Aux Battery Install and Cable Route

430 hp 70

'19 144 Gas Passenger - Carlsbad
I have a '19 144 gas passenger w/out the factory aux battery and have been researching the heck out of wiring the last few weeks. I've searched and read everything that I could find, but I'm still left with some questions.

Here's the plan -

Remove rearmost seat and build side cabinets over the wheel well and under the windows. Add a single aux agm battery behind the lt rear tire inside the new cabinet (vent through new hole in floor) . This will feed a fuse block to power a small 10a water/washdown pump with 17gal tank, 20a air compressor, all the stock, but rewired rear passenger usb ports and the added usb and 12v ports on the cabinets. Power ports will have constant power. I'm building a roof rack and may add flood lights to it as well.

Wiring-

Fab a little jumper inside the 150/1 (pic below) that's attached to the battery and add a 150 amp fuse. Run 2 awg wire from the starter battery to drivers seat base and connect to a si-acr. Run 2awg from acr to new battery. Neg of battery to a tie down anchor point now unusable due to cabinets. 4awg (i have a bunch of extra) from battery to new fuse block.
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Questions-

What is the best path for the 2awg? There is no path through the side wall since it's solid between the two windows. I'd rather not drill through this solid point, I'd rather not go underneath the van and I don't want to go over the window if it's not necessary. There is a gap a little over 3/8 between the flooring and wall. Any reason not to lay the cable in that void? It runs front to back, is covered visually by panels and doesn't have sharp edges.
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My fuse block has a spot for grounds and I plan to run all back to here. Would it matter if I connect this bus bar to the battery negative or to the same battery ground point on the anchor? I read it's best not to connect grounds directly to battery neg. Something about ground loops. I don't understand how connecting it all to the same body ground is any better than bat neg.

Is 2awg enough? It's what the Mercedes pdf calls for when adding a battery under the hood. But I'm guessing that's a <8' run. I'm at 12-14' if I can run in the channel. It will be closer to 25' if I have to go over the window. Blue seas calculator says 2awg is enough for 150a and 3% at 15' and 5% at 25'. I will never come close to 150a draw, but I still can't find max charging amps. If I undersize with proper fusing, is the only downside a slower charge?

Mercedes states no more than an additional 40a on the charging circuit. What is the additional charging amps the alternator would need to supply? Battery will be costco/interstate 95 ah agm. Do I need a limiter? The factory aux battery setup doesn't appear to have one.

There will be more questions and thanks in advance for any help.
 
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Kevsuda

Active member
You could lay the wire in the void, however 2 awg might be a tight fit in a 3/8 gap. If you go that route, consider widening that gap. Lots of us pull the floor to insulate or whatever, you might want to trim the floor on that side. The fuse block ground can run to the battery or grounding point. There are quite a few variables that may determine which path is best. I myself installed a ground bus about a foot from the battery. Only a single negative wire going to the battery and that is the shunt for the monitor. Your 95 AH battery will have it's own resistance and you shouldn't have a problem with the SI-ACR, though will likely exceed the 40amp recommendation at times. Many here do this and there are no reported problems.
 

Kajtek1

2015 3500 X long limo RV
Don't you have space in engine compartment on driver side?
I put 2 house batteries in my Sprinter and you can buy designated shelf for battery there, but I did not want to spend $200, so pulled plastic try on Junk Yard and adopted it for the purpose.
Run cable under the floor mats to the driver seat pedestal, when I have battery separator with fuse connecting it to main battery. Than another cable goes back to the house and 2nd battery under the bed. .
Using space in engine compartment is not only space saving, but helping with weight distribution as well.
 

430 hp 70

'19 144 Gas Passenger - Carlsbad
If you go that route, consider widening that gap..... Your 95 AH battery will have it's own resistance and you shouldn't have a problem with the SI-ACR, though will likely exceed the 40amp recommendation at times. Many here do this and there are no reported problems.
Thanks. A simple rabbit cut on the bottom 80% would probably give it plenty while not widening the top. If I pull the floor to cut the groove wider, I may cut at the seam behind my 2nd bench and remove the flooring in the back and replace with something else.

Good to know about the charging. Someone posted that the 40a was just an old CYA move by mercedes.

Don't you have space in engine compartment on driver side?
I put 2 house batteries in my Sprinter and you can buy designated shelf for battery there, but I did not want to spend $200, so pulled plastic try on Junk Yard and adopted it for the purpose.......
Using space in engine compartment is not only space saving, but helping with weight distribution as well.
I do have space and that is an option from the factory, but like you mentioned the tray is 200 bucks. I was also thinking the opposite as far as weight is concerned. I pulled a rear seat and the back end feels light. Being a 2wd I thought it wouldn't hurt. But, I definitely need to look in to the placement more.
 

Kajtek1

2015 3500 X long limo RV
The rear is light, but it is also long.
I had 2 passengers on my rear dinette, who might be 350 lb plus I think my furniture and water adds 500-700lb and they say the suspension was bottoming out.
I did not feel it in front, but those vans are not build for heavy loads.
 

Kevsuda

Active member
A little more on charging amps. The SI-ACR(I have one) does not have any way possible to adjust charge current. That is all up to your battery, alternator and wiring. The ACE does have limits, but those indicate when the unit will get hot and start to malfunction. That is why battery to battery chargers are becoming so popular. It is a little hard to tell your understanding of the way these things work. A fuse is a poor way to limit current of your system. They should be considered a safety item more than anything. You don't want your fuse to blow every time you have a high spike of current. You want it to blow should your 2 awg cable wear through and short to ground. You could very well see 100+ amps into your depleted house battery for a few minutes. Also, regarding cable size, amps should not be your main worry with anything over about a 6 awg. The %'s quoted in your first post (3% & 5%)are for voltage drop. Voltage decides whether or not your battery is charging whether you have 2 amps or 40 amps available to the battery. If your alternator is producing current at 14.4 volts and by the time it gets to the battery you only have 13.8 volts, the battery won't be accepting much of a charge.
 

Kajtek1

2015 3500 X long limo RV
Been using battery separators for over 30 years and never had issue with house battery charging. Older insulator did have some issues, but once electronic-controlled relays come on the market, that makes pretty bulletproof system.
Suppose nickel batteries require higher voltage, so I can see you might need B2B charger in such case, but why complicate the system with no obvious need?
 

430 hp 70

'19 144 Gas Passenger - Carlsbad
A fuse is a poor way to limit current of your system. They should be considered a safety item more than anything. You don't want your fuse to blow every time you have a high spike of current. You want it to blow should your 2 awg cable wear through and short to ground. You could very well see 100+ amps into your depleted house battery for a few minutes. Also, regarding cable size, amps should not be your main worry with anything over about a 6 awg. The %'s quoted in your first post (3% & 5%)are for voltage drop. Voltage decides whether or not your battery is charging whether you have 2 amps or 40 amps available to the battery. If your alternator is producing current at 14.4 volts and by the time it gets to the battery you only have 13.8 volts, the battery won't be accepting much of a charge.
Thanks for the reply. I am not trying to limit current with a fuse, rather trying to figure out if an additional limiter is needed to keep the charging current under 40a. It sounds like there isn't a concern.

I'm also trying to understand the severity of charging lag in relation to voltage drop. I understand that low current on a large cable will have very minimal drop. But, without knowing the possible charging current, it makes calculating drop difficult. You state that a depleted house bank may see 100+ amps for a short period. 100a on 25' of 2awg is about 4%. Is this a concern? My reading says no, but what will this equate to in additional charge time?

Thanks again for the help.
 

Kevsuda

Active member
OK, I haven't had a problem with the unregulated ACR. Neither have I read where anyone else has. Some take the 40 amp recommendation more serious than others. You have to decide where that fits with your thoughts. My point of high currents(100+) has more to do with your fusing and where I thought you were possibly misunderstanding the role of a fuse. In reality, you shouldn't need to worry about 100 amps of charging into your 95 AH battery. It may happen for a few minutes but will quickly drop and the majority of your charging time will be much less amperage. You don't need to create a system designed to charge at a sustained 100+ amps. Whether you chose a system that can efficiently charge at 100, 75 or 50 amps(based on your calculator) is up to you.
There have been posts here and you can find on you tube where people measure the alternator current to the battery. It starts real high but almost immediately begins to taper fairly rapidly.

Oh and 4% loss is not an efficient system.
 
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