Why Mercedes’ Auxiliary battery package isn’t a good starting point for a conversion

Although lots of people specify the auxiliary battery (option E28) when they buy a van for conversion, or even try to retrofit the pieces that come in that option, it’s not clear that it was designed for the types of use it gets in adventure van conversions. It’s not clear that it’s actually any use for conversion vans.

The intended use of the aux battery package

The aux battery is purely there to augment the starter battery as a power source while the engine is running and to stop the starter battery from being run flat by high power devices that are used for short periods when the engine is off. For instance, wheelchair lifts in accessibility vehicles, or a 12v fridge in a conversion van. Basically, any device that will draw more than 25 Amps.

Hooking all sorts of other stuff like high load inverters to that circuit is outside the design spec. Replacing the aux battery with a higher capacity battery is also potentially harmful.

Mercedes says in their body and equipment guidelines that the maximum battery charging current for all additional batteries should not exceed 40A.

Page 45 of the 2015 Mercedes Body and Equipment Guidelines - aux battery info
Page 45 of the 2015 Mercedes Body and Equipment Guidelines – aux battery info

They also explicitly state that batteries over 100 Ah should not be directly connected to the vehicle’s electrical system. It’s a little ambiguous, but because they go on to say that additional batteries should use a cut-off relay and fuse, it’s likely they mean that you shouldn’t replace the starter battery with anything larger than 100 Ah, but that it’s potentially OK to add other batteries with a higher capacity.

Still, charging and discharging a higher capacity battery located in the aux battery location under the hood has some issues.

First is wire gauge. The aux battery wire is quite long. It goes from under the hood all the way to the driver seat base. It’s not very fat. Pulling a large current through that cable will most likely result in voltage drop and heat build-up.

Second, if you’re replacing it with a lithium battery, the location under the hood is likely to see more extreme temperatures than a location inside the vehicle. Most lithium batteries should not be charged when they are below freezing point.

Third is the relay capacity. It’s designed to carry currents for a 100 Ah battery charging circuit. If you repurpose it for a 500 Ah lithium pack and a 2Kw inverter, is it going to be able to handle the through-current?

Don’t be bound to the aux battery circuit

We didn’t want the aux battery but the van we ended up with (long story) came with one. It makes sense to try and use as much of that circuitry as possible, but we don’t feel that we HAVE to use it if we can make smarter choices without it.

For instance, our house battery system is completely detached from the vehicle electrics. It does not charge from the alternator. It does not use the van chassis for its neutral/return cables. It made sense for us to do things this way because we have a large solar system, lithium batteries, and an inverter that would have boiled the stock aux battery.

Even if you do want to use the stock alternator as a charging source, there are alternatives to the Mercedes auxiliary battery components. RB Components makes a nice battery tray for the engine compartment. You’ll probably want a 200 Amp or 500 Amp relay, or a purpose built automatic charging relay. You might also want to consider following Mercedes’ advice and using a 12v-12v charger that limits the current draw to around 40A (for instance this 50A Sterling model).

“You’re just being a killjoy”

Yes. There are plenty of people who use the aux battery circuitry to power their house batteries. There is also plenty of discussion on the Sprinter forum about how to get this to work properly, what charging rates to expect, and so on. People are obviously (trying to) use the aux battery build option as a charging source for their house battery.

It’s quite possible that Mercedes’ recommendations are extremely conservative, and that it’s just fine to repurpose their 100 Ah system to run a 200 – 500 Ah system using a different battery chemistry.

On the other hand, maybe it isn’t. It’s hard to know whether someone’s alternator died early because of the modifications they made to the aux battery circuitry, or for some other reason. It’s hard to diagnose whether the issues people come to the forum with are caused by their particular install or something specific to the aux battery circuit.

What is clear is that there are plenty of other alternatives for charging your house battery, and given the modifications you’d have to make to the stock aux battery system you might be better off starting from scratch instead.

24 Replies to “Why Mercedes’ Auxiliary battery package isn’t a good starting point for a conversion”

  1. Do you think it’d be possible to add a battery to battery charger connected from the aux battery to a house battery given the aux and started battery are connected in parallel?

    1. Alex, if you attach a battery-to-battery charger between the aux battery and the house battery, then it will work just the same as if you attached the charger to the primary battery. When the engine is on and the alternator is running, the aux battery relay will close and the aux battery will charge. That will trigger the battery-to-battery charger and charge your house battery. When the engine is off, the charger will sense that the voltage has dropped below 14v (alternator not running) and will stop charging your house battery.

      So you don’t gain/lose anything by doing it this way. Again, like this article says, the aux battery doesn’t add anything positive to the equation here.

      There’s also no point running wires from the aux battery under the hood all the way back to the house battery. You can use the wires that run *to* the aux battery from under the driver’s seat if you want, if you calculate that they are large enough gauge to support the power draw of the charger. That’s about the only benefit of the aux battery in this situation.

        1. Ah, you didn’t mention what vehicle you have, so I guess I assumed NCV3 Sprinter. I can’t really give you advice for other models. You might find a major terminal under the driver’s seat. You might have to trace some wires just to find out which location gives you the shortest run. Or just give in and wire it to whatever works best for you!

          By the way, if you *do* have an NCV3 (2010-2018), then that’s your aux battery under the hood.

  2. What if the aux battery is under the passenger seat? Would it be easier to connect to that instead of drilling holes from the engine bay to the cargo department?

    1. It’s not the location so much as the extra length of wires between the alternator and the house battery versus connecting to the starter battery under the driver’s feet.

        1. Connecting to the closest battery to the alternator should allow you to use a smaller wire gauge. It will also mean less overall complexity for any future troubleshooting that you might have to do.

    2. If you decide to install battery in the engine compartment then You don’t drill holes for cables , there is one in the firewall, just remove its rubber cover. Then you drive cables in the floor. Or install batteries under passenger seat as I did in my Sprinter.

  3. Thank you so much for spending the time and effort writing this article. I didn’t know how much amp I can draw from the Sprinter alternator before. However, given the biggest 12v-12v charger I have seen is 50A. I would use no more than that.
    I have 2020 Sprinter with a 70Ah auxiliary batter under the hood. This battery is a temporary power source before my installing 600Ah lithium battery. Once that is done the 70A AGM battery is more of burden than benefit. I would remove it. To charge the house batter from the alternator I would connect the input of 12v-12v charger to the primary battery directly under the driver seat. Let me know what you think.

    1. That’s pretty much the setup I’ve seen people using with a DC-DC converter, so you should be set. There’s undoubtedly more info about individual installs on the Sprinter forum.

      The only *potential* benefit of keeping the 70Ah aux battery under the hood is that you can jump the engine from it if necessary.

  4. Hi Alex, I have a 2015 NCV3 Sprinter (Airstream Interstate). I upgraded the house batteries with 300ah of lithium. I recently checked the AUX battery under the hood and found it was @ 5.5v- completely dead. However everything seems to work on the Airstream, starts fine, other components operate on lithium.
    Replacement for the Original Varta 100ah battery will likely cost $250.
    Ques: do I even need it at all?
    Can I just remove it and cap off the positive cable?

    1. Glen, your house systems are most likely all attached to the lithium batteries you installed so they’ll be fine. The Aux battery typically powers the 12v outlet in the driver’s seat and on the driver’s side at the rear door (D pillar) along with a couple of other items. The only time those wouldn’t work with the aux battery removed is when the engine is switched off.

      So yes, you can remove the battery and cap off the positive cable. And if in the future you find it’s a problem, you can always just add a new Aux battery back in.

  5. I have VS30 and only have the main battery, no aux under the passenger nor in the engine bay and I want to install a “few” things in the front (lights, air compressor, etc) and don’t want to pass a ton of cables thru the firewall. I’ve thought about installing a Trigger controllers but the lenght of the wire is the issue as you mention.
    Is adding a battery in the engine bay a good solution to hook from there? And is yes, which setup? Ask Mercedes to add one? Any ideas welcomed.


    1. Christian, remember that if you put a battery in the engine compartment, you’ll still need cables from the existing battery to charge it. Unless you use that Trigger controller, you’ll also need cables from whatever switches you install for the lights and air compressor so that you can control them from the dash. In other words, you’ll be running cables no matter what solution you decide upon.

      It doesn’t sound like you actually need a second battery for the things you intend to power, so why bother going to that expense and hassle?

      There are 12v sources under the hood, but I’d be wary of taking a large load (for powerful lights or for a compressor) from any of the existing wiring.

      In the NCV3 there’s a pass-through hole under the (driver footwell) battery compartment that you could then run cables from to the engine bay. I’m not sure if the same thing exists in the VS30 but I expect it does.

  6. I have an aux battery under the hood, but now that I’ve added my house batteries I realize I don’t have a lot of real estate for panels and so I am thinking of connecting a 30amp b2b between the starter battery and the house batteries. I get that the aux battery isn’t helping much, but I’m wondering if there’s any reason not to keep it and add the b2b. Thanks!

    1. Andy, I see no issue with keeping that aux battery in place. If nothing else, it’s useful to jump start your starter battery. It shouldn’t get in the way of a 30 amp B2B charger.

  7. Has anyone had experience with replacing the factory AUX battery with two FULLRIVER DC224-6 and using the factory aux set up?

  8. Hello,
    I have 2016 NCV3 with factory AUX battery. I replaced the factory AUX battery with 100AH LI Battleborn & 40A Charge Controller. The system has worked well for me for the last couple years, but would like to increase capacity to 200AH. I just ordered another 100AH LI BB. I plan to re-route the OEM wire going to my current 100AH inside for winter conditions and add the additional battery. I believe given my 40A battery charger will limit draw from alternator I should be ok with current wire. Based on the what I’ve read my ‘potential’ constraint will be the factory relay. Do you see any additional issues with this setup? *Note: I do not have any high amperage draw accessories with exception of coffee maker.

    1. Ryan, I’m pretty sure you’re correct that your 40A charger will limit current draw from the system, meaning you should be OK with the OE wiring and relay. Just check that the total charger draw is 40A, rather than it maybe drawing 50A and passing 40A through to the batteries.

      Rather than the hassle of fishing that wire back through the firewall, why not just buy the couple of feet of cable you’ll need? That way you can return the van to stock if you want later, and you can create a cable run that’s the exact length you need, with the correct terminators for the posts you’ll be attaching it to. You could just detach the OE cable under the seat, and put your replacement wire into the circuit instead.

  9. Your comments about the factory auxiliary battery are generally correct. The real purpose of the auxiliary battery is to provide enough voltage for the chassis CAN data network when the engine is running. Low system voltage intermittently corrupts the data flow between the control modules. The main starter battery can’t always keep up. You can confirm by scanning your OBD. Low voltage fault codes will confirm.
    If properly wired, you cannot jump start a dead main battery from the auxiliary battery. The auxiliary battery connects to the main battery after the engine starts.

    1. Tom, I’ve jumped my main battery from the aux by connecting a jumper cable from the positive terminal of the aux to the jump start terminal under the hood. Obviously that bypasses the cut-out relay under the driver’s seat.

  10. Thanks especially to Diesel for the info. Decades ago, I wired a half dozen commercial Alaska fishboats, and never worried that I was charging multiple 8-D lead-acid batteries with a single Motorola alternator, and never heard any complaints. Mercedes says to limit the extra current to 40 amps, and I have seen a photo of the burnt AUX relay, a few inches from the engine battery positive. Now I understand the problem. I have PV on the roof charging my inside AGM, think I will run a 1/0 wire to the + post under the hood to parallel with my inside battery, not touch the Mercedes charging wiring. An alternate is to use the underhood battery entirely isolated from the Mercedes system, using my PV system on the roof.

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