xj4fun

Yep.. Many folks do . The reasons I choose not to do this were:

1) Two differing battery types (red and yellow top).

2) Even if the batteries were identical, they would fight each other constantly to balance SOC (state of charge). This could result in shortened battery life of both.

3) No way to be assured of reserve power when you need it. You are using 100% of your capacity all the time. Lets say both batteries are 5 years old and (combined) have enough power to start your rig. One of those batteries finally dies, which now mean even with 2 batteries, you can't start your rig.

4) You need your winch in a high draw situation. But your two batteries (combined) don't have the reserve.
As I mentioned my system still has a flaw in the sense there is no way to determine the SOC of the secondary battery. By connecting a battery charger a few times a year to the secondary, I can get a reading on the SOC of that battery. Not ideal, but at least provides a general idea of the reliability of the secondary.

I'm looking into http://www.boatersworld.com/product/196901425.htm as this may be a good solution. Just wish the size was a bit smaller.

Hope this helps
Kent

Mattpar

dual batteries would be nice. Thats another mod to do in the future. But for now I carry around a jump pack cause its nice to have on the trail incase someone needs a jump, or if I need extra amps when winching i just hook it onto the battery.

Daniel_Sach

This is great and all, but I still have no idea how to setup a dual battery setup... somebody please help...

4.3L XJ

I have done this too, also with a red and yellow top. However I simplified the wiring by hooking the two negative terminals together and isolating the two positive terminals with a continuous duty 250 amp relay (read starter solenoid). When the key is on, the batteries are hooked together and charge. When the key is off, I can run my winch or power inverter without running down the primary battery. It is painless to use. If the primary battery runs down, a jumper wire with a couple of alligator clips activates the solenoid and enables me to start. Someday I will wire in a switch there for that.

thelaststarfighter13

Can someone explain in more detail the point of isolating the batteries. I have 8 years experience as a military mechanic (tanks mostly) and have seen systems on combat vehicles with up to 8 batteries in series/parallel. There is only one vehicle that has a battery bank that is charged the same time as the rest of the batteries and only used when it is engaged (too start after hard use of the main batteries). From my experience I've had batteries get old only once during a 14 month combat tour and it wasn't all 6. We test the batteries regularly and the recycled, refurbished crap we get hold up well hooked in parrallel.

On the other hand, the army doesn't always do things the best way, usually the cheapest/most effective while saving cost/easiest to use maintain. Can someone please go into depth why you would spend the extra time to isolate the batteries.

4.3L XJ

There can be several reasons for dual batteries, hooked in parallel. The military hooks them in series to boost voltage. We will set that aside. The biggest reason to have isolated batteries is if you need to winch and the motor is not running. Let's say it stalled while fording and you need to get it out so you can dry it out and then start it. A winch will draw more current than a starter, so by the time you get out of the pickle you are in there is not enough current left to turn the motor over. So if you have dual batteries and can isolate them and dedicate the to specific purposes, then you can winch out on one battery and it doesn't matter if you run it down because the one hooked up to the starter is still fresh. Another reason you can run an inverter off one and let it run down and use the same scenario. I have an apartment sized refrigerator I put in the back of mine. Another safety feature is if you leave your lights on or something and your batteries are isolated, if you have it set up properly you can cut the other battery in, start it up and drive away. But the trick is that to accomplish this you need to isolate them. To do this, you need a relay, which in this case would be a heavy duty starter solenoid.

When you hook them in parallel by engaging the relay, then you have twice as much battery. If you have some heavy duty winching to do, it is possible to draw down a single battery because you can draw 400 amps with the winch and your alternator isn't going to be worth more than 100 charging amps unless you have a power welder or some other after market super alternator. The dual batteries can leave yo enough reserve to restart if you stall the engine.

thelaststarfighter13

yeah, all military vehicles are 24v but most have more then 2 batteries most have 4 or more.

I think the idea of isolating the batteries is very interesting I just wanted to see if the added isolation system which is much more complicated then just hooking them in parrallel was worth it. Obviously so! I have never used a winch on any of the hummv's that I have ever seen stuck. Most of them just pull each other out, or call recovery (thats me).

The battery hooked up normally obviously is charged all the time, do you hook a relay up on the ground side of the second battery removing it from the starting/charging system? It would still be hooked to your winch so it would run that system but by removing its ground to the engine/body it would isolate it from the starting/charging system, correct?

4.3L XJ

The hummers have a hydraulic winch don't they?

Anyway, the way I hooked mine up is with dual cable battery terminals available at NAPA. On the engine battery, one cable goes to the starter relay and the ground for it goes to the engine. The other ground cable goes to the ground of the second battery so that the grounds are hooked in parallel all the time. The other positive cable of the engine battery goes to a 250 amp continuous duty (this is important) relay between the batteries. Then the positive cable out the other side of the relay goes to the second battery positive terminal. Its cables go two places. The top posts go to the winch. The side posts go to the 1750 watt inverter. The relay on the positive cable between batteries is what isolates them. I have it set up so that as long as the ignition is on, the batteries are in parallel and will charge. When the ignition is off, the batteries are isolated. One other note. Heavy duty relays pull a fair amount of power to engage them, so I run another relay to run the relay so the additional current draw on the ignition switch is less than a tenth of an amp.

thelaststarfighter13

the 1114 has a winch that is run off of the power steering pump. Our battalion also had some electric winches that they bought seperate, along with go lights and tons of other JUNK that they wanted installed. All 12 volt, I had to design some electrical systems to run it all and they wouldn't let me add any batteries.

OK I got it now, its a normally OPEN relay, then you flip a switch to run the batteries together, You think it would be better to run a normally CLOSED relay then flip the switch to isolate? I guess that would depend on whether you mostly run it isolated, or not.

4.3L XJ

You got it. But for our purposes, to protect yourself from running out of battery when the motor is not running, you would use a normally open relay. Sorry I didn't state that. I should know better.

Knlin515

Im not an expert but to fix that could you just wire a volt gauge like some Cherokees have to keep an eye on the voltage of the second battery?

TXquad

Has anyone had any experience with one of these "Battery Docotors"? Is there any reason not to go with one of these vs the manual switch?

http://www.ebay.com/itm/Wirthco-Battery-Doctor-20090-12V-75A-Fully-Automatic-Dual-Battery-Isolator-/371054827657?pt=Motors_Car_Truck_Parts_Accessories&hash=item5664975889&vxp=mtr
Sorry if this is a noob question.