1 Volt drop from Alternator to Chassis Battery

[Mike Groves]

For some time now I've had a 1 volt drop (12.8 to 13.2 rather than normal 13.8 to 14.2) from that measured at the alternator to that measured at the battery (and on my Silverleaf and Dash Voltmeter).  How many devices are between my alternator and battery and what are they?

Thanks Mike

[Edward Buker]

Probably just a battery isolator with diodes in it. The voltage drop of the isolator is about .3V. Wire resistance may account for a bit more. Also check for a bad battery pulling current.

Later Ed

[Fred Brooks]

      I agree with Ed. To verify where the voltage drop is, perform this test. Locate the isolator in the battery compartment (blue finned device about 4" by 10" with 3 battery cables going to it). With your multimeter, check the voltage between ground and each cable. The alternator is 14.2 to 14.6. the house battery about .8 volts less and chassis batterys about the same. All contingent on loads applied, clean cables and the age of the batteries.
  Gotta love the mystery of "Tricity"   Fred

[Mike Groves]

Ed and Fred, thanks, I guess the isolator is the only device between the alternator and the batteries.  I'll try probing the voltage when I get a chance (Coach is at the storage facility). 

Some other clues are the following:

Chassis Batteries and alternator (rebuilt) are new and this issue was discovered by the guys that rebuilt the alternator.  I'd had the alternator rebuilt by Oregon Motorcoach, along with new Chassis batteries replaced, in April last year, then went on a cross country trip.  The battery charge rate was in excess of 14.8 (according to Silverleaf).  Prior to alternator rebuild, and chassis battery replacement I'd gotten high voltage alarms through the Silverleaf, but don't remember any after replacing and rebuilding charge system, so I thought all was well.

After reaching the east coast and upon my return west, I got to Wisconsin before I saw that suddenly the alternator wasn't charging any longer.  I fretted for a holiday weekend to end, so that on Tuesday I could get it looked at, but suddenly it was charging again, so I continued, and got all the way to Yellowstone before getting the warning again from the Silverleaf of undercharging/no charge out of the alternator. 

Limped my way through the park using the generator, then stopped in Idaho.  That's where the guys told me that "on the bench the rebuilt alternator was giving 14.2V which was spec for the alternator, but as soon as it was placed in the coach, it would give over 15V and they said that sort of thing would fry the alternator and that's probably what happened.  They verified that the chassis batteries were fine (up to charge), so that wasn't what was calling for more volts.  They weren't familiar with the coach, so what they did was craft an output limiter from the alternator using some sort of electrical wizardry to limit its output to 14.2, and they noted that there was a 1 volt drop to the batteries (13.2 was the max since the alternator was limited to 14.2), but told me by limiting the output of the alternator it would not burn out again.  So, I've had this issue since about June of last year, but have been happy with the charge rate which runs from 12.8 to 13.2 on the Silverleaf without causing an alarm condition.   

I've read others' postings about having outputs of over 14.2, and I've read all the warnings about running the generator in the morning to pre-charge chassis batteries, but even though that would explain the high demand placed on the alternator, it doesn't explain the voltage drop to the batteries, so I guess I need a new isolator to go back to perfect.

I looked at removing it, but boy that looks like either a lot of work (mainly to get to the bolts)  as mine is sandwiched in between two surfaces, or I need specific tools.  Anyone done this?  I'm not sure if a 2000 year coach has the same setup as mine in the battery compartment as to location of the isolator, but it probably does.

You can see the bottom left corner of the isolator barely visible in this jpeg image -

http://coach.quickreckoning.com/BatteryAndServiceBays.jpg - the bolt visible is the easy one to get to. 

Thanks again Ed, and Fred.

Mike

[Edward Buker]

Mike,

It is pretty rare that the isolators go bad and the isolator has no way of raising voltage so if the alternator is producing too high a voltage it is usually the regulator in the alternator or a wiring issue where the ground or sense lead has a bad connection. The isolator either conducts with a small voltage drop or it is open and will not conduct any current. There should be no need to add "external wizardry circuits" to control your voltage. I would probably look for someone who has decent knowledge regarding alternators, isolators, and batteries to sort out what the root cause of your problem is and resolve it. It sounds to me like something is still not right with the alternator or its voltage sensing connections which require a good ground and a good reference voltage connection usually tied to the battery + lead.

Later Ed

[Mike Groves]

Ed,

Thanks for your post.  I brings up two concise questions in my mind that I hope you can answer from your coach experience.  I know its a little different than mine but perhaps the same in these areas.

Where does the battery level sensor measure the battery level to report it back to the alternator?  Where's it coming from?

And two -

What is the maximum voltage output rating for this of this alternator, and how low does the battery voltage have to be to "tell" the alternator to go max rating?  Said another way as an example, if the alternator senses 14.2 volts in the battery does it generate 14.2 volts or does it reduce to 13.8, what does it do? 

Oh, and a third, for anyone, have you metered your alternator at its output, and compared that to your battery?  Are they the same (assuming fully charged batteries)?  What is the difference in yours - mine is 1 Volt difference.

Thanks to all.

Mike

[Larry Dedrick]

Mike
            I had a very similar alternator problem on our previous Beaver. The voltage output was very high, 15.0 to 15.7 at highway speeds. When we were around idle to approx. 1200 rpm voltage was around 13.3. I do know the lead acid batteries were overcharged, electrolyte solution all over the batteries.
             Went to Southeast Power in Altamonte Springs (basically Orlando) FL. and they replaced the sense wire and added a relay to prevent battery power from feeding back to the alternator when coach was shut down. The sense wire went from chassis batteries to the alternator with the relay in there. No more problems, voltage was solid at 14.2 volts.
               Hope this helps.

                   Larry D

[Edward Buker]

Mike,

Some alternators like a Leece Neville with a Duvac system (which were commonly used in Beaver coaches) had a remote sense wire that most likely went to the chassis battery side of the isolator so it was sensing battery voltage close to the battery after any diode voltage drop within the isolator. Most alternator brands do not have the remote sense lead and sense the voltage connection at the alternator, so if someone changed that alternator out you may not have the remote sense. If so that is not a big deal. It would mean maybe .3V less on average gets to the battery +.

There are fixed voltage regulators and adjustable ones in the alternator depending on alternator brand and the era that it was built. I had my Leece Neville rebuilt and the new Leece Neville regulator went from an adjustable version to a fixed version. The new one puts out about 14.4V perhaps a bit more.

The intent is to have the voltage held at a constant and the current output varies to support the charge and chassis use load. That does not mean that you always get 14.4 V at the battery, there is voltage drop that will vary. The higher the load the more voltage drop in the wiring from the alternator going to the battery. At 100 amps you might get 13V and at 10 amps 14.4V. The other factor is temperature, if the alternator is putting out high current and it is internally getting hot, the alternator will derate itself and put out less current which could result in a lower output voltage also. The alternator will probably not get near 14.4V looking at high load of a discharged battery bank and may slowly rise from around 12.8V to the 14V region over time as the battery charge comes up and the load diminishes.

The net of all this is you probably should be seeing between 13V and 14.5V at the batteries with the engine running above idle, no external inverter/charger on (coach not plugged into 120V) as long as the batteries are good and are at a decent charge level, and less voltage possibly if they are not. I would not be looking for an exact voltage of 14.4V or 13.8V, just a usable range and consistent charging performance. 13.8V to 14.4V is typical and I have seen it even a bit higher for for some alternators, perhaps 14.8V with charged batteries.

Later Ed

[Gerald Farris]

Mike,
Assuming that you still have a Leece Neville Duvac alternator, there are 4 wires that connect to the alternator, two large wires (one for output and one for ground), and two small (14 to 16 gauge) wires. one of the small wires is a sense wire that is connected to the chassis batteries, and the other one is from the ignition circuit. If the two small wires are reversed, it can cause the problem that you experienced. However, if they are not reversed, you have a bad connection in the sense wire.

If you are not comfortable in trouble shooting the alternator circuit, find an automotive electrical shop that is familiar with an Duvac motorhome alternator system to have it repaired. With you alternator producing 12.8 volts at the batteries, your batteries will not be fully charged.

Gerald     

[Mike Groves]

Hi Gerald,

When the alternator was last rebuilt, and the gentlemen did that special wiring that prohibits my alternator from putting out 15.8V and burning up in the process, I found that the rear engine door fan no longer worked, so I found a wire unattached by the alternator (which was to drive the fan) and hooked it back up to a post and its now working again, so my assumption is that wire is from the ignition circuit because it now operates the fan as it once had, ie, fan runs all the time, then after engine shutdown it times out after about 20-30 minutes. 

They attached the special wiring getup to the other post I believe so that must be limiting the sense to the alternator and thus limiting its output voltage to 13.8 to 14.2 (of which I read from 12.8 to 13.2 on the silverleaf and if I voltmeter the chassis battery).   I am not sure but part of this wiring change may also contact the positive post on the alternator.

Anyway, my original goal of this posting was to figure out why, if the alternator was pushing 14.2 was the battery only showing 13.2 so that must be across the isolator as there is nothing else between them.  I wasn't comfortable removing that isolator as it was so hard to get to with the tools I had.

Mike

[Edward Buker]

Mike,

I am not sure that you need extra getup wiring here. It may be that your sense wire is now on the wrong side of the isolator (alternator side). 15V at the alternator is OK if it is 13.8 to 14.4V at the battery post.....that is the idea with a sense wire. Maybe the tech can detail the wiring change that he did and you can post it so we can see what is up.

Later Ed

[Mike Groves]

Ed,

I guess I could find the paperwork and give them a call, but its been a whole year since they did this.  They would probably remember the event because they worked mainly on school buses and I brought them a coach, but I doubt they would remember how the patch wire works.  I do remember them discussing diodes.

It does work.  It fixes the output to no more than 14.2 volts and I haven't burned the alternator up in a year which is much better than burning it up in 2 months which it did the first time, and it was bad when I bought the coach, so it probably wasn't a new problem. 

I agree with Gerald that I'm not getting the chassis batteries fully charged but then again, I am now parking more than I am driving so it does get charged when I am plugged in.  One mechanic told me, I'm not hurting myself as 12.8 to 13.2 is higher than 12V and I haven't had any issues starting the coach.

I guess the guys that did the rebuild would disagree with you when you say 15V is ok (mine was giving out 15.8 or so and to them that definitely was not ok), and that's why they fixed it at 14.2 max which according to them was the max output this alternator should be pushing.  I guess I understood that logic as why push more than 14.2 to keep a battery at 13.8 (which is what I've always thought chassis batteries were).

I know when my generator is running, the house battery charge ramps up to about 14.2 or 14.3 during the charge cycle on the inverter, before eventually coming down and leveling off at 13.5V at the float setting.  So I don't know why I'd want the alternator giving more than that.

My issue is more with why the 1V drop as I am happy with the limits on the output.  And I think we've basically all agreed it has to be through the isolator since there's nothing else between the alternator + side and the battery.

Thanks,
Mike

[Gerald Farris]

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

[Edward Buker]

Mike,

The way this is supposed to work is the alternator sense wire looks at the battery voltage and compensates for voltage drop through the isolator. As Gerald pointed out 1V is too much drop for a good isolator. When I said in my post that 15V at the alternator is fine if the batteries are seeing about 13.8V to 14.4V, the 15V will not burn out an alternator as long as the voltage level is good at the battery and the amp load is not too great due to a bad battery forcing extra load.

The reason I bring up the "getup" and understanding it, is lets say they added a series of heavy duty diodes in series with the output lead in order to drop the voltage getting to the battery, that could account for a 1V drop. At this point I guess I am not sure if you have a Leece Neville alternator with a Duvac system, not sure if the sense lead is seeing battery voltage, and what has been added circuitry wise to give you the output that you are getting at the battery bank, which seems a little low but probably in the scheme of things should be adequate. Without knowing what you have installed and how it all is supposed to work makes a lot of this speculation...we know how the original system was supposed to work.

One thing that could be helpful is to take some voltage measurements with shore power off and the engine running. As always only take these measurements if you feel safe doing so and are physically clear of the belts while doing so. Measure the alternator output post to ground (large + lead), sense wire to ground, the center post on the isolator to ground, then each of the two other isolator posts to ground, chassis battery + to minus posts, and the house 6V pairs in series + to minus posts. They should read in the +13V region also. This should tell us where the voltage drop is occurring or if the alternator output voltage is low. Also photograph the "getup" and tell us what alternator leads it is connected to.

You could elect to hunt down a good alternator shop and have them sort this out if you are uncomfortable with these measurements and still want to get to the bottom of this. Hope this helps.

Later Ed

[Mike Groves]

Thanks Ed,

I'll have to wait until I go back to the storage lot, but I can tell you I've already taken two of those measurements, the output voltage is rock steady at 14.2 initially, and the chassis battery is 13.2 hence my initial question of what's in line (in between) the alternator output of 14.2 and where it connects to the battery.

Anyway, for now, I have some action items but they'll have to wait for my return to the coach.

Anyone going to be at Pacific Shores any time from June 9 to June 21.  I'd love to meet some members of the forum, and if also owners at Pacific Shores, that would be great as well.  I am studying all the owner information to catch up on the history of the resort.

Thanks everyone,
Mike