There’s a lot of confusion about the different types of battery that you can find in Sprinter vans, and how to go about connecting to them in order to power things inside the van. Here are some photos to help make things clearer.
WARNING – before you go messing with any of the connections in this article, unhook the battery connection (just to the right of the accelerator pedal) and if you have an aux battery, unhook the positive or negative connector from the aux battery itself.
WARNING #2 – these photos are from a 2015 NCV3 2500 Sprinter van with a factory-installed aux battery. Your van may be very different.
Starter (engine) battery connections on the Sprinter
If you don’t have an auxiliary/additional battery package (no extra battery added by Mercedes under the hood, or under the passenger seat) then the only place you should really get power from is the special set of connectors made for that purpose. You’ll find them under the driver’s seat. This set of connectors is called the “X145 Terminal.” It’s option EK1, so it’s often referred to that way too.
The X145 terminal is mounted on the top of one of the rails that goes from left to right across the top of the seat base. In the picture below, I’ve already unclipped the terminal cover, which is now hinged up and away to the left.
- On the left is a stud with a blue/yellow wire. This provides power when the engine is running (Terminal D+). It has a 10A (120W) maximum current draw.
- In the middle is a stud with a red/grey wire. This is a direct starter battery connection (Terminal 30) so it’s live all the time. It has a 25A (300W) maximum current draw.
- On the right is a stud that provides power during ignition (when the key is in but the engine isn’t running). It’s got a black/yellow wire (Terminal 15) and it’s a 15A (180W) maximum current draw.
You might be able to reach this terminal by sliding the driver’s seat all the way forward and removing the board that covers the seat base. Better still is to completely remove the driver’s seat. It’s only four bolts (the seatbelt can stay attached if you just move the whole seat toward the front of the van).
CAREFUL: undo the wires that thread through the seat base cover board and up to the seat before you remove the board. Reconnect them before you reconnect the battery or turn the key in the ignition. If you don’t you’ll get error codes that can only be reset with a diagnostic tool.
Any wires you connect to these studs need to have their own fuse. The fuse should be rated for the wire diameter and length, not for whatever you’re attaching to the other end.
Even if you do have an aux battery, you might want to make use of these terminals. For instance, we have our electric door step hooked to Terminal 30 so it always has power from the engine battery. That way, if we ever have to take the Aux battery out (or if we run it down unintentionally) the door step will still work.
If you have a factory option package that added an auxiliary or additional battery, you’ll have an extra battery under the hood, or maybe under the passenger seat. This second battery should be of exactly the same type as your starter battery (the one under the driver’s feet).
The aux battery works by connecting to the alternator and the starter/engine battery whenever the engine is running, and then disconnecting whenever the engine is off. This way, you can power some electrical devices without running down your starter battery. You’ll always be able to start the van and drive.
It does this by using a relay under the driver’s seat. It’s the big black box to the right of the picture with the two copper bars going in to it.
The red cable at the front is from the starter battery. The other cable goes to the aux battery.
You do NOT want to connect any additional electrical devices to the bolts on this relay. There’s a special hook-in point for the auxiliary battery. It’s also under the driver’s seat. On the more recent models it’s at the back of the driver’s seat base.
You can attach an additional cable to the connectors inside this box. The cover of the box unclips for easy access. Just squeeze the tabs in the middle.
The red cable comes from the isolator relay. The black cable goes to the aux battery in the engine compartment. Don’t get fooled by the cable colors. The black cable is still positive rather than negative.
We chose to use a secondary fuse panel for all the electrical items we wanted to attach to the aux battery. The Blue Sea Systems ST Blade fuse block with a negative bus gave us everything we needed. It has a cover so the terminals aren’t exposed, and it uses the same style fuses as the rest of the van. Our aftermarket electric seat heaters are attached here, and we’ll probably attach a subwoofer to the fuse block later too.
Having the negative cable on the same fuse block makes it much easier to terminate wires here. There are a couple of points to connect the negative cable to inside the driver’s seat base. Just find the clusters of brown wires where they’re bolted to the metal of the van. You can just see one of those clusters in the picture above (bottom left of the picture).
We ran the wires for this fuse box to the outside of the driver’s seat base. We attached the fuse box to the back of the driver’s seat base where it’s tucked away but still easy enough to access if we have to change a fuse.
Note that the supply cables for our fuse panel are pretty beefy. We had some 2AWG cable left over from our full electrical install so we used it. It might have been overkill, but you could have some pretty serious power running through the cables if you hook up all the fuse positions .
Blue Sea also make 12-circuit versions of this fuse block if you think you’ll be running more devices from the aux battery. Be careful though. It’s only a 95 Amp-hour battery, so you can run it flat quite quickly if you attach a lot of electrical consumers to it.
If you want additional information about how the aux battery works, take a look at the Retrofit guidelines for auxiliary battery in NCV3 Mercedes Sprinter. It shows the location and function of each of the components.