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Old 09-20-2017, 08:37 PM   #1
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2nd Alternator Research

Thought I'd post some of my findings here.

The stock Merc electric system comes with up to 3 different alternators : a 180/200/220. I believe the 200 is typically provided with the Aux Battery kit...or it can be ordered separately. The 220A (M46) is usually provided with the Aux battery + rear A/C options. Someone who knows these things better might have more info...(this is a thread about adding a 2nd alternator, not Merc options.)

Per the upfitter, there is a 100ah limit on the amount of battery that can be placed on the Sprinter electrical bus. This corresponds with the factory aux battery @ 100ah.)

The upfitter manual states (1) do not directly connect more than 100ah to the vehicle electrical system (2) that no add'l batteries may be installed w/o limiting the charging current to 40A.

This is presumably to prevent the alternator bus from being swamped/voltage collapse in the case of a highly discharged battery. I know from my own testing, that a large bank can sink well over hundreds of amps if sufficiently discharged....so this makes sense. (Typical lead acid RV banks can sink 180-200A easily). As lead acid batteries charge, they quickly start accepting less current for a given charge level than lithium....but that is another discussion.

Per the Merc BEG, the stock alternator produces around ~75A at idle and tops out around 200-220A @>3000 engine RPM depending on alternator temp. Based on info from others, the stock electrical MB demand can exceed 130A at idle. (I have not confirmed this yet).

Edit: ~75A is wrong. It produces ~116-~140A at idle


Regardless of the stock demand & alternator size (mine is a 220A M46), I will need a secondary alternator to supply power for my rear electric AC and lithium bank.

Cheapest way is to order an 2nd alternator bracket from MB and then install the balance of parts from a 3rd party.

Most commonly used 2nd/aux alternator appears to be sourced from Nations Alternator.

My undertstanding is that SMB uses this alternator.
https://www.nationsstarteralternator...-Van-s/773.htm

This is a '280A' alternator in a relatively small case that can produce up to 280A when the engine is around 3000RPM or higher.

Nations was supplying this alternator to a few other high-end conversion companies with very large lithium battery banks.

However, I've heard/read 2nd hand anecdotal info about the Nations units not being able to handle continuous duty applications (like those imposed by large lithium banks, car stereos, etc). Large lithium batteries banks can sink all the current alternator can provide. When coupled with other DC loads like inverters and/or DC air conditioners, the alternator may be at 100% for quite a while. Traditional automotive alternators are not designed for this use case.

I also looked at a few of the more well known 'high amperage alternator companies'. most seem to cater to stereos, motor sports, etc. None really seemed to focus on industrial/institutional clients (suppliers like a DelcoRemy, Denso) with products used in military,
or OTR applications. This bothered me as I am familiar with all the hype/games/crap prevalent in the 12V DC world (like stereos & lighting).
https://www.dcpowerinc.com/
Mechman Alternators

MG Industries Inc.


This led me to other alternator suppliers and possible larger case sizes. There are some companies (Advanced RV) who discovered this early with their massive lithium battery banks. They switched from Nations and other similar products to use a massive 40SI Delco-Remy used in semis/OTR trucks



My research led me to several niche companies who deal with large electrical vehicle loads (the high-rel world of ambulances, armored vehicles, goverment/DoD/Homeland applications).

Two companies produced specialized alternators of all sizes (including the massive Delco Remy 40SI units) - none would install a 40SI, an alternator of that size onto a Sprinter sized engine.

They claim there is not enough mass/strength in the block & pully system for long term reliability. The idling behavior of our diesels, dynamic loads (A/C compressors, etc) placed on the belts etc - they claim are too much for such a large alternator on such an small engine.

I spoke to a few bracket suppliers/manufacturers as well - they confirmed these concerns. Without working with the engine designers, retrofitting large loads can be problematic with the block - and on high RPM diesels the shock loading can be very high.

This conclusion is confirmed by the BEG, stating the max size & weight of a PTO is 8-8.5kW and around 15# or so. The 40SI alternator is more than 2x this weight!

A couple of the more specialized alternator companies mentioned an 'alternator clutch' is req'd on high amperage alternators used on small diesels. Advanced RV calls this a 'dynamic torque converter'. The industry calls this an Overrunning Alternator Decoupler pulleys (OAD) or an alternator clutch - it has been used for years and is simply a device that reduces the shock loading in the belts. It has a rubber/polymer core that reduces shock/vibe on the belt/alternator system. Nothing really new here. What are OAPs and OADs? And How Do They Fail?.

I have settled on American Power Systems Inc to supply a '380A' alternator. I'm wtg on a formal quote before proceeding. They have a decent warranty and have a relationship with a bracket supplier (should my secondary A/C bracket not work properly).

As I get more info, I will share.

Link to 2016 MB BEG - see p 49 for alternator curve. I can't upload the doc here.

If there are any alternator experts out there, would like to hear your perspective on the above.
Attached Files
File Type: pdf mercedes-dualkit-instructions.pdf (1.68 MB, 3 views)
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Old 09-20-2017, 10:14 PM   #2
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Excellent write up...
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Old 09-20-2017, 10:16 PM   #3
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Originally Posted by VtSoundman View Post
Per the Merc BEG, the stock alternator produces around 130A at idle and tops out around 200-220A @>3000 engine RPM depending on alternator temp. Based on info from others, the stock electrical MB demand can exceed 100A at idle. (I have not confirmed this yet).
That's insane! That's a lot of demand.

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Originally Posted by VtSoundman View Post
I have settled on American Power Systems Inc to supply a '380A' alternator. I'm wtg on a formal quote before proceeding.
That's also insane! That's lots of juice.

Lots of good info here. Thanks for sharing.

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Old 09-20-2017, 11:22 PM   #4
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That's insane! That's a lot of demand.
What is insane is trying to run the AC compressor from batteries.
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Old 09-20-2017, 11:37 PM   #5
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It may seem insane...here is why...
1) I work in utility scale solar & large scale energy storage systems. My van is also used a mobile office & conference room ... rolling out a genset/hearing/smelling one for a 30-90min mtg on a hot job site...nah.
2) My dogs need to be kept cool for a few hours while I traveling and I'm in a store/in an office/etc
3) I hate gensets

But let's get to the engineering:
A 380A alternator doesn't provide 380A of usable current...

380A is a peak value assuming the alternator is cold and the engine is running at full throttle. Alternator based power systems are typically designed assuming idle to slightly above idle-current conditions. Otherwise the battery will be discharged/wiped out. A good baseline is to assume ~33-40% of the full HOT current rating. (See attachment in this link as a reference - correction factors per some automotive electrical engineers).

The permissible alternator case size (physical case) for sprinter engines doesn't lend itself to constant duty applications...and therefore the alternator (electrical) size needs to be oversized it at all possible.

My load is a ~1.2-1.3kW DC Air con + a DC fridge + a few other small appliances (heater)+lights...and a 400-600ah Lithium battery stack. The battery stack, when fully discharged will heavily tax the alternator. The primary issue is that I have not been able to find a field current regulator that can be controlled remotely via a BMS ... so as to regulate the current into the lithium stack. At 24V the HVAC is less efficient and support hardware (inverter/chargers) are more expensive. I would also need a buck converter to run any 12V only appliances.

Therefore, I am designing the system for a nominal load of less than a third the full rating of the alternator ... Anything above that, will simply dump into the Lithium batteries...or not used. The Nations unit at 280A at cold conditions would be right around 45-50% of its full load rating. The Nation's datasheet claims ~190A at idle - but have been told by many that doesn't make a great deal of sense assuming a full load capability of ~280A (this means its idle current is 67% that of full load).

For example: the sprinter's '220A' alternator will produce *60-75A* at idle (Stop light, in traffic etc). Per the MB BEG, idle current is *~34-40%* of full rated output.


EDIT : *indicates some wrong info...

Merc BEG states alternator output
116-137A @650RPM
125-150A @ 700 RPM
137-162A @800RPM.

This supports the 130A base load at idle that I've seen/heard about.
However, it supports the assertion that idle rating is <50% of full rating. I've been told the Merc alternator is a very good unit.

If the vehicle load exceeds *~75-80A* , the vehicle will begin to draw from the battery and will eventually kill it. This is why MB requires an add'l battery for add'l electrical loads, especially when a rear A/C is added.

Alternator systems are a balance between nominal RPM and idle loads. At idle, I will be running a DC powered HVAC system (stop light, heavy traffic, etc). This consumes ~100A on high (~1.2-1.3kW). The batteries will also be sinking current - as much as the alternator can produce. The alternator's regulator will respond by increased the field current to maintian voltage...but the alternator cannot produce the necessary power...so the voltage will stay suppressed. This can lead to smaller alternators overheating & failing.

380A is possible at 25degC / 20degF at full engine/alternator RPM. However, that's not the correct size/think about alternator systems. Alternators produce current that follow a curve dependent upon engine RPM - and its ability to produce current is dependent upon a couple of factors (look at p 49 of the MB BEG attached above):

1) temperature
2) field current
3) RPM

Worst case would be a partially/fully discharge lithium battery and running rear DC powered A/C on high. The A/C consumes ~100A at full bore. Figure I will have another couple of small items running at the same time. So at idle, the alternator will be required to produce a min of 120A to supply the AC, a few other devices WITHOUT continuing to discharge the batteries....and not KILL itself in the process.


Wire sizing:
High temp, high flex cable will be run and sized for ampacity at 380A assuming a 2-5% drop (https://www.wirebarn.com/Wire-Calculator-_ep_41.html) As I am not concerned about voltage drop at full RPM, I will size the conductors a bit differently. Perhaps two 1/0 or maybe a single 2/0 should be plenty. I am more worried about voltage drop a mid & low RPM when the system will be most taxed. (ex: MB did not side the alternator 'Y' cable assuming 220A & optimal voltage drop...it is quite a bit smaller)

Does this make sense?
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Old 09-20-2017, 11:42 PM   #6
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Originally Posted by 86Scotty View Post
That's insane! That's a lot of demand.



That's also insane! That's lots of juice.

Lots of good info here. Thanks for sharing.

I know...I need to verify the 130A load...it does not support the alternator rating at idle...
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Old 09-20-2017, 11:52 PM   #7
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F-550, 650 and 750 trucks as well as the V10 on the Econoline can be ordered with an optional extra cog in the tranny that acts as a PTO. The PTO can drive a generator whose housing mates to the tranny housing such that it looks like a single unit. The system comes with a high idle. This setup is used in some ambulances and in FD mobile command center vehicles to power the huge lighting systems that are set up to illuminate the scene around fires at nighttime. The PTO/cog, generator, and high-idle gear total around $6k, which actually is about the same as Onan diesel generators carried on Class A motorhomes. (The same PTO/cog can drive water pumps on FD pumper trucks.)

With the number of Sprinters being used in these type applications, I am surprised MB has not considered (though they would put out less juice) a similar system on a smaller scale. It really does sound like a job for a generator rather than an alternator.

I wonder if instead of an aux diesel tank some of us should think about on-board propane and a propane compatible small generator. Or maybe fewer house battery amps.

(Note: VtSoundman made his last 2 posts during the time I was typing this. You've given me more to think on.)

Edit: Correction, on the V10 gasser the power comes from the torque converter, not the extra gear cog, and the system cannot run at idle like the higher F series diesels.
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Old 09-21-2017, 12:01 AM   #8
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Originally Posted by TomH View Post

(Note: VtSoundman made his last 2 posts during the time I was typing this. You've given me more to think on.)
I also fixed a ton of typos in my first post. Sorry about that folks.
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Old 09-21-2017, 12:11 AM   #9
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WILL SMB install this 380 amp alternator? Are you sure it will fit? How much is the mechanical load going to lower MPG?
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Old 09-21-2017, 12:13 AM   #10
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Originally Posted by VtSoundman View Post

For example: the sprinter's '220A' alternator will produce 60-75A at idle (Stop light, in traffic etc). Per the MB BEG, idle current is ~34-40% of full rated output.

If the vehicle load exceeds ~75-80A , the vehicle will begin to draw from the battery and will eventually kill it. This is why MB requires an add'l battery for add'l electrical loads, especially when a rear A/C is added.
This is incorrect. Had made an error in my One Notes....my first posting also has some incorrect.

Merc BEG states
116-137A @650RPM
125-150A @ 700 RPM
137-162A @800RPM.

This supports the 130A base load at idle that I've seen/heard about.
However, it supports the assertion that idle rating is <50% of full rating. I've been told the Merc alternator is a very good unit.
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