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Old 04-15-2019, 08:15 AM   #15
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I agree that most alternators are not sufficient. Mine is 320 amps which seems to do the job.

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Old 04-16-2019, 07:42 AM   #16
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Originally Posted by Mr_D View Post
I agree that most alternators are not sufficient. Mine is 320 amps which seems to do the job.

You are correct.

Even with a total running light and headlight conversion to LED, plus a refrigerator conversion to a extremely efficient DC compressor, the math still comes in at or near 300 amps being the safe size alternator to operate at night in the rain.

The only reason I didn't ultimately choose the 320 amp Delco Remy was it is another 2" longer and it only comes in a pad mount. Fabricating a mount would have caused me to need to reroute the turbo to intercooler pipe, adding a bend that might have negatively affected the overall performance.

So my maximum is 300 amps AND I have 590 watts of solar on the roof that produces about 100 watt/hrs of power. So far it seems sufficient to keep my conventional start batteries and the LIFEPO4 house batteries topped off.

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Old 04-16-2019, 08:21 AM   #17
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I believe the average motorcoach RV'r doesn't realize that a low voltage condition leads to an over amperage condition. The over amperage condition leads to electrical system(s) failures. I know this all too well. We purchased a used motorhome and encountered a steep learning curve in the discovery of what was causing all of these electrical issues. The problem: The OEM 130 amp alternator was completely incapable of meeting the normal load demands if we started a trip at night.

We were having trouble with our engine driven air conditioner, but at the time I didn't think it was related to a low voltage issue - I would later discover this error.

So to combat the problem I started troubleshooting by turning all the lights on, including the interior DC lights, the engine driven airconditioner and using a clamp meter on the primary alternator wire, I measured the amp and voltage. I discovered the voltage was 11.8 VDC and I only had a 10 amp load with the engine at flat idle. That was completely baffling.

Revved it up to 1400 rpm and the voltage came up to 12.8 VDC and the load grew to 50 amps.

Continued this process by revving it to 2000 rpm and it topped out at 13.2 VDC measured at the batteries and it was 110 amp load on the primary wire.

Then, over the roar of the engine, I could literally hear the alternator trip off, the load go to zero, the voltage drop back to 12.5 VDC and start dropping fast.

Shut it all down. Put a battery charger on it overnight and charged everything up.

Started the same diagnosis the next day and basically got the same results, albeit with higher battery voltages.

Then I measured the line load on the batteries. It was over 110 amps.

I left things alone for several weeks and came back to the same test before departing on another trip. I got some startling results.

Now take into consideration all of this was BEFORE installing solar a more powerful alternator and LIFEPO4 House batteries.

Disconnecting the alternator primary lead to see just how much of a load the coach pulls resulted in 90 amps load on the battery primary wire.

This means the RV consumed 90 amps of DC load from the batteries when they were fully charged. This amperage draw increased as the voltage in the batteries dropped. The rate of amperage increase was startling.

I estimated that if the alternator failed at night, in the rain, the batteries would be too low to operate the coach for more than 20 minutes.

Turning off everything unnecessary would lengthen the time, but not by a sufficient margin to give me a warm and fuzzy.

Hooking everything back up and starting the engine I realized the OEM alternator was functioning at 100% load almost constantly.

Turning off the lights helped quite a bit, but the alternator still operated at or near its maximum output until the battery voltage reached 13.2 volts. The OEM alternator was incapable of putting out more than 13.8 VDC under any reasonable condition I could create.

Six months ago we decided to fix it all.

We started with upgrading the coach from 30 amp service to 50 amp service.

The house batteries were upgraded from conventional golf cart batteries to LIFEPO4 batteries.

We added 590 watts (potential) of solar power to the roof and a MPPT controller.

We upgraded the alternator to a Delco Remy 40Si 300 amp with remote sense.

Then we changed the AC compressor and drier, precharged the system and it wouldn't engage.

So it is at this very moment at the RV center getting a diagnostic rundown on what wire, fuse, relay or whatever is smoked or got killed during the under voltage running condition we had coming back from Auburn Alabama last fall. It was that trip that started all of this, but between the fall and now we haven't needed the engine driven air conditioner.

Obviously with the onset of spring we need the AC working, so since AC systems in an RV are even more complicated than one might expect, I took it to the professionals to sort out.

Hopefully some time later this week, and several thousand dollars later, we will have a system we can operate in basically any environment without the worry that we are operating anything at 100% capacity - like it was designed.
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Old 04-17-2019, 07:32 AM   #18
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Originally Posted by jacwjames View Post
The only issue I had was I did find that even though I had all the battery disconnects off I was still getting voltage back to the alternator, found my solar was back feeding, pulled the fuses and this isolated the alternator from all voltage.
Our coaches are different, but where did you find the solar fuse for the chassis battery? The fuse for my house batteries is at the battery terminal of one of the house batteries, but I can't find the one shown on the wiring diagrams for the solar to chassis batteries. Chasing that output wire from the solar controller I lose it in a bundle of wires. Thanks.

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