dual battery proposal, need help


here's the short story.
2004 Jeep GC, 4.7L v8. 136A Alternator.
Currently has autocraft gold battery for main.
Tripp-Lite 1000w inverter
Got it "used" from a buddy who killed his battery with it on first use and sold to me for $75
Have a 15W solar panel from work that was surplus.
Have a large grp 31 marine deep cycle battery also from work that was surplus.

Here's my problem
I'm familiar with boat power, but not so much with the intricacies of auto power.
In the boat, we have our starting battery which is connected to the starter and the alternator and that's it. There's another battery which is the deep cycle and is connected to everything else, so that is either from a second alternator, ideal, or is controlled via switch. It's a dumb switch, so 1, 2, both, off.

Potential solutions
1. Run a marine switch and only have the inverter on the marine battery, and switch to both to charge if the engine is running while the inverter is running. Have a solar charger for topping off and a shore charger if power is available
2. use a solar charger, and have a switch on the input to switch between a constant 12v line, and the solar line. Solar charger should handle the constant 12v from the car with no problems. Correct me if I'm wrong. Would charge slow, but that's better for the batteries anyway, and this is relatively limited use anyway.
3. run it like the boats. Have a diode set up to where the starter battery is connected to the starter and alternator, and everything else is run by the deep cycle battery. Diode would prevent batteries from draining each other, and having the deep cycle would essentially eliminate the potential of dead battery from leaving lights on etc. Would put priority on starter battery for charging. I would really prefer to run this type of setup, but wanted to make sure that the vehicle wouldn't go ape shi!t not having the starting battery drop voltage at ignition. I.e. the starting components other than the starter are designed to run at 7v or whatever since the voltage on the battery goes way down on starting. If this setup is possible, I'm going to put the deep cycle battery up front, and run the starter lines to the spare tire compartment with the starting battery.

To make matters worse, I would prefer to have a solar/shore setup to charge the aux battery when camping and what not, as well as keep everything topped off if it's sitting for a while. Originally this was all going to be mounted in my trailer, but realized that the timeline for the trailer is much longer, and having the system in the jeep is better suited for normal use.

Call me crazy, and help me pick which one to go with. I don't want one of the automatic dual battery setups with the solenoids and what not.
Thanks guys!

diagram for #3 would be like this
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It seems that most people don't run diode based battery isolators any more. I'm using a Voltage Sensitive Relay (VRS) to do what you're describing. They are also called things like "intelligent solenoid", basically a device that connects the main and aux batteries once the main has recovered to full charge from the alternator. Then the alternator charges the aux battery.

Take a look at the following wiring diagram, but I think you could eliminate the switch that isolates the main battery in your case.

Connect your solar charge controller to the aux battery.

One last thing, watch out for the inverter. They can draw power even if nothing is connected to them when turned on (standby current).

Hope that helps...
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the inverter is one of the nice ones with a remote switch, so it'll stay off unless I need it, but I was seriously considering using the deep cycle to run the car and just have the main as the starter.


I am utilizing a 1/2/both/off battery switch in my Van. (make before break type)

I use the stud on the Battery switch for the second battery as a buss stud. From this stud I have a short length of 6 awg cable to a 10 bay ATC fuse block. This fuse block then distributes power to everything I do not want directly connected to the engine battery.

I start the engine on battery 1 (engine battery), then turn switch to both. Then all batteries are being charged. Sometimes I turn the switch to just 2( house batteries) so that no alternator current goes to, or comes from the starter battery. This can allow higher engine/ alternator rpms to produce higher amps into the 2 depleted house batteries compared to having the nearly fully charged starting battery in the alternator charging loop.

When I park overnight, I turn the switch back to either 1 or 2 to effectively isolate the engine battery. Usually 1. By the next morning, the only thing that used any power from my starting battery, is the engine computer memory.

Not fully automatic, but I like dictating where my alternator amps go, and which battery bank my loads come from.

I've been using this system for nearly 10 years now.
I've never run down my engine starter battery but obviously this method is not for everyone.

If I built a system for someone else, i would use a HD solenoid activated by the blower motor circuit for total automatic
combination/ separation of the battery banks.


well, here's a nice situation to be in. I just acquired 50 feet of 1/0 welding cable for free. Advantage of having buddies who work with the stuff. Apparently there was a nick on the shielding somewhere in the middle and work said it had to be trashed. So he snagged it for me. Very nice.
Anywho, with this in mind, here's the plans.
Rear mount two batteries in the spare tire well. Not sure what to do with the front of the engine compartment though. Jeep is total 15 feet long, so I'm planning on 40 feet of the cable to be used to run the positive and negative wires up and back for the primary battery, then run the aux battery with the remainder. Will use a 3 bank battery isolator.
Basically that.
No need for emergency jump start option, and if I need to jump myself I always carry jumper cables and can run those in the back if need be. Obviously avoiding this is ideal. I was originally going to go for a two battery option, but here's why I'm thinking not.

I'm building an expedition trailer which will have it's own independent battery system. I was originally going to trickle charge it through the 7pin connector, but with the extra wire, I may run a full lead from the isolator and that way it can get a full charge quickly from the vehicle when it's running. Would mean a second connector, but it may be worth it.

Now to find battery terminals for this size wire...

don't think i'm going to have to worry about overheating due to current though :-D
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Tail-End Charlie
Diode isolators drop the voltage to the secondary battery, usually by around a half a volt or so. Half a volt low = aux battery never fully charged.

Bad news: 15w of solar ain't jack. That's not enough to qualify as a charger - at best it's a maintainer.
15 watts / 12 volts = 1.25 amps.
Figure when the panel is hot you'll be lucky to get even a full amp out of it. Then figure the battery is only 80% efficient. Then figure the voltage mis-match from a PV that puts out say 17v or so into a battery at 12v (or less). If you actually get more than 1/2 amp per hour into that battery then you've been blessed by the electro-gods and have a miracle on your hands.

Using a solar charge controller to take the alternator/voltage regulator output and feed it to the aux battery might be okay (Morningstar, for one, allows their charge controllers to be bench tested using a regulated power supply). Depends on the charge controller and how it affects the operation of the voltage regulator.

If it's just a straight-through PWM controller, then you won't see any benefit from using it since all it will do is keep the primary and secondary tied together until the voltage of the secondary rises enough to be fully charged - which is exactly what a dumb solenoid and voltage regulator will do anyway. The main purpose of a solar charge controller is to keep the battery from being overcharged...which isn't going to happen unless the alternator is somehow fooled into keeping the voltage too high for too long...which would end up overcharging the primary battery as well and the solar charge controller won't protect that battery anyway.

I'd recommend: Run a dumb solenoid energized from the ignition to tie the batteries when the engine is running (that's what I do) and then hook the solar charge controller to the aux battery.

That is, hook the solar charge controller to the aux battery *after* you get a real solar panel that can actually charge the aux battery in a reasonable time.


ok. The solar was just something that I have laying around, I knew it would be slow at best, but better than nothing.

Thinking more and more about the ACR's. Just keep seeing everything with the emergency jump start thing, and I know I won't need that. If done properly it shouldn't happen, and if it does happen, everyone should have jumper cables anyway, so good to go.
Thoughts on an ACR instead? I know it would likely give me the jump start option, but it isn't a selling point. I really just don't want to have two different batteries directly connected, especially when they are both very very different. Hence the isolator with the one way flow
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Tail-End Charlie
ACRs are handy, but all they are is a little computer brain which energizes/de-energizes a dumb solenoid. Some will wait until the voltage on the primary side reaches a set point, to get some juice into the cranking battery before tying in the secondary. Some will energize the solenoid whenever the voltage on either side rises to a "okay, there's some charging happening here" point.

As for jumping - I've done it a few times; Both my batteries are under the hood, so I've just used my cables.

But, if you have a dumb solenoid which comes on with the ignition (never wire it to ACC), it's ultra simple to rig a momentary switch to power it from the aux if you wanted a self-jump capability. Though your primary battery would have to be pretty much totally flat to ever need it - even a little charge in the primary will be enough to energize the electromagnet in the solenoid and as soon as it lights up, the batteries are tied and it's being powered by the aux anyway.

Connecting two very different lead-acid batteries *during charging* is not a bad thing IF the charger is a "constant voltage" type charger (alternator/voltage regulator setups are) AND the batteries require roughly the same charge voltage. Flooded and AGM batteries do require roughly the same, but GELs require a slightly lower charge voltage so you never mix them with anything other than other GELs.

It's only when the batteries are rigged in series or in a permanent parallel bank (or when using a multi-stage charger), that they really should be identical.

I run a normal cranking battery and a larger deep cycle with a dumb solenoid. The only real problem is that if you run the aux down and don't drive enough (takes a LONG time to fully recharge from a normal alternator/voltage regulator setup), which I do all the time - then you WILL need a shore power charger. I like to park in the shade and stay a few days or a week at a time, so instead of a solar panel, I tote along a small generator and a charger and top off the aux battery with that.


i have the same rig as you, i went with dual sears platinum's and the blue sea ACR for my setup...

the blue sea for me had the right price, great specs, a seemingly reliable brand behind it, and just the right mix of "smarts" and low-tech latching relay to make sense to me. so far it's worked rock solid, the ACR switches on and off very reliably. i haven't had to self-jump a completely drained main yet, but i have had a sort of slow crankover after a long hot day with the fridge on and so i flipped the switch to manual combine just to see, and sure enough it would crank like normal when combined.

if you want to add in the solar, you'll of course need some sort of charge manager too.


I too run dual Sears Platinum marine batteries and the Blue Sea ACR. It's simple, effective and priced right for the capability you get (2500 amp surge current with properly sized conductors).


yeah, with the freebie golf cart batteries and what not I'm thinking this is going to be a removable power pack setup. Have the charging option from the alternator, but be able to move it to the trailer or wherever if I need to or want to. The acr seems like the best solution. Run the welding cable inside the frame rails and put the bungs on the spare tire carrier to leave it in there methinks.
What has me though is the spare battery is going to be removable so I definitely want the switch.

Definitely leaving the main battery in the engine compartment, no sense in removing it with my limited welding experience and what not. Trailer has a scuba compressor if I really need some serious air, and I've always got scuba tanks in the jeep for tire inflation etc.

Running the welding cable from wherever the switch ends up through the drive side frame rail and all the way back. Any reason not to leave it inside the frame rail? Saw Kristoffer ran his along the frame, but if I can leave it in the frame I'd rather do that. Run the wire thru the spare compartment and have some sort of quick disconnect system to remove the aux battery setup. Quick connectors would have to include the main power wire, as well as the power wires run to the jeep. Thinking about 2 AC plugs up in the passenger compartment. Likely run the same location as the E brake wire and have them coming out of the center console, and two up the back on the rear wheel well. Switch for the inverter which is a LAN line, and will be run to a switch panel up front. I'm thinking I can probably get away with a custom 12plug molex type connector for all of the power and the switch, then an anderson connector for the battery cable.
Can't get over the cost of the anderson connectors though. It's stupid. $10/side for a piece of injection molded plastic and a metal barb is nuts.
Looking for high amperage plugs. Would rather a plug and receptacle than a connector, but beggars can't be choosers.

Fuse blocks inside of the battery housing, and up at the relay. obviously.

Using golf cart batteries means that I have two batteries, but that'll be better in the long run.
Other option with this is since the batteries are massive, is to run a plug to the back of the vehicle *other reason for not choosing an anderson connector, because that'll look like crap on the bumper*, and just hook the trailer up to that for any power needs, and put a smaller deep cycle battery in the trailer for when the Jeep is gone and power is still needed. Would have a Marine battery switch on the trailer and would be able to chose between batteries that way, and leave them together for charging.
Anyone have any suggestions for shore chargers? Not seeing a lot of info on that. Would opt for an inverter/charger in the trailer, but that's $$$
Will see if that is still 50% off when I end up getting to that point
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