Log In | Register
Skip to main content
Topic: Coach battery alternator charging (Read 365 times) previous topic - next topic
0 Members and 1 Guest are viewing this topic.
Coach battery alternator charging
I've got a question about charging the coach batteries from the alternator. Is the voltage regulator driven from the aggregate state of charge from all three (3) batteries (1 truck/chassis, 2 coach)? For example, assume that the truck/chassis battery is fully charged, whereas the coach batteries are 75%.

Normally, in a passenger vehicle, the voltage regulator/exciter wire would energize the rotor sufficient to run normal operating requirements, while the chassis battery provided real-time balancing and would perhaps be periodically 'topped off.'

However, in a LD, how does the relay know that the coach batteries are in need of charge? Are the coach batteries simply wired through some kind of diode connector to the chassis battery, so that the exciter wire is energized sufficient to have the alternator run both the operating vehicle, along with re-charging the coach batteries?

I guess the real point of my question is this: let's say the coach batteries are beginning to fail and are having difficulty holding a charge. Assume the RV has been charged with both a dedicated battery charger for the chassis, and household shore power through the converter for the twin coach batteries.

You take off assuming all three batteries are fully charged, but in actuality, the coach batteries are 75% (while chassis is 100%). Does that mean the alternator is going to be working over-time by running at full throttle if the voltage regulator/exciter wire is being driven by the aggregate battery voltage?

Or is there some kind of relay that first uses input data from the chassis battery, and once its ok, switches over to check the state of the coach batteries? It's essentially the same problem, in that the alternator rotor will continue to be energized in an attempt to bring the coach batteries up to 100%

So, the bottom line is, if your coach batteries are going, does it place a load on your engine/alternator? And the reverse also being true: if all three batteries are indeed charged 100%, then the vehicle/alternator can basically lollygag sufficient to produce just enough DC to run the vehicle itself?
Re: Coach battery alternator charging
Reply #1 –
   Hi Snerf, so that last paragraph is true. Your alternator is always applying current to keep the engine and everything charging. The Diode isolator makes the chassis battery charge through one diode, and the house batteries through the other diode. If the house batteries are more discharged, usually the case when you hit the road, they will drag down the voltage, and the alternator will pump most of the current into those batteries. Because the wire going to charge the house batteries isn't that big, there will be some voltage drop across that which will decrease (by very little) the maximum charge rate. The load on the alternator isn't that high, and really low (say 10.5 volt) house batteries won't affect the rest of the coach very much. As the chassis battery gets back to maximum (pretty quickly), almost all of the current will go to the house batteries. I have a voltmeter function as part of my scan gauge, so I can watch that voltage go up very gradually as the house batteries charge. It takes about 4 hours of driving to get a full charge, and the solar is helping if it is daylight.   RonB
RonB (Bostick) living in San Diego
Original owner of "Bluebelle" a '99 TKB
Re: Coach battery alternator charging
Reply #2 –
Instead of a relay, your rig has a diode isolator, which basically contains a pair of diodes whose anodes are connected together and to the alternator output. One diode cathode goes to the coach batteries, the other to the chassis battery. The alternator voltage sense connection is to the chassis battery, to compensate for the voltage drop across that diode. Now, the actual voltage drop across each diode will depend on the current draw from the batteries in that leg - more current, more voltage drop. So, if the chassis battery is almost fully charged, it will draw very little current, and the voltage drop across that diode will be perhaps 0.5V. The output of the alternator will then be about 15V, at the anode of the diode to coach batteries. If they are discharged to say 12V, then the voltage drop across the diode will be about 3V, and a lot of current will be delivered to the batteries. This might correspond to about 20A to the batteries, which means that diode is dissipating 60 Watts of heat. This is why the diode isolator has a big heat sink on it.

Steve
2004 FL
2013 Honda Fit
Re: Coach battery alternator charging
Reply #3 –
Thanx to both Ron & Steve for excellent answers. While I know the very basics about the diode isolator, I'm still not sure how the alternator is being energized in response to (semi) discharged coach batteries.

If the engine alternator is energized based on readings coming from the chassis battery sensing wire, won't it decrease the output accordingly to satisfy just the basic vehicle  operating requirements once the starter battery is fully charged? (As in normal passenger vehicles.)

If not, is there an auxiliary voltage sense coming from the coach batteries that then re-energizes the alternator to achieve full output? (Which is then directed to the coach batteries via the diode isolator.)

Or, does LD make a modification by connecting the alternator voltage sensing wire to the diode isolator? In that fashion, the alternator would still have a single source of voltage regulation? It's just that the first/primary reading would come from the chassis battery, and then when it reached full charge, the diode would act to supply sensing input to the voltage regulator from the coach batteries. If the voltage sense is then coming from the coach batteries, this in turn would direct the (renewed/higher) output charge to the house batteries via the same diode isolator?

Re: Coach battery alternator charging
Reply #4 –
Quote from: Steve  - 
This might correspond to about 20A to the batteries, which means that diode is dissipating 60 Watts of heat. This is why the diode isolator has a big heat sink on it.

So, here's a hypothetical: you're camping (w/o generator or solar) for 2 days and the coach batteries get down to 60%. (Or, assume the coach batteries have been charged from gen/shore, but are beginning to fail holding a charge.) The drive that day across the desert is expected to hit 100 degrees. Where you're going next has shore power, so you can easily power the coach and/or re-charge while you're there.

Question: do you simply disconnect the house batteries so that the alternator is working at bare minimum to simply run the vehicle - and the diode isolator stays cool - during the drive? That is, turn your RV into a very heavy passenger vehicle?
Re: Coach battery alternator charging
Reply #5 –
The diode isolator is designed to handle the power. You should not need to do anything. The alternator does not sense anything but the chassis battery voltage, and puts out whatever voltage it needs to produce that voltage at the battery. The more discharged your coach batteries, the more current will flow to them from the alternator. I don't know the Amp rating of your alternator, but voltage drops across the diode and the wiring run, fuses, etc, will be the limiting factor.

Steve
2004 FL
2013 Honda Fit
Re: Coach battery alternator charging
Reply #6 –
I've got a new Delco CS130 (105 amp). The '93 G30 itself has limited electrical needs, so the alternator easily handles the vehicle power requirements.

My concern was putting an unnecessary load on the system if there was constant flow to the coach batteries in an attempt to re-charge them from a discharged or failing state. (ex 12.25 VDC)

Your example of a 3+- volt drop producing around 60 watts @ 20 amps and subsequently heating up the diode fins raised my initial concern. Logically, it seems that if all 3 batteries (2 coach, 1 chassis) were fully charged, then the only output required from the alternator would be for the rig itself.

Keeping the discharged and/or failing coach batteries hooked up seems similar to adding a power hungry stereo system or similar that keeps the current flowing near maximum.

Thanx again for your time & input.
Re: Coach battery alternator charging
Reply #7 –
Hi Snerf;  I wouldn't be concerned about batteries reading 12.25 V.  Once leaving Las Vegas for Bryce Canyon, I noticed on my Scangauge that the voltage was 12.5 with the engine running. Stopped and checked chassis battery at about 13 volts. House batteries were 'anemic' and measured 10.5 volts. One battery was 4 volts.  I took the caps off and dipped the test probes in the acid (be careful not to get the drips on anything). and measured 2.2 volts between cells, except for one cell that was shorted, at zero volts. Next stop was St George Utah, that had a new Costco. Replaced the two batteries, put the 'cores' back in the shopping cart to get my deposit back, and was on my way. The only complication was it was very windy that day.   RonB
RonB (Bostick) living in San Diego
Original owner of "Bluebelle" a '99 TKB
Re: Coach battery alternator charging
Reply #8 –
Quote from: RonB  - 
one cell that was shorted, at zero volts. Next stop was St George Utah, that had a new Costco. Replaced the two batteries, put the 'cores' back in the shopping cart to get my deposit back, and was on my way. The only complication was it was very windy that day.   RonB

Exactly. My local Costco has over 20 of their 'golf cart batteries' in stock for $90/each. My current coach batteries are over 5 years old. Worse, we use our rig very irregularly, and I'm not very diligent about battery maintenance anyway (water levels). So, flip them and forget it.

That being said, I appreciate the feedback from you & Steve. It's always good to refresh one's recollection of how various technical elements work.
Re: Coach battery alternator charging
Reply #9 –
Hi Snerf; Keeping your rig plugged in can cause overcharging depending on how 'smart' your converter is. I put in solar and that keeps the batteries ready to go. A good controller will treat the batteries well.  It doesn't have to be a lot of Watts, just enough to keep them charged up. And it keep that LP gas detector happy.  A bigger system isn't near as expensive now as mine was 18 years ago. It was one of the best improvements though in retrospect.  With the Trik-L-Start I put in a few years ago, the chassis battery is kept up also. Now all the batteries are ready to go.    RonB
RonB (Bostick) living in San Diego
Original owner of "Bluebelle" a '99 TKB
Re: Coach battery alternator charging
Reply #10 –
Quote from: RonB  - 
Hi Snerf; Keeping your rig plugged in can cause overcharging depending on how 'smart' your converter is. I put in solar and that keeps the batteries ready to go. A good controller will trat the batteries well.  It doesn't have to be a lot of Watts, just enough to keep them charged up. And it keep that LP gas detector happy.  A bigger system isn't near as expensive now as mine was 18 years ago. It was one of the best improvements though in retrospect.  With the Trik-L-Start I put in a few years ago, the chassis battery is kept up also. Now all the batteries are ready to go.    RonB
Interestingly, until this year my '02 was in covered storage with 120v available and I always kept it plugged in and the solar charger (HP22B) on "shore power". Never had a problem in 15 years with over charging. Couldn't use solar. Original converter.

Chris
Formerly: 2002 30' IB
 
Re: Coach battery alternator charging
Reply #11 –
Just a quick update to the thread.

First of all, I dug out my old original LD owner's manual, which describes two different types of diode isolators, depending on whether it is a Ford or Chevy chassis. (Note, the manual pertains to my 1993 G30.)

Ford has a 3 pole isolator (alternator, truck & coach batteries), while the Chevy has 4 poles: alternator, truck and coach batteries, and 10 amp exciter wire. I had vaguely sort of recalled there were 4 poles for the Chevy, which is why I was asking Steve above about the role - if any - of the exciter wire possibly working in conjunction with the voltage regulator in terms of alternator output.

Second point is I got two new golf car batteries from Costco, and the change is quite obvious. I can tell why my voltmeter was always reading low even though I would charge the old batteries for 12+ hours. They were over 5 years old, and pretty neglected. As to that, we store our RV outdoors, so it's really dependent on me going over there now & then to start the engine and let it run for a 1/2 hour or so.

We're taking it to Pomona this week to the Winternationals, which is why I've been working on bringing it back up to speed. This is the pattern we've had for almost 7 years, where we'll use it for a couple of events/trips, then let it sit for months. It's why I keep spending $ to keep the old rig going rather than spring for something newer, because it really isn't our style to exclusively camp/RV travel.

On that note, our neighbor who bought a brand new Airstream last year, and used it maybe 3 nights, sold it a few weeks ago. Another guy down the street has a brand new 35-40' class A that he keeps nice and shiny, but never used. It now has a 'for sale' sign on it. So, I guess it's better to keep plugging $ hundreds for something that works the way we want (and use), rather than wasting $ thousands on something that doesn't fit.