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.
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.