What is DUVAC?

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What is DUVAC?

January 11, 2018, 10:38:08 pm

Not to hijack another thread where someone may be stranded....
Did both a Google search, and a search here regarding alternators. I've seen the term DUVAC mentioned here time and again, and should I have problems (knocking hard on living room table) it would be nice if I understood exactly what a DUVAC is.
Links are fine.

Re: What is DUVAC?

Reply #1 – January 11, 2018, 10:48:21 pm

This is how I understand it, The LN alternators need an excite wire (on with ignition) to excite the alternator into making electricity when it rotates. They also need a sense wire that tells the alternator the voltage at the start battery. This voltage level tells the alternator how many amps to put out. Most LN alternators have an adjustable voltage which, if you have a diode based isolator, will be set about 0.8 volts more than the voltage at which you want to be charging the batteries. This is the level of voltage loss across the diodes in the isolator.

Roger

Re: What is DUVAC?

Reply #2 – January 11, 2018, 11:18:10 pm

I'm starting to think I need a lesson in RV charging period. But thats why I'm here.
Ok, excite wire makes sense. It gets the electrical "pump" working so to speak. Generators have magnets to help make electricity, alternators need current in the windings to make power.
Sense wire also makes sense. Its the "throttle" to the alternator, with out that the alternator can just keep making electricity.

Isolator I'm a little fuzzy on. Its keeps the coach and engine batterys seperated. With out that, during a "demand" period - like dry camping you run off the coach battery with out drawing down the engine battery. Or, if engine battery is low, hit the "boost" switch for additional power to crank as both banks are tied together, forming one big old battery. What function does the isolator serve for charging? I always assumed that there were two "battery" wires coming off the alternator, one going to each bank of batterys.
I've been told to turn on the boost switch when on shore power if I want to charge the engine batterys - in fact it mentions that in the manual. Same if running on the genset. Why not when I'm driving down the road?

Re: What is DUVAC?

Reply #3 – January 11, 2018, 11:31:11 pm

Quote from: Moby – January 11, 2018, 11:18:10 pm

I've been told to turn on the boost switch when on shore power if I want to charge the engine batterys - in fact it mentions that in the manual. Same if running on the genset. Why not when I'm driving down the road?

Best way to understand your electrical system is to get the wiring diagram out and study it. There are two 12 volt charging sources on your coach - the alternator and the AC powered battery charger (stand alone charger or combo inverter/charger). The alternator output is connected to the center post on the isolator. From there it is distributed by the isolator to both battery banks. The battery charger output is only connected to the coach batteries. The only way battery charger output can reach the engine start batteries is through the boost solenoid. This is all very clear if you just study the wiring diagram. This is also why it is so critical to have the correct wiring diagrams for your particular coach model.

There's a good simplified diagram of a DUVAC alternator setup at bottom of this page:

http://www.dixie-electric.com/tsb/TSB_704.pdf

Re: What is DUVAC?

Reply #4 – January 12, 2018, 12:16:46 am

OK, think we're getting somewhere.
Sticking with the plumbing analogy, the isolator is a one way valve which prevents current from going between the two banks of battery's, which is what I kind of knew already (the diagram helped, Alice did not come with one).
If I understand the diagram properly, if the engine battery is not up to par, it will send an incorrect voltage through the sense line causing the alternator to produce excessive current with the possible result of over charging the coach battery's. This would be why people talk about replacing both banks with equivalent battery's. I've heard similar and the diagram helps.
So, what happens when I flip the "boost" switch? Am I just triggering a big solenoid to bridge the two banks?

Re: What is DUVAC?

Reply #5 – January 12, 2018, 12:52:58 am

Let's back up a second. Number one study your electrical diagrams and verify that the previous owner hasn't made a bunch of changes. Mine had. Seriously Think about installing a second voltmeter on the dash that is connected to the house batteries, that way you know how to make an intelligent decision about if and when to use the Boost solenoid. If the house batteries are low and you use the boost solenoid you can take energy away from the start battery. Due to my setup I can start the tiny genset first and add power back to my Engine start battery. If I do something stupid like leave the lights on or the mirror heaters in my case.

Re: What is DUVAC?

Reply #6 – January 12, 2018, 01:14:52 am

Phred,
Your point is well taken, however I do not have the original diagrams to compare reality with. That's why I'm asking all the theoretical questions, so I have a better understanding of what's going on.
The point about a second volt meter is well taken, however. Thanks for that.

Re: What is DUVAC?

Reply #7 – January 12, 2018, 01:23:33 am

Give FT a call to see if they have your coach wiring diagrams, from what I have heard it's kinda hit and miss. And some charges should be expected of course.

Re: What is DUVAC?

Reply #8 – January 12, 2018, 05:21:47 am

Here's the deal. Duvac has nothing to do with charging your batteries. It is "dual voltage acceptance". When you drive your car, your alternator charges the battery that starts your car. Same with your motorhome. So.......how do you charge the house batteries while you drive when your alternator sees the start battery as always fully charged? DUVAC. It shares the charging ability of the alternator with both banks through the isolator with a modification to the regulator.

Now you don't need to study your diagrams.

Re: What is DUVAC?

Reply #9 – January 12, 2018, 09:12:04 am

This is what James Triana from Fortravel wrote in the Winter 2009 Mororcader Magazine.
"Foretravel coaches are equipped with what are termed "externally excited" alternators. These are not the same alternators used in the large trucks that run on our nation's highways. This fact is often misunderstood in a majority of large truck repair shops. Often Foretravel customers must utilize these shops when they experience charging problems on the road. Most of these truck repair facilities are only familiar with what are termed "internally excited" alternators which maintain a single bank of batteries (12 volt supplied to the large output post continually).

Foretravel, as well as other coach manufacturers, utilize a battery isolator in the alternator charging circuit. The alternator must be able to maintain two separate banks of batteries, one bank for the house batteries and one bank for the engine batteries, at a particular set voltage. When a battery isolated system is used, the alternator has to be turned on,'excited, from an ignition source. That process is termed "externally excited". During testing, with the main engine not running, there will be no voltage present on the large positive output terminal of the alternator.

The alternator that Foretravel employs has two large terminals, one positive and one negative, on the back, as well as two small terminals. The two small terminals tell the voltage regulator what to do. One small terminal is labeled DUVAC and the othersmall terminal is labeled IGN. The DUVAC terminal monitors (senses) the output voltage. The sensing wire runs from the DUVAC terminal directly to the engine battery side ofthe isolator on 2000 year model Foretravel's to present. Earlier models sensed from the remote start panel. The IGN (ignition) terminal turns on (excites) the alternator to start charging. This circuit is powered up when the ignition key is turned on. The only way the ignition terminal voltage will affect charge output voltage is if no voltage is present. It takes minimal voltage to start the alternator charging (7-7.5 VDC).

When checking the output voltage directly at the alternator while the alternator is charging, the output voltage will read higher than at the batteries. This is because there will be approximately a 1 volt drop from the output terminal of the alternator to the actual charge voltage at the engine battery terminal. This drop occurs in the isolator and associated wiring to the batteries.

Example: When reading the output voltage at the alternator with fully charged batteries and a properly functioning alternator, your readings should be approximately 15 volts dc (+/-), this in turn will allow for a maximum voltage at the batteries to be 14.1 VDC (batteries allowing for a normal 1 volt dc drop in voltage through the isolator).

If you have questions about which model alternator to use on your particular coach, please contact our Parts Department to place an order or Technical Services to obtain further information."

A bit longer but what I said. The start battery voltage will be lowest just after you start the engine. The alternator is excited to get going and sees the lower voltage at the start batteries and puts put appropriate amperage to recharge them quickly. As their voltage rises the amps go down. All the while the house batteries are likely at a higher voltage initially. They are getting charged too through the other side of the isolator. If both sets of batteries have equal voltage and equal capacity to accept charge then they will get an equal share of the alternator output. If the house batteries are at a lower voltage than the start batteries then (generally) they will get proportionaly more of the amps coming from the alternator.

Most alternators, like the one in your car, are connected directly to the single battery and sense its voltage directly. With an isolator the alternator is isolated from the batteries so you need to provide it a way to sense the voltage at (generally) the start battery. The Leece Neville alternators also need to be excited in order to start working. Not all alternators need to be excited in this way and not all alternators have a separate sense terminal.

Roger

Re: What is DUVAC?

Reply #10 – January 12, 2018, 09:49:34 am

Quote from: Moby – January 12, 2018, 12:16:46 am

1. If I understand the diagram properly, if the engine battery is not up to par, it will send an incorrect voltage through the sense line causing the alternator to produce excessive current with the possible result of over charging the coach battery's.

2. So, what happens when I flip the "boost" switch? Am I just triggering a big solenoid to bridge the two banks?

1. I suppose it is possible such a scenario might happen, BUT, in normal operating conditions not likely. Normally, your engine start batteries will remain fully charged (one way or another - see "Trik-L-Start" threads) until you start your coach. The start battery voltage will be depleted by the starting routine, but as soon as the engine starts they are quickly recharged by the high initial charging current from the alternator. Although the coach batteries are also seeing the same current (through the isolator), they have much larger capacity (at least double?) and should easily accept the high initial charging current. As soon as the engine batteries are (mostly) recharged, the sense wire signals the alternator (voltage regulator) to reduce the charging current, and both battery banks will receive the same lower (constant) current as long as the engine is running.

2. Yes. Boost Switch

Re: What is DUVAC?

Reply #11 – January 12, 2018, 10:52:46 am

Here are a couple of good sites for adding DUVAC to a standard Leece-Neville/Prestolite alternator as well as an overhaul guide to the alternator.

Alternator bearings usually fail because the belt(s) are too tight. You should be able to use your fingers and turn the belt(s) 90 degrees midway between pulleys. Every year or so, pull the belts and spin the alternator. It should be smooth with no rough spots or dry bearing sounds.

Prestolite - Leece Neville

http://news.prestolite.com/drupaldocs/Form4801.895.pdf

Pierce

Re: What is DUVAC?

Reply #12 – January 12, 2018, 11:05:27 pm

A word of caution...
The part number on the Leese-Neville alternator of my 295 was the part number for a NON-DUVAC alternator (2800JC). It had additional metal stickers stuck on it which said "Modified For DUVAC Service.".
The Tech at the Truck repair shop didn't see or didn't understand what that modification implied and neither did I (in spite of the same words you are reading now)
He thought he was good because he ordered the part which had the number he saw.
The new non-Duvac alternator is/was a 2800JB unit @$370

All sorts of grief.....
Limped into Madison, WI
The old-line alternator/starter guy who fixed it said "yep, that's about the right price for that alternator. The DUVAC model is in the neighborhood of $650!! "
"But... he could convert it -- the only problem is, If it ever had to be serviced whoever worked on it had to know it had been modified". Sound familiar?
He removed the new wrong alternator, installed a L-N 2824LC regulator ($131.45) to modify it for DUVAC , and reinstalled it ($160 labor). The alternator now has "converted from 2800JB to 2824LC" written with magic marker on the pulley and a tag tie wrapped to the battery cable.

I was so pleased by that time I could have kissed him.

Pierce has instructions (see previous) if you want to do it yourself.

I can't recommend Mike Harder at FINIKI Auto Electric in Madison, WI enough

You should probably tattoo "The Alternator I want is a 2824LC" on your left palm.

,.

Re: What is DUVAC?

Reply #13 – January 13, 2018, 01:51:10 am

Topic is "What is DUVAC?". After much searching around on the internet I found DUal Voltage Alternator Control. Not so much dual voltage as it is dual batteries with an isolator between the alternator and separate batteries which is what we have on our coaches.

Re: What is DUVAC?

Reply #14 – January 13, 2018, 03:11:16 am

You can buy a newer Delco Remy 160 amp for under $300 but you have to also change the isolator to work with it,much improvement charging at idle.

Re: What is DUVAC?

Reply #15 – January 13, 2018, 07:23:04 am

Here is some literature
http://www.dixie-electric.com/tsb/TSB_704.pdf
Prestolite - Leece Neville
Foretravel Alternators and Isolated Battery Charging
http://www.alpinecoachassociation.com/tech/Alternator%20Diagnosis%20and%20RR.pdf

Re: What is DUVAC?

Reply #16 – January 13, 2018, 08:54:27 am

Quote from: John44 – January 13, 2018, 03:11:16 am

You can buy a newer Delco Remy 160 amp for under $300 but you have to also change the isolator to work with it,much improvement charging at idle.

The Delco Remey will work with the OEM isolator. No need to change it although there are some very nice zero voltage drop isolators, some of them "smart". You do not need an excite wire connection. The start battery sense wire connects to the sense post on the Delco Remy. Even with a diode style isolator with the sense wire connected the Delco will adjust its voltage output to get the necessary voltage at the battery.

Medium & Heavy Duty Alternators | Delco Remy

A Victron ArgoFET 200 amp isolator is less the $120. Zero voltage drop.
Victron Argofet Battery Isolator

Re: What is DUVAC?

Reply #17 – January 13, 2018, 09:15:05 am

This is all unnecessary. To modify for duvac all you do is open up the alternator and rewire one of the posts on the SAME regulator and put it back together. Mine was done by a guy that actually knew what he was doing. 50 bucks.

Re: What is DUVAC?

Reply #18 – August 25, 2021, 02:56:06 pm

I've been working with American power systems to add or redo my system. Currently my house lithium Bank is not charged at all by the alternator. I would like to change that and have been researching different ideas. This is the email I received today

Bad news on "simply" upgrading the OE alt currently on the coach. This is apparently a "duovac" setup and not changeable. Whatever the original setup was at the time is not conducive to any known updating we can find. So looks like continuing with the generator and repairing the dash air is only options.

Then this after inquiring about duvac

This Duvac system that Leese Neville came up with was a completely integrated dual battery/isolator setup that was "state-of-the-art" in '99 but soon was obsoleted by newer better technology. Because of the setup the output was limited and if we could put a larger output alternator in there the isolator would not allow the extra power. And you can't take the isolator out of the system. And even if we could come up with a secondary bracket/alternator setup you would be faced with a major re-wiring of the entire coach to accommodate that.

I'm not sure this is accurate though.
Any input appreciated.
Working on the dash air issues.
Bob

Re: What is DUVAC?

Reply #19 – August 25, 2021, 03:52:39 pm

Not a guru on the subject by any means but several have done this by switching to a Delco Remy Alternator and since I have solar like you I needed to switch out my Isolator when I installed the lithium batteries with a DC to DC charger. Although to date I haven't used Xantrex charger or the alternator to charge my batteries on a regular basis. Solar has taken care of my needs so far.

I chose Victron. This is how I wired with input from Roger

Adjusting the voltage on alternator is only necessary if you are using the OEM alternator without the isolator.

Re: What is DUVAC?

Reply #20 – August 25, 2021, 04:45:28 pm

DUVAC is needed if you decide to stick with the OE diode-based battery isolator.

Other more modern and other more KISS solutions are available.

Re: What is DUVAC?

Reply #21 – August 25, 2021, 05:55:43 pm

I'm thinking the "adjust voltage" notes are intended for a L-N alternator. The Delco has no manual voltage adjustment - instead has a sense terminal which should be used and connected to the start battery "+" terminal. This will automatically adjust the alternator to correct voltage with or without an isolator.

Re: What is DUVAC?

Reply #22 – August 26, 2021, 11:18:04 am

Sorry that is correct about the voltage adjustment. I installed the DCDC first and replaced the alternator about a month later when it started giving me wild ranges of voltage on my cig lighter voltage readings. Will ad to original post

Re: What is DUVAC?

Reply #23 – August 26, 2021, 09:31:59 pm

Tommy D, next time we're through your neck of the woods we might have to spend an extra day.
I could use a teacher with a pointer saying "leg bone connected to the ankle bone".

Always been a visual (slow) type of learner, at least that's what the drill instructor yelled at me.

P.S.
Those batteries are working GREAT, bourbons gone though.

Re: What is DUVAC?

Reply #24 – August 27, 2021, 10:38:05 am

Quote from: Phranko – August 26, 2021, 09:31:59 pm

Tommy D, next time we're through your neck of the woods we might have to spend an extra day.
I could use a teacher with a pointer saying "leg bone connected to the ankle bone".

Always been a visual (slow) type of learner, at least that's what the drill instructor yelled at me.

P.S.
Those batteries are working GREAT, bourbons gone though.

Don't know how good of a teacher I would be but Ive made a lot of mistakes and they say that is when you learn the most. I would be happy to show you what I have done and I know where to get more Bourbon, you bring the pointer!

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