It’s not a good idea to use the van’s starter battery to power the electrical items in your conversion. It’s too easy to leave yourself with a flat battery and no way to start the engine. The van’s charging system can be used to help charge a second battery that you isolate from the starter battery, but there are some constraints. If you’re trying to charge a large house battery, you may need some additional equipment.
You can think of the van’s engine like a big generator that’s producing electrical power you could use to charge your auxiliary or house battery. How much power you can take from the engine is governed by the size of the alternator. You have to be careful because the stock alternator isn’t designed to produce much more electrical power than the van uses already. Not to worry – there are other options for charging bigger batteries from the engine too.
How the auxiliary battery works in a Sprinter
Our van came with the optional auxiliary battery installed. The auxiliary battery is a 95 Amp Hour battery, exactly the same as the starter battery. It’s OK for powering a couple of lights and maybe recharging a phone and laptop, but it’s not really suitable for a full van conversion.
We have almost no use for the factory aux battery because we added a separate 750 Amp Hour house battery. However, if you are planning on using the engine to charge your house battery, the way the factory aux battery hooks in to the van’s systems is pretty much the same as lots of people use for any small to medium size second battery.
The aux battery is disconnected from the rest of the van’s electrical system until the ignition is on. Then, a relay closes that connects the aux battery to the starter battery. Now, the two batteries both take a charge from the engine alternator. Also, whichever of the two batteries has the higher charge will start “charging” the other battery until their voltages are equalized.
All the stuff that makes this happen is under the driver’s seat. We wrote a description of how to hook in to the Sprinter engine battery or the factory-installed Aux battery. The photos in that post will help you understand what’s needed to hook in to a charging source.
Overall though, the factory-provided aux battery setup isn’t really designed to be used in the way you’ll want it for a conversion van. You might be better off using a different approach. We’ve listed several below.
Ways to set up an aux battery charging system from the engine
Minimal dollars, minimal functionality
The really cheap-and-nasty version is to just add a solenoid between the starter battery terminal and the auxiliary battery that you add, with the control wire for the solenoid being triggered by a cable that’s only hot when the ignition is turned on You can find this at the EK1 terminal. The wire you want is the blue/yellow one. It’s “hot” when the engine is running. You are only taking enough current from the wire to open a relay, so you’ll be well within its 10A limit.
That works OK if the aux battery you add is not too much bigger than the starter battery. In the Mercedes Body and Equipment Guidelines it says the maximum charging current for the starter battery and any other batteries should not exceed 40 Amps.
The downside is that the battery isn’t really receiving the type of charging current and voltage it wants. Lead-acid batteries want to be charged in three stages (bulk, absorb, float) and need different voltages/currents during each of those stages. A solenoid just closes a switch between the starter and aux battery. Most of the time that’s OK, because the aux battery will first equalize its charge with the starter battery, then both will charge at the same rate if they are the same size.
More dollars, more like the factory version
You can replicate the Mercedes way of doing things if you want. There isn’t much difference between this and the dirt cheap version, other than if you use Mercedes parts you’ll be more likely to fit the stuff in the original locations.
You can read what’s required in the Retrofit guidelines for auxiliary battery in 2007+ NCV3 Mercedes Sprinter or Retrofitting an auxiliary battery with isolation relay in a pre-2007 T1N Sprinter. These guidelines show how the auxiliary battery is fitted in Sprinters, and list the parts needed to do a factory-style job.
Of course, you don’t need to use Mercedes parts. Blue Sea systems make a nice battery combiner, the Blue Sea ML-ACR 7620 for around $180. It is magnetic latching, which means it takes no current to keep the “relay” inside the battery combiner open. You could still use the Stinger relay mentioned above in this situation too.
Again, this solution is only really useful if your auxiliary battery is the same type and size as the starter battery.
Upgrading to a DC-to-DC charger
Sterling make several DC-to-DC chargers from 20 Amps to 120 Amps. This type of device works by taking a DC input from your engine battery and alternator, then ramping it up or down to make it suitable for the aux battery that it is charging. In other words, it’s a proper battery charger that happens to work on a 12v supply rather than the more usual 120v supply. The 55A version is probably the largest you should use in a Sprinter, and even that might be too much for the alternator in the long run.
The benefit of using a proper DC-to-DC charger is that it can sense what state of charge the aux battery is in, and change its output accordingly, independent of the engine battery. That means the aux battery can still be in bulk charging mode even if the engine battery is in float. This works much better for aux batteries that are a different size or type than the engine battery. Obviously that means your aux battery can be bigger than the starter battery, but you still need to be careful how much power you draw from the engine alternator overall.
Getting smart with inverters
Dave Orton on the Sprinter Forum pioneered the use of a 12v to 120v inverter to take 12v power from the running engine and turn it into 120v, then send that 120v power to wherever the house battery is placed. The 120v runs a charger (or runs through an inverter) to recharge the house battery.
Why would you do this? The inefficiencies are crazy. Even a good inverter is only around 90% efficient. Then the inverter/charger at the other end is probably only 80% to 90% efficient as well. That means you’re losing a good 20% of your energy before it gets to your auxiliary battery.
The real benefit is in the gauge of wire you need. 120v wires can be 10, 12 or even 14 gauge – a regular extension cable will set you back less than $20. If you are running them from the front to the back of the van, that’s a massive savings over the equivalent 12v cables you’d need; probably 25 feet each of + and – 2/0 gauge flexible welding wire at $155.
For some people, the inverter-to-charger approach is likely to work really well and be a very cost-effective solution. Be careful though. Even though Dave and many other people have used this approach with relatively large inverters, anything over a 400W inverter is likely to draw more amperage from your engine alternator than Mercedes allows in the Body and Equipment Guidelines.
Adding a whole new alternator
At the beginning of this article, we said that the limit on how much power you can draw is based on the stock alternator. The alternator is pretty tightly wired in to the rest of the van’s electrical system so you aren’t likely to just rip it out and replace it with a larger one. Instead, there’s space on the engine block to add a second completely independent alternator.
Avanti on the Sprinter forum added a Nations alternator with a Balmar MC-614 regulator. It also appears that Advanced RV offers this type of setup on their builds. The alternator provides 270 Amps of charging current (200 Amps at idle) that isn’t being used for anything else. Also, once the battery is charged, the alternator basically “disengages” so it’s not slowing the engine down.
This type of option costs around $2k plus installation. It involves adding a second alternator on a bracket in the engine bay, and running wires from that second alternator into the van (or to wherever the extra batteries are).
The benefits of this system over a solenoid, combiner or DC-to-DC charger are that you aren’t limited to 40 Amps of current, so you can charge any size battery of any type. The Balmar regulator can be programmed to support most types of lead-acid and lithium batteries.
Ignoring the engine all together
This is the approach we took. We were going to use the Dave Orton inverter method to get 800W of power from the engine to the back of the van. However, we’ve found that our solar panels will pretty much recharge our van while we are parked or driving without much assistance from us.
We might still add the engine inverter in the future, if for instance we find that the solar panels can’t keep up over the winter. But for now, we haven’t bothered to hook into the engine powered electrical system for any of our charging needs.