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Old 03-05-2016, 12:50 PM   #1
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Did charging dead RV batteries driving kill my Jeep Alternator?

Question first, then the scenarior.

Can charging 2 6V batteries while driving kill an alternator and battery?

I have two 6V batteries. I left them at the RV repair to check/charge (they were dead)...and they didn't get around to it. They told me I could charge them up while driving 8 hours from San Diego to Phoenix. I drive a 2007 Jeep 3.7L, 6cyl, 2 WD.

About 4 hours into the drive, the red battery light when on, on the dash. I didn't do anything about it (face palm).

I parked in Phoenix, then used the car for about another 60 miles, not pulling the trailer for the next two days. The battery light was on 1/2 the time.

The car ended up malfunctioning (eg wouldn't start or drive well and had lights flashing intermittently, and gave a U?141 code on the ODB), so I had the dealer check it out. They diagnosed need a new alternator and battery. The alternator is original (175K on the car), the battery is 1.5 years old.

So here is my question: can charging those 6V while driving kill an alternator and battery?

Thanks! Love the answers on this forum.

Erica from San Diego

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Old 03-05-2016, 12:59 PM   #2
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As long at you connected the batteries in series to make 12 volts it should be OK (unless batteries are done/damaged). If you charged the batteries in parallel (6 volts) then yes it would damage your alternator and Jeep battery...

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Old 03-05-2016, 01:13 PM   #3
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They were in series. One of the batteries could likely have been charged up. The other the service center had partially charged up. Thanks for that info!
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Old 03-05-2016, 01:23 PM   #4
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You simply overloaded the alt/charging system by asking it to handle 3 batteries two of which needed charging BEFORE you left.

Look at the alternators max output.
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Old 03-05-2016, 01:34 PM   #5
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With 175,000 miles, could have just been coincidence or a combination.

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Old 03-06-2016, 11:57 AM   #6
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I vote for coincidence. The size of the wire going from the alternator to the trailer batteries limits the current flow. I don't think the wire size would allow enough amps to overload the alternator. The regulator should limit the alternator to its maximum rating.

Even with the alternator maxed out for hours, it should still be okay. It's designed to do that. It's possible that the diodes or brushes were getting to the end of their respective lives and failed due to the heavy load.
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Old 03-06-2016, 12:17 PM   #7
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To late now but anytime you have a/2 low/no charged batteries it is always best to give them (time considered) as much of a charge as possible prior to connecting to a charging system.
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Old 03-06-2016, 12:34 PM   #8
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Best case: coincidence; Worst case:it pushed an about to fail alternator to a slightly earlier demise.
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Old 03-06-2016, 12:41 PM   #9
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Damaging the diodes in an alternator is possible if you use jumper cables to jump a very dead/shorted battery with the vehicle running, however it is unlikely you could damage the alternator charging the RV battery via the aux line for two reasons.
1st. The aux pin in the Bargman connector goes through a fuse in the fuse block which would blow before you damaged the diodes.
2nd As mentioned above, the wiring between the fuse block and the battery is not heavy enough ( as opposed to jumper cables) to carry sufficient current to damage the diodes.

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Old 03-06-2016, 02:35 PM   #10
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Thanks, all! I really enjoy learning the details. After getting Jeep Service center to look at (thought it could be control module), and they quoted $1250 for alternator and battery, I took to a local shop and got it all done for one hour labor cost + 25% markup on parts prices. On the road again.

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