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Old 07-04-2019, 08:00 PM   #1
Join Date: Oct 2018
Posts: 59
Alternator killing my House Batteries??? Ideas??

When I bought my MH, I figured out that the alternator wasn't charging my house batteries. (they would charge on shore power, or generator, otherwise, no). I realized that the batteries (duracel marine/deep cycle) were probably shot, but wanted to get the system operational, before replacing the batteries.

Troubleshooting, I found the 'hot' coming from the solenoid/alternator was disconnected at the battery post. So, of course, the alternator wasn't charging the batteries.

I reconnected the cable, started the MH diesel, and one of the house batteries started gassing (major) after a few minutes. I stopped the MH, disconnected all cables, and removed the batteries. Hydrometer and voltmeter confirm 2 shorted cells. End of life.

So, I replaced both batteries (easy on me, now) with the same model duracel Marine /deep cycle as before. :-)

I hooked up the alternator/solenoid wire when installing the batteries. I went on a 3 day trip, everything seemed to be OK, when I got home, I checked the batteries, and a bunch of cells were low on electrolyte. [mod edit]. I started the MH, let it idle, and sure enough, I started smelling sulfur. It was boiling the batteries.

I'm starting to figure out, why the alternator/solenoid wire was disconnected when I purchased the MH......

Has anyone else had problems resembling this?

One additional fact, the chassis battery is fine through all of this. It is so old, I cant make out the roman numerals for the date code, but it is holding at 12.65 and showing no signs of weakness.

I know there are some really smart guys lurking on this forum, with lots of experience....I'm looking forward to your responses!! Thank you!

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Old 07-04-2019, 09:44 PM   #2
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What is the voltage measured at the house batteries with the engine running?

2004 Monaco La Palma 36DBD, W22, 8.1, 7.1 MPG
2000 LEXUS RX300 FWD 22MPG 4020 LBS

Don't know where I'm going, spose I'll get there anyway.
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Old 07-04-2019, 10:07 PM   #3
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You could use a “smart” combiner like a echo charger to maintain the house wile driving. Does your alternator have a single 12+ or 2 12+ outputs?
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Old 07-04-2019, 10:15 PM   #4
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A few more questions,

Is this a Chevy chassis?

What type of battery isolator do you have, relay or diode?

If it is a Chevy Chassis, does your alternator have a sense lead at the connector plug (I am not sure what year they started having them)

And again, what is your charging voltage measured at each of the batteries.


p.s. I had alternator output voltage issues on my coach when I bought it due to a disconnected alternator sense lead
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Old 07-05-2019, 09:24 AM   #5
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Thanks all, for your input. More info:

The voltage at the house batteries, with the engine running is 13.9 vdc open circuit (cable from the solenoid disconnected from the battery) and 13.8 vdc when connected to the house batteries. The house batteries are 100% charged when I checked voltage.

I'm not sure how to tell if the alternator has 1 or 2 outputs. A little more info on my setup: I have a 48vdc solar system, which has an inverter that supplies 120 volts to my rig, if I choose to run it when driving. So, if I do run the 48v inverter, it is powering my 12v converter, which will charge the 12v house batteries. (so, I do have a 2nd set of house batteries (48v)). I really don't need to solve this problem. I am just concerned, when something isn't working as expected, it may be indicative or a precursor to another problem. So long story short, the 2nd charger would be a solution, but isn't necessary.

The Bus is on a Freightliner chassis, Cummins/Allison typical pusher package. I have a fuse on the output of the 12v house batteries, feeding the 12v inverter, but I'm not sure of the isolator from the alternator. I know it works, though, since the 12v converter doesn't backfeed, and charge the chassis battery.

Thanks again for your ideas and questions, folks!!
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Old 07-06-2019, 09:59 AM   #6
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it is powering my 12v converter, which will charge the 12v house batteries.
Does this converter have 3 stage charge profile?
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Old 07-06-2019, 07:00 PM   #7
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Yes, the converter is completely programmable, 3 stage, it does a great job.
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Old 07-06-2019, 07:34 PM   #8
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Confused here... You have a 48v solar system - does it have its own 48v battery bank or is it going through a charge controller to charge your 12v house bank?

Seems like something is not set up ideally if you are using solar to power an inverter which then runs a 12v battery charger. Each step along the way has built-in inefficiencies, so why not just charge the house batteries from the solar charge controller?

All of this doesn't address your primary question though, why are your house batteries being overcharged by your chassis alternator. If your alternator is putting out 13.8v that seems about right. It shouldn't do much damage to your house batteries, but if the house batteries are fully charged they really don't need that voltage. They would do better with a float charge of around 13.2v if they are charged. Your chassis alternator's regulator is reading the voltage of the chassis batteries, and will adjust the voltage output tailored to the chassis batteries.
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Old 07-07-2019, 04:50 PM   #9
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Yes, I have a separate 48v battery bank. It is way higher capacity than the lead acid house batteries (7KW). My solar charge controller charges this bank, which feeds a 48v pure sine inverter.

indeed, I do lose efficiency, since I have overhead of the inverter and 12 v converter and charging loss. But I don't want to invest in additional 12v batteries, while I have lithiums on 48v. (They will not work as 12v).

As you say, this is separate from the 12v house battery question. It seems if the 13.9 is too high from the alternator, that it would hurt the chassis battery, too. No?

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alternator, batteries, house batteries

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