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Old 06-03-2023, 12:58 PM   #1
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To Use A DC-DC Converter Or Not? ...In My Diesel RV (2004 with ISC-350)

I talked with on solar tech who specializes in RV customized solar systems. He has been in this business for over 10+ years, and he is a Vicrton dealer-installer. He also is an Onan dealer-installer and I really was surprised when he told me this: "I no longer will install a DC-DC (in RVs) because I have seen too many alternator failures over time."

This comment really got me thinking, but I don't have enough experience to confirm or deny the above, but I think these points are valid. Are they?

1) I know my 2004 RV, with a Cummins ISC350, uses a 160A Delco alternator, because I replaced the OEM Leece Nivell Alternator about 4 years ago. I also think this alternator maybe rated for 160A, but for efficiency reasons the engine battery will only see about 50-60% of this, which means my 160A alternator is only capable of delivering 80A to maybe 96A.

2) I believe, but don't know, if my RV chassis battery current draw is in the 40-60A amp range, but I think this is correct. So, this would suggest I only have ~20A maybe 30A to spare, safely.

3) Also, the gauge of the alternator wire that is connected to my chassis battery seems quite small for the amount of current is carries. Maybe 14 gauge? IDK.

==> That said, if I add a DC-DC to charge a bank of 100AH or 200AH (LiFeP04) batteries, is it absolutely necessary to run a thicker gauge wire from my alternator to the chassis battery?

4) 7 years ago, I added a KeyLine Voltage Sensitive Relay (VSR) that combines my 420AH, FLA house batteries to my 2-950CA engine batteries; and this setup works great!

So, already, I think I am using additional amps off the alternator to charge my house battery bank, which probably reduces my RV's available alternator amps to only 10-15A, instead of the 30A I calculated earlier without the VSR in play, but this is just a guess, as I have no way of knowing how many amps my alternator is putting out? I.e., I have no way of knowing how hard my alternator is currently working or how hot it is getting?

QUESTIONS

Assuming the above parameters are correct: Why would anyone install a 30A or 40A DC-DC Converter? Heck, I'm not even sure I should install a Victron Orion 12/12 - 18A DC-DC converter?

So, is the professional tech right? Should I avoid installing a DC-DC converter, or can I get away with a small 18A DC-DC converter, without risking a premature alternator failure down the road?

Fundamentally then: Does installing a DC-DC regularly lead to early alternator failure in large RVs which use 300HP+ motors and presumably 160A alternators?
...Or is this only a problem for my setup, because I am already using a VSR to charge my house batteries off the chassis battery which is being charged by the alternator?
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Old 06-03-2023, 01:36 PM   #2
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Quote:
Originally Posted by 2004Horizon View Post
I talked with on solar tech who specializes in RV customized solar systems. He has been in this business for over 10+ years, and he is a Vicrton dealer-installer. He also is an Onan dealer-installer and I really was surprised when he told me this: "I no longer will install a DC-DC (in RVs) because I have seen too many alternator failures over time."

This comment really got me thinking, but I don't have enough experience to confirm or deny the above, but I think these points are valid. Are they?

1) I know my 2004 RV, with a Cummins ISC350, uses a 160A Delco alternator, because I replaced the OEM Leece Nivell Alternator about 4 years ago. I also think this alternator maybe rated for 160A, but for efficiency reasons the engine battery will only see about 50-60% of this, which means my 160A alternator is only capable of delivering 80A to maybe 96A.

2) I believe, but don't know, if my RV chassis battery current draw is in the 40-60A amp range, but I think this is correct. So, this would suggest I only have ~20A maybe 30A to spare, safely.

3) Also, the gauge of the alternator wire that is connected to my chassis battery seems quite small for the amount of current is carries. Maybe 14 gauge? IDK.

==> That said, if I add a DC-DC to charge a bank of 100AH or 200AH (LiFeP04) batteries, is it absolutely necessary to run a thicker gauge wire from my alternator to the chassis battery?

4) 7 years ago, I added a KeyLine Voltage Sensitive Relay (VSR) that combines my 420AH, FLA house batteries to my 2-950CA engine batteries; and this setup works great!

So, already, I think I am using additional amps off the alternator to charge my house battery bank, which probably reduces my RV's available alternator amps to only 10-15A, instead of the 30A I calculated earlier without the VSR in play, but this is just a guess, as I have no way of knowing how many amps my alternator is putting out? I.e., I have no way of knowing how hard my alternator is currently working or how hot it is getting?

QUESTIONS

Assuming the above parameters are correct: Why would anyone install a 30A or 40A DC-DC Converter? Heck, I'm not even sure I should install a Victron Orion 12/12 - 18A DC-DC converter?

So, is the professional tech right? Should I avoid installing a DC-DC converter, or can I get away with a small 18A DC-DC converter, without risking a premature alternator failure down the road?

Fundamentally then: Does installing a DC-DC regularly lead to early alternator failure in large RVs which use 300HP+ motors and presumably 160A alternators?
...Or is this only a problem for my setup, because I am already using a VSR to charge my house batteries off the chassis battery which is being charged by the alternator?
You have a lot of ifs. How about measuring your various consumers currents (amp meter) and the charging amps of your alternator? And then recalculate.
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Old 06-03-2023, 01:49 PM   #3
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I may have to do this, but I was hoping someone else already has done this evaluation.

There are a lot of large diesels out there with similar configurations. Maybe I should have summarized this way:

If you installed a DC-DC converter, how may miles have you been running on it without failure and without any consequences?
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Old 06-03-2023, 02:14 PM   #4
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To Use A DC-DC Converter Or Not? ...In My Diesel RV (2004 with ISC-350)

Another option is to install an alternator temperature monitor. I just finished mine yesterday, simple normally closed 120degC (248f) thermal switch KSD 9700 and epoxy to alternator case, it provides ground to a green led on dash powered by ignition circuit.

If led goes off reduce load on alternator. In my case I would shut off my roof a/c running on inverter.

Member Mechengrsgh on here came up with this design so its proven in use and less than $25 diy.

Up until now Iíve assumed up to about 50% -60% load on alternator is ok and with my roof a/c pulling about 150 amps from a 340 amp alternator plus normal loads ive spot checked the temp and itís usually around 210 but I will be more comfortable now with the monitor as loads and ambient temps change.

If yours is overheating, you could go with a larger amp alternator, I went from 160 to the 340 large frame and only had to modify the bottom bracket and longer belt.

I still carry the old alt, belt and bracket just in caseÖ
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Old 06-03-2023, 05:38 PM   #5
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Quote:
Originally Posted by 2004Horizon View Post
I talked with on solar tech who specializes in RV customized solar systems. He has been in this business for over 10+ years, and he is a Vicrton dealer-installer. He also is an Onan dealer-installer and I really was surprised when he told me this: "I no longer will install a DC-DC (in RVs) because I have seen too many alternator failures over time."
.
.
.
Fundamentally then: Does installing a DC-DC regularly lead to early alternator failure in large RVs which use 300HP+ motors and presumably 160A alternators?
I completely disagree with properly installed DC-DC chargers causing alternator failures !!

Thousands have been sold. I doubt there have been thousands of alternator failures.
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Old 06-03-2023, 07:06 PM   #6
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Love your user name: theoldwizard.

I catch your point, but alternators are not an infinite source of amps. So where is the line drawn?

Are you saying anyone can install a 40A or 60A Renogy DC-DC Charger in their Diesel 160A alternator and not worry about a thing? Or are you just the man behind the curtain?

Since you "completely disagree" ...I need more to go on than just your namesake. Moreover, just because everyone else is using a DC-DC Converter doesn't back up your position.

Here's what I have to offer based on my research. WHat have you got?

The 'free' power that can be continuously taken safely from the alternator is small.

Causing an alternator to run at maximum output for long periods is the #1 cause of diode and voltage regulator failure.

Trying to charge a completely dead battery by letting your engine idle is even worse. Why? ...You’re forcing the alternator to put out lots of current at a low speed. That’s the best way to fry the diodes.

About wire thickness?

A: A 20A load over 30 feet (out and back) on 8AWG copper wire will have a voltage drop of ~0.37V which is just over 3% of a 12V system.

Many folks use <3% as a target for "critical loads". Use this calculator to determine the right gauge wire to run from your alternator to your house batteries:

Here's a calculator to determine how thick of a wire you should use:
https://baymarinesupply.com/calculator

PS: Everyone is a "wizard" until they are not. Let's see what happens when someone asks a question that has not been asked before?
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Old 06-04-2023, 06:51 AM   #7
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Exactly what your system can support depends on many issues. There are many possible failure modes. The primary one is overheating and cooking the alternator components. Some have been photographed and published showing the windings glowing. They don't last long at that temperature.

Jon's suggestion of monitoring temperature is a good one. Hopefully you will recognize what temperature is too high.

Monitoring current out of the alternator is a second best method. Or, better yet, do both.

A 160 amp alternator is not particularly large. It may already be operating near its maximum under specific conditions. Adding a larger wire or drawing more current may push it over the edge at some unforeseen event.

The engine technician may have been suggesting that the specific engine system is not capable of more output. Some systems can support a B2B converter. All engine systems are not the same.

The design of the B2B converter also matters. Designers can make their creations walk, talk, and blink their big blue eyes at you. Some are better for one application. Some are better for another.

Some B2B converters are capable of pulling full rated output from the engine system even when voltage is low and alternator is dropping voltage due to high output. Most alternator systems are not protected against high output.

Trust the technicians experience.
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Old 06-04-2023, 08:16 AM   #8
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Quote:
Originally Posted by 2004Horizon View Post
Love your user name: theoldwizard.

I catch your point, but alternators are not an infinite source of amps. So where is the line drawn?

Are you saying anyone can install a 40A or 60A Renogy DC-DC Charger in their Diesel 160A alternator and not worry about a thing?
YES !

Following the manufacturers (Renogy) instructions the DC-DC charger. A 60A charger consumes about 750W. At 10V input (lower than normal) that is about 75A input, which is quite a lot ! The only question is, does your vehicle alternator have this much spare power ? Maybe you should use the 40A charger.

Renogy DC-DC chargers do shut off if the input gets too low (8V).

(30+ years of automotive electrical engineering.)
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Old 06-04-2023, 08:24 AM   #9
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Originally Posted by 2004Horizon View Post
Love your user name: theoldwizard.

I catch your point, but alternators are not an infinite source of amps. So where is the line drawn?

Are you saying anyone can install a 40A or 60A Renogy DC-DC Charger in their Diesel 160A alternator and not worry about a thing? Or are you just the man behind the curtain?

Since you "completely disagree" ...I need more to go on than just your namesake. Moreover, just because everyone else is using a DC-DC Converter doesn't back up your position.

Here's what I have to offer based on my research. WHat have you got?

The 'free' power that can be continuously taken safely from the alternator is small.

Causing an alternator to run at maximum output for long periods is the #1 cause of diode and voltage regulator failure.

Trying to charge a completely dead battery by letting your engine idle is even worse. Why? ...Youíre forcing the alternator to put out lots of current at a low speed. Thatís the best way to fry the diodes.

About wire thickness?

A: A 20A load over 30 feet (out and back) on 8AWG copper wire will have a voltage drop of ~0.37V which is just over 3% of a 12V system.

Many folks use <3% as a target for "critical loads". Use this calculator to determine the right gauge wire to run from your alternator to your house batteries:

Here's a calculator to determine how thick of a wire you should use:
https://baymarinesupply.com/calculator

PS: Everyone is a "wizard" until they are not. Let's see what happens when someone asks a question that has not been asked before?
An alternator with a voltage sense wire was the answer to low voltage 70 years ago. The voltage is sampled at the batteries instead of at the alternator output.
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Old 06-04-2023, 08:31 AM   #10
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The 'free' power that can be continuously taken safely from the alternator is small.

Causing an alternator to run at maximum output for long periods is the #1 cause of diode and voltage regulator failure.
More correctly, it is the heat as Jon_C pointed out.
Quote:
Originally Posted by 2004Horizon View Post
Trying to charge a completely dead battery by letting your engine idle is even worse. Why? ...Youíre forcing the alternator to put out lots of current at a low speed. Thatís the best way to fry the diodes.
Actually, no ! The alternator is specially designed to handle this load.
Quote:
Originally Posted by 2004Horizon View Post
About wire thickness?
The DC-DC charger should be mounted as close to the battery bank as possible. The charger itself makes up for any voltage loss (long engineering explanation).
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Old 06-04-2023, 08:37 AM   #11
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An alternator with a voltage sense wire was the answer to low voltage 70 years ago. The voltage is sampled at the batteries instead of at the alternator output.
More specifically, the alternator (or more likely on vehicles built in the last 20+ years, the PCM) sense the vehicle battery voltage and adjust the output voltage of the alternator to properly charge the vehicle battery. They actually cut back the charge voltage once they sense that the vehicle battery is fully charged.

A DC-DC charger does something similar, which is why it should be mounted close to the battery bank (some DC-DC chargers have a remote battery voltage sending wire if yo can not mount it close by).
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Old 06-04-2023, 08:44 AM   #12
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More specifically, the alternator (or more likely on vehicles built in the last 20+ years, the PCM) sense the vehicle battery voltage and adjust the output voltage of the alternator to properly charge the vehicle battery. They actually cut back the charge voltage once they sense that the vehicle battery is fully charged.

A DC-DC charger does something similar, which is why it should be mounted close to the battery bank (some DC-DC chargers have a remote battery voltage sending wire if yo can not mount it close by).
Totally agree. And due to lack of reliability there are kits to put the sensing back to a regulator instead of the pcm with certain makes.
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Old 06-04-2023, 08:57 AM   #13
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I’m with theoldwizzard, this “solar expert” you talked to doesn’t know his stuff. Not installing a DC2DC charger can burn up your alternator. Not the other way around.
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Old 06-04-2023, 10:22 AM   #14
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All good comments. Thank you for sharing.

The "solar expert" says he will no longer install DC-DC converters, because he says he has seen so many alternator failures and does not want to be "on the hook" after a solar/lithium upgrade for making good on a DC-DC failure... which takes out the alternator and costs him big bucks to remedy. Not to mention who pays for the cost of an RV tow.

So, yes, all these concerns got my attention since I am considering how to add 800W of solar and 200AH of LiFeP04 batteries to my rig, and up to this point I had not given much thought to adding a DC-DC and I had not considered what the risks are if I do?

In my world, field experience counts just as much at design experience. So, I'm still on the hunt for answers. And that's what I like about this forum. So, many thanks to all who share.

Victron has a nice video that has become a solid reference on what happens to to "over taxed" alternator at low RPM when charging lithium batteries, so I do think this is a concern.

I have also read comments about how hot Victron DC-DC Converts can get. And Renogy uses a fan to cool down their DC-DC charger. (Does anyone have any comments about this?)

In any event, it sounds like I will need to take some actual measurements of my alternator's charging system to know how much "Amp-Headroom" is left in my alternator, before I select which 20/30 or 40A DC-DC converter to install as I do not want to experience a premature alternator failure down the road, and I certainly don't want to pay for an expensive tow.

NEXT STEPS

I will turn off my existing 400W of solar; and take the following measurements with my existing 12V-FLA house batteries at 50% SOC:

1) Measure the alternator's amp output under full load at idle and 1500RPM.

2) Drive for about 30 minutes and measure alternator temperature and amp output.

If anyone has already done this, please share your findings as I will not be able to take measurements on my RV charging system for another 30 days or so.

Thank you!
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