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.
Linear Mode
