Electrical Question

I have a 2016, NCV3, purchased new. I ordered it with the E28 auxiliary battery option. The auxiliary battery, from the factory, only powers two 12 volt sockets, one at drivers pedestal and one in the rear left pillar. To the auxiliary battery load I've also added one dual USB port in the dash, one 12 volt socket at the back of the drivers pedestal, and a Amp Research power step that opens with every door opening. The USB ports are used regularly for device charging and the 12 volt sockets occasionally for a portable light or a small 12v marine pump for a simple souse vide hot water set up as seen here: https://sprinter-source.com/forum/showthread.php?t=76094&highlight=simple+hot+water

I primarily ordered the E28 option because the MB service manager said they've had so many low voltage related error codes from loads added to the start battery from people using the 12volt sockets for everything. With the auxiliary battery, they rarely have that problem.

When I did my build, I installed two AGM 220ah, 6V, deep cycle (Full River), wired in series as house batteries for a 12V system. Coupled with 300W of solar, CTEK monitoring and control, alternator charging, shore charging, and a magnum 1,000W inverter. This system has worked great for my loads, which are primarily a D2 heater, ARB 60Qt refrigerator, Maxx Air fan, and a couple of small LED lights. Also, occasional use of the 800W Souse Vide heater and a 700W coffee maker. Never get below about 70-80% SOC, even in winter at trail heads for long periods. At the time, I intentionally isolated this system from the auxiliary battery system.

My cooking is almost entirely outside propane, but I'm considering adding a single conduction burner and possibly a crook pot for occasional inside use. These loads are all 120V, so it's not possible to put any of this load on the auxiliary battery. My question is if I should consider combining the house battery system with the auxiliary battery system, or keep them separate? The auxiliary battery is the factory AGM, 90ah, 12V deep cycle. Would combining them pose problems with incompatible batteries? I've also considered the Adventure Wagon bracket to allow two batteries in this location and then directly linking them to my two batteries underneath. Advice appreciated, thanks.

It's a bad idea to simply parallel batteries of different "chemistries" (AGM isn't the same as your deep cycle flooded lead acid).
They have different charging curves (which stage they're in at what voltages).

The MB "Body Builder's Guide" specifically says "do not parallel another battery with the MB-supplied Auxiliary battery"
(this can overload the charging system, among other issues).
(you can find various years' BBIGs at http://diysprinter.co.uk/reference/ )

The one way that would be "supported" would be to use a battery-to-battery charger ... it draws a limited amount of current from the "source" battery to charge the "client". 30 or 40 amp units would be the maximum if the Aux battery was to be the "source".

--dick

The mismatched battery bank is issue when you leave them idle for long time.
In my years of RV-ing , I usually use "what I have" meaning when I go on vacations, boat, tractor, car batteries go into RV.
AGM are lead-acid batteries FYI.
With constant use the batteries will equalize anyway and not likely you will overcharge double battery bank with 200amp charger?

Autostaretx- Thanks for that information, totally makes sense and is what I suspected. Any recommendations on a battery-to-battery charger? I can easily get to the auxiliary battery tap at the driver pedestal and from there I have a spare #12 (only good for 20A) wire running to the back where my CTEK D250SA and Smartpass https://smartercharger.com/collections/on-board/products/140a-off-road-bundle. Which I believe I can route a charge through. Otherwise, I have to tear into the finished walls, which I don't want to do, or run new wires underneath. Your thoughts? Thanks, Chris

There have been (and still are) numerous threads bashing the "which B-to-B?" question to a bloody pulp.... i don't use one, so i can't make a useful recommendation.

Ummm... how long is your #12 feed? You may suffer a "more than recommended" voltage drop across it.
That would affect which end is better for the B-to-B (i'd assume the "client" (i.e House, not Aux) battery end).
On the other hand, too thin a wire will merely create "losses", and if you have power to burn it's merely inefficient, not a real problem.

--dick

Funny how fast people can get paranoid.
I also own Truck Camper and the TC forum has lot of disputes on the subject as well.
When some owners run designated #2 cables from truck charging system, the factory set is using trailer charging wire, who from front truck panel goes to bumper plug, than camper cable carries it to front of the camper, where internal wiring carries it to batteries placed in the rear. So we are talking ca 40 ft of wire between alternator and camper battery. All done with #10 wire (at its best).
When factory charge wires have their limits and take several hr of driving to get camper battery to full charge -B to B chargers are unknown.

Dick,
I have about 12' of #12 from the pedestal to the left back corner where my power center is located, it likely is even less than that. The "source" battery would be the auxiliary battery under the hood, charging the "client" batteries under the vehicle, about mid way between the front and rear of vehicle. In most situations, we'd be driving, fully charging the auxiliary battery. Park for the night, or several, and consume off the house batteries, which would benefit from having some additional charge from the auxiliary battery.

Thanks for your advice, Chris

In its simplest form, 12 gauge wire is 0.00162 ohms per foot.
So 12 feet is 0.01944 ohms
Times 20 amps is 0.3888 volts drop.

Which, at 12 volts, is 3.24% loss. (only 2.86% at 13.6 volts)
Using 10 gauge would reduce that loss by almost 40%.

Many people use 3% loss as their "do not exceed" design goal.

I'm assuming the negative path is via the frame, so i'm not going to try to calculate that path's loss (hopefully it's less than the wire's, but it will add (or subtract, in this case) something.

--dick

Dick,

Thanks again. I will likely research chargers and just run new, heavier, wire underneath. This would also give me the option of running directly to the house batteries and not run through the CTEK controls. This would also shorten the run by about 6-7'. When the auxiliary battery is spent, I will likely go with two batteries under the hood and also try and mount the battery charger there. Any recommendation for a battery to battery charger in this application? Don't be concerned about strong opinion, either way, from other forum members, I'm wont' be. Chris

Nunataq1955, I am just getting started with my house battery. When you wired you house AGM to alternator charging, is your start battery also AGM? What size cable did you use? Did you measure the amperage from the alternator after the house battery was connected? I am planning to wire AGM batteries to my start battery for alternator changing. The run is 6' and I want to use 1AWG or 2AWG. I did not config my van with an AUX battery. Thanks in advance for your feedback!

Lilypad,
My house batteries (two Fullriver AGM 220AH 6v, wired in series to produce 12V) are connected to my alternator via #4, stranded copper welding cable (slightly higher amperage rating and lower cost for me than battery cable and with better jacketing). The start battery is also an AGM, but it is not connected to the house battery system in any way. For alternator charging, I tapped into the main fuse panel right at the start battery (installed with MB OEM fuse) where you can pick up a positive alternator charge, and also used the grounding post right under the accelerator pedal for my negative lead. Both of these run back to my panels in the rear and are integrated into my solar and shore charging using a CTEK DS250SA and SmartPass. It has worked quite well. I have a Magnum 1,000W inverter that I regularly run 800-900W loads on for water heating and boiling water with an instant hot kettle. Hope that helps. Chris

Nunataq1955 said:

For alternator charging, I tapped into the main fuse panel right at the start battery (installed with MB OEM fuse) where you can pick up a positive alternator charge, ...

Click to expand...

Got a picture?
I see big connection board at my main battery and old wheelchair lift cable connected to it, but no fuses?
The lift cable has breaker installed later on under the driver seat.

I don't have a picture in my files. I can't seem to find one on the forum right now, though I've seen them several times. On my 2016, the terminal strip that I'm referring to is attached directly to the positive lead of the start battery and slips down along side the battery as you attach the positive lead. Yours might be different.

Nunataq1955 said:

Lilypad,
My house batteries (two Fullriver AGM 220AH 6v, wired in series to produce 12V) are connected to my alternator via #4, stranded copper welding cable (slightly higher amperage rating and lower cost for me than battery cable and with better jacketing). The start battery is also an AGM, but it is not connected to the house battery system in any way. For alternator charging, I tapped into the main fuse panel right at the start battery (installed with MB OEM fuse) where you can pick up a positive alternator charge, and also used the grounding post right under the accelerator pedal for my negative lead. Both of these run back to my panels in the rear and are integrated into my solar and shore charging using a CTEK DS250SA and SmartPass. It has worked quite well. I have a Magnum 1,000W inverter that I regularly run 800-900W loads on for water heating and boiling water with an instant hot kettle. Hope that helps. Chris

Click to expand...

Is there an isolator or switch between the alternator and the house bank?

OrionN,
Of sorts. The #4 positive lead is tapped onto the battery via a fuse on the MB fuse terminal and then they run through CTEK, which I believe also has internal protection.

Nunataq1955 said:

OrionN,
Of sorts. The #4 positive lead is tapped onto the battery via a fuse on the MB fuse terminal and then they run through CTEK, which I believe also has internal protection.

Click to expand...

What is a CTEK?

https://smartercharger.com/collections/on-board/products/140a-off-road-bundle. Mine is the Smartpass combined with the D250SA, so four years old and slightly different spec's.