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Old 06-02-2015, 10:29 AM   #1
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Second Alternator for Charging House Batteries

A few of the manufacturers are adding second alternators to the Mercedes Sprinter to charge the house batteries instead of needing a generator. The engine kicks on automatically if the batteries get below a certain point and it charges them relatively quickly. Seems like a great solution especially when you're camping in places that do not allow generators or you want to avoid the noise of the generator. I'm curious if it's possible to add this to the Airstream Interstates and if so, who could do it?

Here are some links for background:
Go Farther Off the Grid: Advanced RV Redesigns Battery System - Advanced-RV
Dual alternators, why or why not .... • Class B RV and Camper Van Discussion Forum
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Old 06-02-2015, 10:37 AM   #2
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A few of the manufacturers are adding second alternators to the Mercedes Sprinter to charge the house batteries instead of needing a generator. The engine kicks on automatically if the batteries get below a certain point and it charges them relatively quickly. Seems like a great solution especially when you're camping in places that do not allow generators or you want to avoid the noise of the generator. I'm curious if it's possible to add this to the Airstream Interstates and if so, who could do it?
With the BIM, that automatically detects charging current and connects the house and chassis systems, can't you already charge the house batteries from the existing engine alternator? I don't know of anyone who does, but in theory it's possible. You would have to "high-idle" the engine to do it, but you'd have to do that with a second alternator as well.

As for who could do the install, in theory any authorized Sprinter upfitter could do it. Airstream (Jackson Center, not individual dealers) is an authorized upfitter, but you might want to check the Sprinter webpage to find someone more local to you.
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Old 06-02-2015, 10:53 AM   #3
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Hi Protag. Yes, you can do that, but my limited understanding is that it doesn't charge nearly as quickly as the second alternators that they are adding purely for charging the house batteries. If you've followed davydd's "Advancing Alvar" posts on the ClassBForum, he's reporting that he can charge his 800ah lithium batteries from about 30% charge in about 2 hours.

You're right about needing to be in high idle.
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Old 06-02-2015, 11:03 AM   #4
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Originally Posted by migriffin View Post
Seems like a great solution especially when you're camping in places that do not allow generators or you want to avoid the noise of the generator. I'm curious if it's possible to add this to the Airstream Interstates and if so, who could do it?
If the campground will not allow a generator it will not allow you to run the engine to charge the coach batteries either.

There is a crossover point in the total battery amperage you have where a second alternator makes sense.

Any good generator/starter or good mechanic shop could do the mode, it is a simple bolt on change.
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Old 06-02-2015, 11:57 AM   #5
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With the right brackets and belts it can be done. It is common to have 2 alternators in tool sales trucks with an on board show room (Mac, Snap-On, S-K) and high power custom car audio. I imagine ambulances, bookmobiles, mobile shoe sales vehicles, mobile hearing test vehicles, and others have 2 alternators.
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Old 06-02-2015, 06:19 PM   #6
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An alternator designed specifically for charging a deep cycle house battery bank is very different than the standard automotive alternator. It needs a special regulator that will provide the proper three-step charging routine (bulk, absorption, float) just like a three-step battery charger/converter. It also needs to be "hot rated" to withstand the type of duty required of it. This kind of setup is routine in cruising sailboats and powerboats, but pretty rare in the RV market. A m.hony mentions, it also is found in mobile applications where demand is high (ambulances, utility line trucks, etc.) and where large battery banks power inverters for running respirators, power tools, etc. It's a specialized and expensive setup, but certainly worth the effort if you're serious about self-sufficient boondocking, which is pretty much like "anchoring out" in a boat.
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Old 06-02-2015, 07:03 PM   #7
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Originally Posted by Protagonist View Post
With the BIM, that automatically detects charging current and connects the house and chassis systems, can't you already charge the house batteries from the existing engine alternator? I don't know of anyone who does, but in theory it's possible. You would have to "high-idle" the engine to do it, but you'd have to do that with a second alternator as well.



As for who could do the install, in theory any authorized Sprinter upfitter could do it. Airstream (Jackson Center, not individual dealers) is an authorized upfitter, but you might want to check the Sprinter webpage to find someone more local to you.

I agree with Protag about the BIM - it is adequate for charging the stock Interstate house batteries. Now if you install a much larger battery pack having a second alternator with separate charge profile might make sense.


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Old 06-02-2015, 10:15 PM   #8
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Check out the Roadtrek E-trek. It uses a chassis driven generator.
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Old 06-02-2015, 11:02 PM   #9
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If anyone is interested in adding a second alternator specifically for battery charging, look up Balmar.

They have a huge presence in the marine industry.


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Old 06-03-2015, 11:27 AM   #10
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Good referral, Lew. When I was installing high-demand charging systems in boats, I used Balmar products a lot.

Welcome to Balmar
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Old 06-05-2015, 12:20 AM   #11
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Going the other way

I am sure this has come up before in one of the many threads about batteries, but can any one tel me for certain (Portag) In a 2012 with the stock triplite charger (and yes I know there are those that would have me replace that first) does it charges the start battery as well has the house batteries?
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Old 06-05-2015, 12:31 AM   #12
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Originally Posted by bennu36 View Post
I am sure this has come up before in one of the many threads about batteries, but can any one tel me for certain (Portag) In a 2012 with the stock triplite charger (and yes I know there are those that would have me replace that first) does it charges the start battery as well has the house batteries?
That is less about the type of charger than it is about the BIM. When the BIM detects that one bank of batteries is charging and the other isn't, it's supposed to connect both battery banks together so they can both charge. Regardless of which is charging or the source of the charge; engine alternator, generator, or shore power. Solar panel doesn't really provide enough charging power to make the BIM do its trick, though, so don't count on solar to keep the chassis battery charged.

When the BIM detects that one battery bank is discharging and the other isn't, it's supposed to disconnect the two banks so they don't both discharge together.

I don't recall the exact thresholds where the BIM connects or disconnects the battery banks, though.
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Old 06-05-2015, 12:41 AM   #13
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Ok the reason for the question

Like many I have spent the last two years (how long we have had it) looking at that spot in the over head and thinking something should go there. So I am going to put a ham radio up there. I am currently studying for my exam and hope to pass before the end of summer. Any way these radios are some what complex and to use most of the functions would take more attention that someone driving should spare. However it states in the installation instructions operation of the radio with out the engin running can deplete the battery in short order. So I was hoping that being plugged in to shore power or running the generator would keep the start battery from being depleted.
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Old 06-05-2015, 12:47 AM   #14
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As Protag said, the generator, shore power, or the engine alternator will keep both the chassis batteries and the house batteries charged.
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