coach battery question

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New member
May 29, 2019
Hey Everyone!

So over the last week or two my coach battery has slowly gotten worse and doesn't hold much of a charge anymore. Which is to be expected since it's 6 years old and I've been using it a lot lately. :)

This weekend I bought a new AGM battery and hooked it up. My interior panel showed 3/4 charge and I walked away, thinking that was that. Then yesterday when I went back to the coach for the first time since I hooked it up, it was DEAD. Not a flicker from an interior light. It occurred to me when I hooked it up that I put it in the wrong way. The coach builder, in their helpful wisdom, decided it wasn't necessary to mark the leads. Here is what I've got:

- Three black cables that go to one post.
- A single white cable that goes to the other post.

With only that to go by, which would you guess is positive, and which negative?

Yes, I could take the battery out and flip it around, but (a) that's hard. It weighs 50 lbs. and you lose finger skin getting it in the box, and (b) if it's in the right way and I messed something up, I don't want to make it worse by putting it in the wrong way. Yeah, I checked all the circuit breakers and nothing got tripped. Also, I drove the rig 15 miles after I first hooked it up and still got zero power to the smallest light. I freaked out and just unhooked the battery.

(If it's in the wrong way, that doesn't solve the mystery as to why the inside power meter suggested the battery was 3/4 full when I originally hooked it up. Unless a fresh battery can just do that, even if the polarity's reversed.)

Thanks in advance for any/all opinions!



Well-known member
Sep 14, 2009
Cedar City, UT
When you buy a new battery it is fully charged,( or should be) that's what you were reading.. Then you killed it from there...Remove the battery and charge it fully ,Figure out what the wires are by choosing one and trace it to a frame grounding point, that's half the battle.. Now place the battery in the tray so the ground strap will reach the negative post.. The rest should be obvious.. (If not you need help) >>>Dan


Well-known member
Nov 17, 2018
Albuquerque, NM
There are a lot of unknowns here.

Unhooked, does the battery show any voltage with a voltmeter?  "Everything dead" could be a dead battery, or a popped main breaker or fuse.  Or even a mis-tightened cable termination.

Usually when presented with black and white wires in RV, white is negative and black is positive.  But you should prove that by ohming each to chassis ground and see which one is a dead short.

Fall back and regroup.  Make believe you're starting from ground zero and reaccomplish the installation, testing as you go.

Mark B.
Albuquerque, NM


Arch Hoagland

Well-known member
Jul 11, 2014
Clovis California
I would assume the three wires went to the positive terminal. 

However I myself wouldn't connect them until I did a resistance check on the wires. I would measure the resistance to ground on the single cable and also on all the other cables.

There should be zero resistance to ground on the negative cable.

What year make and model RV do you have?

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Well-known member
May 7, 2015
Western Kentucky
Welcome to the Forum!

Your battery cables - one goes to the ground - chassis - nearest metal part and is the negative.  One (probably with 3 wires) goes to the fuse box, levelers, slide outs, etc.  Check ohms between ground and each battery cable.  The zero ohm terminal is ground.

Before reattaching any cables, read the battery voltage.  A fully charged battery should read 12.6V.  A reading of 12.0V is a nearly depleted battery. 

Reattach the cables.  Do interior lights work?  Plug into shore power if possible.  Check voltage at the battery.  Do interior lights work?
If the converter is working. you should have interior lights when on shore power.  If the battery reads 13.6V or higher, then the converter is sending power to the battery.  If the battery reading is the same as before, then converter power is not reaching the battery.

If the battery was indeed connected backwards, there are fuses on the converter that should blow to protect the converter.  These are physically located on the converter.  Correcting wiring and replacing fuses may get you going.

It may be the converter is not functioning, and may be the cause of the old battery not getting charged.  This could be several things including a breaker feeding the converter is tripped, the fuses mentioned above, loose wiring or a bad converter.

Good luck in your search.

Gary RV_Wizard

Site Team
Feb 2, 2005
West Palm Beach, FL
From your description, I would not necessarily conclude the battery is hooked up wrong. In fact, I doubt it, since your interior display showed a valid reading.  Many RVs have reverse polarity protection on the converter/charger anyway, blowing a pair of fuses if hot & ground are reversed.  You may have a short, or possibly just a broken wire somewhere.

Check the cables to be sure - the 3 go to the positive terminal.  Also check the converter/charger to see if it has a pair of fuses (usually 30A) for reverse polarity.  Then recharge the battery, preferably with an external charger (leave the cables disconnected if you have another charger). 

You checked circuit breakers, but those are nearly always strictly for 120v power. Your 12v circuits typically have fuses.
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