Author Topic: 1 Volt drop from Alternator to Chassis Battery  (Read 14081 times)

Mike Groves

  • Guest
Re: 1 Volt drop from Alternator to Chassis Battery
« Reply #15 on: May 03, 2015, 05:52:15 PM »
Mike,
I have never seen an isolator that had enough resistance to cause a 1 volt drop in charging voltage without failing. Also the resistance would have to be the same on both sides of the isolator which is almost impossible or your house batteries would be charging at 14 volts while the chassis batteries are at 13 volts.

Voltage output is not what burns-up an alternator. Amperage output is what burns-up an alternator. However, if the high voltage is reaching a battery bank, they will take more amps so the issue is how many volts are reaching both banks batteries. I still say that your alternator needs to be properly repaired.

Gerald

Ok, let's assume then that my isolator is not working properly.  While running down the road, with my monitor on (the CMP), the coach batteries are not getting any more than 13.2 volts as well, so that value matches my silverleaf chassis battery value.  Those two values do seem to parrot one another.  Does this mean the isolator is simply not working at all?  Is that what you're saying? 

I may have confused you when I talked about the coach batteries charging at 14.2 or so while running the generator or plugged into 120V.  I never said that they were charging at 14.2 from the alternator while running only the engine.  Rather, to be very clear, they MATCH what the chassis battery is showing - ie 1V less than alternator output which varies from 13.8 to 14.2 but never more then 14.2 (rarely see less than 13.8 and only for a short period - pretty steady in between those two measures, which I consider normal alternator output).

For clarity, obviously I don't have a real-time sensor for the alternator output, but under the conditions noted on my monitor gear (13.2), when I have gone to the back and measure alternator output to ground, it has always yielded the 14.2v.  So, I am in no way saying that I can measure all simultaneously all the time as I can with chassis and coach batteries using the CMP and the silverleaf.

Thanks,
Mike

Gerald Farris

  • Guest
Re: 1 Volt drop from Alternator to Chassis Battery
« Reply #16 on: May 04, 2015, 04:56:25 PM »
Mike,
The purpose of the sense wire on the alternator is to deliver the desired voltage to the batteries. With a Duvac alternator, any resistance in the isolator is disregarded because the alternator voltage regulator is reading the voltage at the batteries instead of it's output voltage at the alternator. Apparently the shop that worked on your alternator disabled the sense wire circuit on your alternator if it exist. Most coach owners have no idea what the alternator output voltage is at the alternator because they are looking at the gauges that read output at the batteries which should be around the 13.8 to 14.2 volt range.

Your isolator is working if it is passing current through from the alternator to the batteries, not getting hot, and not letting one battery bank discharge the other bank. However, if the cable between the alternator and the isolator is good with good connections on both ends, and both battery banks are receiving the same voltage at 12.8, your sense wire circuit is your problem if you have a Duvac alternator. The key here is with your alternator, is it designed for a Duvac electrical system.

If you do not want to repair your sense wire circuit, you can replace the battery isolator with a good quality "Bird" system that will directly tie your alternator to your chassis batteries and remove the need for the isolator and sense wire. I would recommend a "Bird" system like the Blue Sea system instead of the Intellitec system that Monaco used on late model Beavers. A "Bird" system will allow you to use the less expensive alternator like trucks and busses instead of the more expensive Duvac system used on motorhomes.

Gerald

Gerald

Mike Groves

  • Guest
Re: 1 Volt drop from Alternator to Chassis Battery
« Reply #17 on: May 04, 2015, 05:52:23 PM »
Gerald,

I think we agree that other owners have no idea what their alternator is outputting. 

They only know what their batteries read, and those values are as high as 14.8 from what I've read.  And that's about what mine read when I had my chassis batteries replaced and alternator rebuilt in April of 2014.  Based on what I'd read, I felt this was normal voltage to see on the silverleaf.  I now think that the alternator output at that time was about 15.8V, and I would not be surprised if other owners measure their output voltage that they would see something similar.

Since V=IR, and assuming resistance is a constant, a higher voltage will mean a higher amperage as they are directly proportional mathematically.  My newly rebuilt alternator burned up during a 2 month trip across country. 

Since being rebuilt the second time in 2 months (rebuild again in June 2014), the gents who repaired mine the 2nd did measure the output voltage (something not done typically because the rebuild is farmed out to someone else by a coach maintenance facility and therefore can not be tested on the vehicle by the rebuilder), and found it to be in the 15 to 15.8 range but only 14.2 on their bench test, I do know what mine was outputting, and also after they did the "fix" I know its now outputting 13.8 to 14.2.

So, I am still wondering if any owners will put a meter on theirs and see what the output post is reading.  I wonder if theirs will show 15V or more, and if perhaps, this is the reason there has been so much talk of alternator rebuilds/replacements for these coaches.  I do know that the other theory is that dry camping causes too much current to be generated by the alternator and thus burns out the alternator and that the fix for that was to run the generator in the morning or while driving to mitigate this issue.  This was an actual precaution I used during my 2 month cross country trip, but it didn't save my alternator.

Thanks,
Mike