engine running. Cycling deep discharges requires high power recharging when back on the alternator .
In addition to the Voltage sensing relay, you may want to consider a "smart" alternator controller (Balmar, Xantrex) which will automatically sense the needs of the battery and adjust alternator output accordingly.
I would suggest for your house bank that you use a marine deep cycle AGM battery such as the Deka Marine Master or the Die Hard Marine AGM. Either a Group 27 or even a 4D would be ideal for powering a winch or overnighting with lights, TV and Wii going full blast. AGMs will mount in any position (even upside down) and tend to last longer than spiral batteries in deep cycle applications.
Be sure to size your cables as big as you can. Minimum cabling for the battery leads will be 2 or larger. Genuinedealz.com sells marine grade wire, cable,connectors, and accessories for clean nautical grade installation. Let me know if you need further assistance.
1966 109 5 door wagon 300Tdi "spermaceti fueled"
1994 RRC LeWiB "ruining the air behind me"
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