dual battery options

[KC2MMI]

Some modern alternators will not start up if there is less than ~10.5V on the battery. This is a safety factor, they say, to make sure you have a professional find out why your battery died. Don't know if your jeep has that problem, one hopes Jeep has a better attitude.

But don't bother with a battery ISOLATOR, that's a set of diodes and that means voltage drops and the alternator voltage sense lead can never be connected properly. Obsolete technology.

The simplest way to do this is with a battery COMBINER, i.e. Yandina (aka West Marine) or Hellroarer or Blue Seas. Also sold as an "echo charger". These all work similarly, when they see 13.3V (some 13. at your primary battery, they connect the secondary battery and allow the normal alternator to charge both batteries in parallel.

That's not a perfect way to charge two batteries, but in reality it works out very well. It is simple, cheap, reliable. The only catch is that both batteries should be of the same type, i.e. both wet acid or both AGM or whatever your choice is, designed to charge by a car alternator at 13.8-14.4 volts. Your deep cycle battery will last longer if you also arrange some other means for better charging, occassionally using a 3-4 stage charger on it or bringing it up to a real 100% charge once in a while. A car alternator never really does that. Be generous, use something like a 4AWG cable for the second battery, and instead of grounding it to the jeep, you may want to run a ground cable back to the same grounding point that the primary battery uses. (Your choice, that's debateable.)

Also use a suitable FUSE on the secondary battery, normal breakers arc over and weld around 3000W and a battery can aesily do that in a dead short. You want a proper ignition-protected fuse at the second battery, and a disconnect switch or other protection where the primary battery cable goes to the combiner. Among other things, so you can disconnect the secondary and ensure "no surprises" if you dop it off for service someplace.

After 8 years of running this way I finally had to replace the secondary battery. That's not bad battery life, it works.

[N6AJR]

While on batteries in general, you need a starting battery to start the car, and usually a deep cycle for long term draw on the radios and accsys. I use something called a constant duty solenoid to control the charging. This works like a ford starter solenoid, but is made to stay on for long times, the ford starter relay will burn out in about 30 minutes of use, ( don't ask how I know !!) They run about 15 bucks from an autoparts store.

You have the big in and out terminals  for the actual charge circuit and then 2 small  connections for the control wire and a ground.  I hook the control wire up to a dash mounted switch,  and then feed the switch from a fused outlet on the Key switch which on get power when the key is on.  so if you for get to turn off the  solenoid, it will automatically disconnect itself when you turn off the key.  How I used it was to switch off and start the car, you can see the main battery charge as the dash gauge drops to  12 v.  then whee you switch in the solenoid, it shows the charge on the second battery.   you need big enough wires to carry the current being used when mobile, and run a good common ground  to both batteries. Put a ground strap from the front battery neg post to the chassis and then a good ground from the neg post on the second battery to the chassis, and a wire from the ground on the radio ( or even a case screw) to ground.  this stops any ground loops .  get some cheap heavy wire from cutting the ends off some jumper cables, or such.

[K3GM]

.....But don't bother with a battery ISOLATOR, that's a set of diodes and that means voltage drops and the alternator voltage sense lead can never be connected properly. Obsolete technology......

The Hellroaring Battery Isolator has a voltage drop of 5 millivolts on a fully charged battery.  I've used mine for several years with great success.  I run an AGM for the the equipment and a traditional flooded cell battery for the the vehicle's electrical system.

[W8JX]

.....But don't bother with a battery ISOLATOR, that's a set of diodes and that means voltage drops and the alternator voltage sense lead can never be connected properly. Obsolete technology......

The Hellroaring Battery Isolator has a voltage drop of 5 millivolts on a fully charged battery.  I've used mine for several years with great success.  I run an AGM for the the equipment and a traditional flooded cell battery for the the vehicle's electrical system.

As I stated earlier you can simply use a 12 volt relay whose coil is wired to ignition power and it closes circuit to charge. You can also install a switch for a manual override or to disable it. Such a relay would not be hard to find as GM  and others use a lot of them in cars these days and can be found cheap in bone yard socket and all and even new relays are only about 10 bucks or so.