More Alternator 101

[GallenH]

Questions. So what does it mean from a battery charge perspective if your RV has a single alternator of a specific amperage? I'm going to make it specific to my RV.

My alternator = 136a. I'm assuming that when I'm driving, some is going to the engine battery and some to the cabin battery (a single 100AH LifeLine).

Is the alternator output getting divided equally between the two batteries or is it somehow dependent on the DOD of each battery?

The alternator output goes to an isolator (I understand its function) and the output going to the cabin battery has a 40A circuit breaker (that's never flipped). What's the function of the circuit breaker?

I believe the recommended charging amps on my 100AH LifeLine is 20a. How does this integrate with the 136A alternator and the 40A circuit breaker?
Is the battery placing a 20a demand on the alternator?

thx.glenn

[JohnnyFry]

The function of the circuit breaker, or any circuit breaker, for that matter, is to protect the wiring from overheating and possible fire. The fact that you have a 40 amp breaker going to your battery indicates that you probably have 6AWG wire. If you have had #4AWG you could use 50 or 60 amp breaker.

[mkguitar]

the alternator does not "push" or "force" current


it responds to demand placed on it.

if one battery is charged to capacity, it will not draw more current, no matter what the other battery is doing

the rating of the alt is what it will output without failing- but will actually output more IF the demand is there

in the event of a fault like a shorted circuit- the alternator might output hundreds of amps- the fuse and the thinner wire gauge will fail first, hopefully before the diode bridge blows apart ( which will go before the windings)

the thinner wire which might burn up is in a way a protection against a catastrophic failure as is a fuse or breaker

your 40 amp breaker is there to protect alternator and battery isolator should your battery or the things connected to it short out or draw more than designed


automotive designs sometimes use "fusable links" which are "weak links in the chain" deliberately weak section of what appears to be wire, made to fail.

sometimes you want to design a weakness to preserve the greater, more expensive, hard to service part

mike




in steamboats the drive system often included gear assemblies with WOOD engaging steel gears- the idea was that if something jammed ( like the paddlewheel hitting sandbar) the easily replaced wood gear teeth would shear off- the paddle wheel would freewheel and the drive from the steam engine would be relieved of damaging stresses.
then the teeth replaced

[avanti]

We can't really even begin to have this discussion until it is specified whether we are talking about an old-school voltage-regulated alternator or a modern LIN-controlled "smart" alternator. The behaviors you are asking about are entirely different in the two cases.

[GallenH]

Originally Posted by avanti

We can't really even begin to have this discussion until it is specified whether we are talking about an old-school voltage-regulated alternator or a modern LIN-controlled "smart" alternator. The behaviors you are asking about are entirely different in the two cases.

Since my RV is 1997 and it appears that the alternator is from that vintage, I'll give it a try based on my service manual. It states that the voltage regulator isn't a separate component but a voltage regulating circuit within the powertrain control module. There is also a temperature sensor in the battery tray that feeds the electronics. So my take on that is that the alternator I have isn't "smart." It's still relying on externals for regulation.

Hope that helps. glenn

[booster]

Lots of things going on in the system that contribute to what happens. I will assume about a 50% SOC on the battery as at that point it will be accepting about as much as deeper and less than shallower discharge.


Assuming the alternator has a typical regulator in it at about 14.1-14.4 volts, that is what the starting wet cell will see, as will the engine and body electronics. It will take what it needs for amps, but will top off quickly in most cases. The engine running will take probably 15 to 40 amps depending on what your have running like AC blower on high, etc.


The coach battery, through and isolator will be seeing somewhere around 13.4 to 13.7 volts as the isolator has voltage drop. The coach will see the same. How much the Lifeline will accept at that voltage is unknown as the normal charge voltage would be more like 14.4v. At 14.4v a 100ah Lifeline at 50% will accept over 40 amps maximum and can handle 40 amps continuously without over heating in most applications. A call to Lifeline might be in order to find out what it would accept at the lower voltage as it would be good to know.


A stock 136 amp alternator would probably not like to be charging at much more than about 50 amps for long periods as it will get hot and have a shorter life. It appears you would be within this range as long as the Lifeline doesn't accept too much.


The 40 amp breaker is likely on a #6 wire from what I have seen in other applications. Our 80 amp one was on a #4. You might have tripping of the breaker when the coach battery is accepting the max it can, but we don't know that yet at the lowered voltage.


Of concern also is that the lower voltage will not give the Lifeline a good charge, leaving it short of being full. Going to a separator instead of the isolator would take care of that by eliminating the voltage drop, but would also make breaker tripping more likely.


Probably the best and easiest way to go would be to not upsize the wiring if you are going to keep with one battery as it is a bunch of work. What would address the tripping and charging questions would be to go to a separator instead of the isolator, and then get a 20amp 12v to 12v charger to put in the charge cable to Lifeline in the coach. I think that would solve all the issues you might have.

[mkguitar]

Originally Posted by booster

seeing somewhere around 13.4 to 13.7 volts as the isolator has voltage drop

exactly what I see- and I have no problems with my old 2006 technology, but i know it's there and it bugs me.. I want my accessories to see the 14.7 VDC which is commonly supplied
mike

[GallenH]

So...would I use the Victron to measure incoming volts and amps?

[SteveJ]

Originally Posted by GallenH

So...would I use the Victron to measure incoming volts and amps?

It would be interesting to see the voltage numbers. Before I replaced my diode isolator with a two way Blue Sea separator I would get voltage readings in the mid 14 range on both feed lines from the isolator with a voltage in the low 15 range on the input to the isolator. This on a 2000 Chevy.

With the Blue Sea separator I'm reading low to mid 14 volt range at the cabin battery when first charging tapering to mid 13 range as the battery gets charged.

My thinking is that there is a pick up area somewhere in the harness that tells the voltage regulator how much voltage to put out. Case in point: We had a 94 Olds that was cooking the battery and electrical system on an intermittent basis with voltages as high as 20 volts. The repair shop, noted for being competent with auto electrics was able to isolate the problem to a corroded wire connection. It was giving a falsely low voltage reading to the voltage regulator, hence the higher system voltages.

To others who have or had a diode isolator, what are/were the voltage readings at the isolator terminals?

[GallenH]

Originally Posted by booster

Probably the best and easiest way to go would be to not upsize the wiring if you are going to keep with one battery as it is a bunch of work. What would address the tripping and charging questions would be to go to a separator instead of the isolator, and then get a 20amp 12v to 12v charger to put in the charge cable to Lifeline in the coach. I think that would solve all the issues you might have.

Right. In my current setup I actually have no problem........that I'm aware of. I'll have to check the alternator output (why I began this thread) to make some measurements and will probably return for your advice.

But most of my query has, instead, to do with upgrading the system in anticipation of going from an absorption fridge to a compressor. I'm trying to figure out how to do it; if it's practical. You and others have given me some good advice on necessities in other threads. I'm just trying to connect all of the dots.

Really it boils down to if I add 200-300AH of lithium, what else do I have to do. I'm guessing that my current 136A alternator won't be up to snuff. Upgrading the alternator's output = also upgrading wiring. But if I only go from my current 100AH to 200........?

Like I said, you, GeorgeRa, hbn (sorry H, I can never remember your full handle), marko, rowieb, mkguitar, avanti and many others have given me continual guidance. I appreciate it. I'm just feeling this out. In 1997 I bought by BIL's 62 vette and tore it down to the X frame and completely rebuilt it so I can deal with challenges...........with the help from friends.

cheers.glenn

[rowiebowie]

All of the following advice is written on the assumption you will not be upgrading your stock alternator.

Installing a DC to DC charger between your engine battery/alternator and your coach batteries will accomplish 3 things: 1) battery isolation between your two different battery chemistries; 2) proper charging profile for your expensive lithiums; and, 3) limit the charge demand on your alternator to a level it can handle (in your case probably only 20-30 amps for the coach batteries).

Remember, the alternator will need to push more than your rated DC to DC charger rating. Due to inefficiencies, a 20 Amp DC to DC charger could require 25-30 amps from your alternator (in addition to what is supplies to your engine battery and for other electrical needs).

I have a 160 amp alternator and 200AHs of lithium. I chose a 40Amp DC to DC charger thinking (but not knowing) this was what my original OEM battery charging was only based on the fact I had 4 ga. wiring and my house Tripplite charged the batteries at 40 amps when plugged in.

As I've mentioned on this forum previously, my alternator never seems to get hot, but I've also noted occassions when I've been running the ac on high and then stop at a light or when stopped in traffic and have noticed the engine battery voltage drop 1 -2 volts (to as low as 11.9 volts) briefly. Voltage recovers once off idle and never drops when driving. I'm assuming on these rare occassions that my alternator is not keeping up with demand of my DC to DC charger and is briefly pulling current from my engine battery in these cases. To me, this is a sign I'm at (or slightly over) the capacity of my alternator.

For the reason above, and the fact your alternator is rated significantly less than mine, I'd recommend only a 20-25A DC to DC charger in your case. Theoretically, it will give you 100 amps of charging in five hours of driving which would be 1/2 to 1/3 of your battery capacity. I'm always above above 50% capacity overnight (usually 75-80%) so this would keep me topped up since I drive at least that long daily.

[markopolo]

Originally Posted by SteveJ

It would be interesting to see the voltage numbers. Before I replaced my diode isolator with a two way Blue Sea separator I would get voltage readings in the mid 14 range on both feed lines from the isolator with a voltage in the low 15 range on the input to the isolator. This on a 2000 Chevy.

With the Blue Sea separator I'm reading low to mid 14 volt range at the cabin battery when first charging tapering to mid 13 range as the battery gets charged.

My thinking is that there is a pick up area somewhere in the harness that tells the voltage regulator how much voltage to put out. Case in point: We had a 94 Olds that was cooking the battery and electrical system on an intermittent basis with voltages as high as 20 volts. The repair shop, noted for being competent with auto electrics was able to isolate the problem to a corroded wire connection. It was giving a falsely low voltage reading to the voltage regulator, hence the higher system voltages.

To others who have or had a diode isolator, what are/were the voltage readings at the isolator terminals?

GM CS series alternators ( CS-130D & CS-144) have a voltage sense wire that connects to one of the outer isolator posts. You end up with 5 wires on the 4 post type isolator. The center post is around 0.75V higher than the outer posts. 15.25V center post and 14.5V outer posts on mine on a cool alternator with already fully charged batteries.

I think Dodge vans of that vintage use a 3 post isolator and would have the 0.7V or so drop. Someone with an older Dodge could let us know.

[SteveJ]

Originally Posted by markopolo

GM CS series alternators ( CS-130D & CS-144) have a voltage sense wire that connects to one of the outer isolator posts. You end up with 5 wires on the 4 post type isolator. The center post is around 0.75V higher than the outer posts. 15.25V center post and 14.5V outer posts on mine on a cool alternator with already fully charged batteries.

I think Dodge vans of that vintage use a 3 post isolator and would have the 0.7V or so drop. Someone with an older Dodge could let us know.

Thanks.

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