I have a 2006 Jeep Wrangler with a stock alternator.

I just built a off road teardrop trailer with its own 12v AGM battery and 12v refrigerator --the Jeep battery is an Optima. Using solenoids, I have it set up to charge either battery separately. When charging the Jeep battery the alternator puts out the required 13.8-14.2 volts +/-, but when I switch to the trailer battery it starts overcharging slowly up ti 18+ Volts. I shut it off so as not to ruin the alternator. I used #4 wire for this installation.

Question is why? If the alternator is internally regulated why the difference? or is the alternator regulated bu the Jeep computer? I am new here but this looks like a great site. Any help would be appreciated.

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    Welcome to the site. Do you have a diagram so us non-Jeep, non-trailer folks can understand what the #4 wire is?
    – Zaid
    Commented Oct 3, 2016 at 17:38
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    Is it the gauge on the dash what is showing the voltage reading going up to 18v or is it what's measured at the battery? The reason I'm asking is, I'm wondering if there is some type of resistance between the alternator and the secondary battery (like the relays) which are causing the issue. It could be the alternator is reading the battery as still being low and so continues to try and rise the voltage to compensate. Check the voltage at both places and at any place where there's a connection (like the relays) and see if you can tell where the difference may lie. Just a theory. Commented Oct 3, 2016 at 18:05
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    @Zaid OP means 4 gauge wire I think.
    – Ben
    Commented Oct 3, 2016 at 20:01
  • I suspect the control circuit that tells the alternator when to charge and when to stop charging is still monitoring the battery in the vehicle, not in the trailer. Commented Oct 4, 2016 at 8:33
  • Do the two systems share a common ground? Voltage is measured relative to ground. The gauge and the battery must have a common ground in order for the gauge to be correct.
    – 3Dave
    Commented Oct 4, 2016 at 14:26

3 Answers 3


My first thought is your ground wire to the trailer is not large enough causing a voltage drop to the rear battery.

However after some research turns out your alternator is controlled by the PCM and is basing it's output on the truck battery which you have disconnected from the alternator. It keeps increasing the alternators output but never sees the batteries voltage increase so it keeps increasing the output which is causing you to see the 18 volt reading.

But it could also be an issue with how you have wired it up. One thing that is particularly concerning to me is that depending on how you have wired it you may be causing a voltage spike in the system when you are switching between batteries, enough of a spike that at least in theory could damage one of the computers on the vehicle.

Please post a wiring diagram of how you have wired this up and where exactly you have tapped into the Jeep system and installed solenoids.

Also I am not sure why you want to charge the batteries separately. If you have a specific reason for this let me know and I will give you another way than the way I am suggesting below.

You're best option IMHO is to use an charging relay, or battery isolator. I don't like using the battery isolator because they have voltage drop across the isolator of around 0.7 volts. Charging relay does not.

The simplest way is to have one relay that comes off of the positive post of the main battery and runs to the rear battery positive post. The relay is activated by ignition power so when the vehicle is off the batteries are not connected together, that way the trailer battery can never run down your vehicle battery. When the vehicle is running (well ignition turned on) the batteries are connected in parallel and the alternator will charge them at the same time.

enter image description here

This is similar to what GM does with Auxiliary batteries in trucks with options for the slide in campers.

Here's the wiring diagram for one of them

enter image description here

You can also use an automatic charging relay that will only allow the trailer battery to be connected to the vehicle when the vehicle is running.


I'm assuming the engine in your Jeep is a 4.0. The voltage regulator is a part of the PCM, this is a PWM system that operates up to 100Hz and can field the alternator at 100% by fully grounding the circuit. The PCM determines how much to field the alternator through a fused B+ line to the PCM.

I think you just need to run a fused wire from the rear battery wire to the PCM. Without seeing how the trailer battery circuit is wired it's hard to say though.

Maybe something like this.

enter image description here

  • Wired this way will always have the batteries connected together through the PCM sense wire bypassing the solenoid and likely blow the fuse. Commented Oct 3, 2016 at 22:14
  • @MoveMoreCommentsLinkToTop You're right you would need a relay for the sense wire.
    – Ben
    Commented Oct 3, 2016 at 22:16
  • That revision will allow the alternator to charge both the trailer battery and the vehicle in the battery when in the trailer battery position, again likely blowing a fuse. Also that increased current flowing through that wire will likely cause enough of a voltage drop to cause the PCM to increase the alternator output beyond a safe level. Commented Oct 3, 2016 at 23:51
  • @MoveMoreCommentsLinkToTop Trying again, I think this works.
    – Ben
    Commented Oct 4, 2016 at 0:53

Without knowing specific details and being unsure to having fully understood your wiring I will just post my guess:

If I understand your question correctly you basically detach the alternator from the primary battery and also detaching it from the engine electrics?

Since now the primary battery alone has to handle all the power requirements (radio, fans, sparks, lights etc.) the voltage will inevitably drop. I assume that the alternator is regulated by the engine ecu, which tries to counter the sinking voltage by raising the alternator output. Since the alternator is disconnected from the engine power line there is no feedback, and the alternator output rises to maximum levels.

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    I concur about the battery isolator. If the trailer battery is a deep cycle battery it could toast the alternator.
    – Old_Fossil
    Commented Oct 4, 2016 at 5:22

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