Tapping in to the Sprinter van’s battery

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.

Power tap point under driver's seat

  • 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.

Be sure to add a fuse!

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.

Auxiliary/Additional batteries

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.

Aux battery relay under driver's seat

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.

Aux battery tap point under driver's seat

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.

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).

Blue Sea 6-fuse block

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.

34 Replies to “Tapping in to the Sprinter van’s battery”

  1. Love your website. Very informative, well written and with good photo documentation. I do have a specific question relating to the factory aux battery relay. You recommended not connecting any electrical consumers to the relay directly. I have a situation where I want to do this, and I think it would be fine, but I am interested in your thoughts. I have a 2016 crew van with the factory auxiliary battery installed. I am upgrading the audio system. I am actually doing the speaker upgrade as you did, but additionally I am adding an aftermarket amp and sub amp as well as a subwoofer and equalizer. Normally, with this configuration, you would connect both amps to the starter battery with 4 ga cable and a 100 amp fuse. However, I want to take advantage of what the auxiliary battery was intended for, to power additional components without drawing down the starting components. In this case, playing the radio while parked in a campground or at a outdoor flea market. The 30 amp connection under the driver’s seat works fine for small consumers. (I like the Blue Sea fuse block that you used) However, the 30 amp limit will not work to power my amplifiers. You say the charging circuit to the aux relay is 100 amps. I think this would work fine for my amplifier connection. Although the circuit is protected by a 100 amp fuse, the absolute maximum both amps could ever draw together is probably 50 amps, and the average consumption for both together is listed as about 8 amps. I would appreciate your thoughts on using this relay connection for the amplifiers. (100 amp fuse would be within 6-12″ of the relay.

    Thanks!

    Greg

    1. Hey Greg, for your use that sounds fine. Like you say, this use is very much what the aux battery was designed for. Remember that you are fusing the wire, not the components when you choose fuse size.

  2. If I were to want to add solar where would
    You recommend to connect the output of the solar controller to the battery? At the Aux battery mounting points you described?

    1. Hi Michael,

      Without knowing the amount of solar you intend to add, and whether it’s going to the standard aux battery or to an upgraded size battery, it’s hard to give you specific advice.

      If you connect to the aux battery terminal (distinct from the EK1 terminal), then the solar will only charge the aux battery and not the starter battery while the ignition is off. That’s probably what you want to happen.

      Unless you had a really, really large number of solar panels, the cable size at this point is sufficient to take the solar charge current.

  3. First thank you very much for all you amazing work. We have the automatic Sprinter step for our side door. I would like to put a switch on it so I can leave the step out when we are camping. It takes the battery down and is annoying.

    1. Hi, is your step the OEM Mercedes version or an aftermarket one? If it’s the Mercedes version, it might be hard to add a switch without causing the vehicle to get upset.

      As far as I know, the OEM step is controlled by the Parametric Special Module (PSM) and so it reads messages about the doors being open or the ignition being on from the CAN bus and then tells the step whether to extend or retract based on these messages.

      It’s probably possible to find the wire that brings power to the step and put a switched relay in that circuit, but it might cause the van to throw an error code if it tries to retract the step and nothing happens.

      If it’s an aftermarket step, you can almost certainly add a switched relay to the power circuit feeding the step.

      To be honest, I’m surprised the step takes enough power to draw down your battery. Are you sure it’s the step, and not something else that’s attached to your starter battery while you are camping?

      1. My 2014 sprinter van has the PSM…with 200 amp batteries. My van has been sitting outside while it is torn apart. Is there a way to keep my OEM batteries charge….Would you recommend on of those window solar charges? Thanks Again. Have a happy new year!!!!
        Melanie

        1. If you aren’t using the van, you could just disconnect the batteries. They will keep their charge for several months. I’ve not used any of the solar chargers for stock batteries, so I don’t know how good they are. You could ask at the Sprinter forum.

          You say that you have 200 Amp Hours of batteries. If those are stock batteries, I’m assuming you have the auxiliary battery and the starter battery installed (each one is 100 Amp Hours).

          When you are camping, you should only be using the auxiliary battery, not the starter battery. The two cigarette sockets that come from the aux battery are the one on the driver’s seat base and the one in the back door pillar. If you use any of the other cigarette sockets, you are in danger of running your starter battery down. Then, you wouldn’t be able to start the van. That would be bad. So, really, you have 100 Amp Hours of battery for camping, which means 50 Amp Hours of usable charge because you should not run the battery down beyond 50%.

          That’s one reason why the auxiliary battery isn’t particularly good for van conversions.

  4. Wow. Your site,is awesome. Glad I stumbled upon it.
    I have a very basic question about running power off the blue Sea fuse box. I plan to run some minor components off a fuse box (lights, fan).

    If I understand correctly you ran a + 2awg wire off the aux connector + post to the fuse box and then ran a – 2awg wire from fuse box to the grounding under the seat. Did I follow that correctly?

    Did you also run a Mega Fuse (instead of the copper strap) across the connectors under the seat?

    Do you think it is a good idea to run a kill switch between the connector posts and fuse box?

    Thanks in advance.
    Tim

    1. Thanks Tim!

      Yes you’re right about the wire gauge and where I attached the positive and negative cables. I used 2AWG because it’s what I had lying around. Depending on your loads, you may choose to use a different wire gauge.
      I did not run a mega fuse, but I should have done and will do next time I have the seat off.
      A kill switch is an awesome idea. That way you can isolate the entire fuse panel very quickly in an emergency. We are’t using the Blue Sea fuse box as our primary power source, so I skipped that step.

  5. Hey dieselfumes (and Tim),
    Curious what size fuse you will use for Mega-Fuse? 100 amp?
    I’m setting up same fuse block with 4AWG and will be running ARB fridge periodically.
    Thanks for all your documentation.
    Phil

    1. Hey Phil, I’m trying to work that out. If I was just fusing the wire I’d added, I’d use a cable calculator based on the AWG and length of the wire. The thing is, the fuse is in-line with the (1 AWG?) factory cable to the aux battery. I would need to measure the length of that cable and fuse it appropriately. Then, the fuse might be too big for the wire I added. So, I may just end up fusing the cable I added rather than/as well as adding a mega-fuse.

      UPDATE: Looking at page 6 of the battery retrofit guidelines document I linked to in the post, it’s pretty clear that the wire that runs from the main starter battery to the aux battery via the relay and the electrical connector is already fused with a 150A fuse at the starter battery. So, if you’re adding a fuse, you need to add it on the branch you take off this connector rather than across the connector terminals.

      In other words, you need to bolt a fuse on to the electrical connector, then bolt your cable to the other side of this fuse. Of course, you might put a kill switch in the circuit first.

      Doing things this way just seems right to me. Now you don’t have to worry about the length of the aux battery cable. You just have to worry about the length and gauge of the cable you use to connect from the electrical connector to your fuse box (and back to a grounding point).

      If the aux battery cable shorts out or bad things happen to the aux battery, the 150A fuse at the starter battery will blow. If bad enough things happen to your additional fuse box, that 150A fuse will also blow. But before it does, the properly rated fuse that you add to your fuse box cable will cut out first.

      Adding a fuse to replace that copper bar doesn’t help protect your additional fuse box. If you rate that fuse correctly for your additional fuse box, it’s likely to be undersized for the supply from the starter battery to the aux battery. And it wouldn’t stop your additional fuse box from being supplied by the aux battery, even if it blows. It would just take the starter battery out of the equation (there’s a battery on either side of the electrical connector).

  6. Thanks for the response gentlemen. Yes, it’s a guess when estimating the length of the cable from aux battery to relay…probably 6-8 feet. My run of 4AWG is 5-6 feet. Considering my potential loads, I think I’ll start with a 50 amp fuse. Would rather be wrong with only a melted fuse.
    Thanks again for all your advice.
    Phil

  7. Hello….Thank you so much for all of the information in these posts. I feel that I am learning so much. My husband installed two maxair fans on the roof of our van. We would like to wire them through the auxiliary battery connection as described. We are waiting to receive our BlueSea fuse box. I think I understand about connecting a positive wire to the auxiliary connector post under the driver’s seat and running it to the fuse box….and then run a negative wire from the fuse box to a ground.
    My question is about where we would place the Mega-fuse….and what size fuse. And where would we place the kill switch on this set up? And, any recommendation on brand of kill switch?
    You can probably tell by my questions that I am just learning about this stuff…and Dieselfumes…you have my utmost respect for your van build….and mostly for your patience in posting so much information and in so much detail. Thank you taking the time to share your knowledge with everyone.
    Teresa and Luis Rauda

    1. Hi Teresa,

      There is a copper strip inside the auxiliary battery terminal where you’d connect your fuse box. It is bolted between the two posts in that terminal. Some people replace this copper strip with a Mega-fuse. We have not done so yet.

      The reason we haven’t is because it didn’t come with one from the factory. I think that the fuse Mercedes installed at the end of this wire is probably sufficient. If you want to add one, that’s fine too. It won’t hurt anything and it might help.

      What you will need is a fuse on the wire that you use to connect from the auxiliary connector post to your fuse box. You can calculate what size of fuse by adding up the positive wire distance from your auxiliary connector to the fuse box and the negative wire distance from the fuse box to ground. Once you know that, and the maximum current (amps) you would ever pull through the fuse box, you can use one of the online wiring calculators to work out what size wire and what size fuse to use.

      As for the type of fuse, these Blue Sea 5191 fuse block terminals are very compact. Remember to order the right size fuse too. The one problem with these fuses is that they aren’t as common as other types. If one was to blow, it might not be something that local stores stock. However, because you’re basically fusing a fuse box, something pretty bad would have to happen for this fuse to blow.

      You also asked about kill switches. It’s probably best to put this switch in a position that lets you easily change fuses when it’s “killed”, so it would make sense to put it in the circuit between the auxiliary connection and the fuse. That does leave a very short section of wire unprotected by a fuse, but that should not be an issue. As for brand, I really like the Blue Sea stuff. Their M-series switches are sort of overkill for your needs, but they are very well made. People sometimes mount these through the side or rear of the seat base so they can easily reach them. There’s no way you’d accidentally knock this thing on or off. It has a very solid “clunk” when you switch it.

      You say you’re just learning about this stuff, but you’re asking good questions. Remember: I’m not an electrician. I had to learn this stuff too. There might be some other folks out there who disagree with what I’ve written and what I’ve just told you. However, if you keep reading and learning until you are confident that you know what you’re doing (rather than just copying something blindly) then you should be just fine!

      Enjoy your fans, and enjoy your van. If you just cut two massive holes in the roof of the van, everything else you do will be easier from now on.

  8. Thank you so much for your quick reply. The information you provide is more than helpful! I, too….am not an electrician! But I can learn! Yes, it took nerve to cut those holes in our van (2015 Cargo van)….Not only the holes for the fans….but big holes for windows on both sides and rear doors. Our van is definitely a work in progress…..Your write-up keeps my husband and I up late at night—reading and re-reading. We are in awe! Again—-we can’t thank you enough! Teresa and Luis, Apple Valley, CA.

  9. First off, great blog. I’ve got a question regarding the aux battery system. We installed a max air fan and planned on hooking it up the factory aux battery system. We swapped the factory connection point with the same fuse box you have pictured above. When we connect our wires to the fuse box…nothing. In order to make sure the fan worked, we used jumper cables and connected them to our aux battery direct to ensure our wiring was correct which it was. Anything special that has to be done to the system before using it? Or recommended trouble shooting? Next step is going to be to check the cut off relay to make sure there aren’t issues there. Any tips or suggestions would be greatly appreciated

    Thanks
    Dan

    1. Hi Dan,

      Good job troubleshooting by jumping directly to the aux battery. That narrows the problem down to the fuse box you added.

      When you say you “swapped” the factory connection point, what do you mean? Hopefully you didn’t change anything in the stock van wiring setup. All you should do is connect the positive wire that runs to your fuse box to this connection point (on the bolt that the black wire is connected to) and run the negative wire from your fuse box to one of the bolts under the seat base that has brown wires attached to it.

      You also need to insert a fuse in the new fuse box for the circuit you connected your fan to, and make sure you have your positive and negative wires connected the right way round. Check that the fuse is good (not blown).

      It’s very unlikely that the cutoff relay isn’t working, because your aux battery had enough power to run the fan. Even if it wasn’t working, the power coming from your aux battery would still drive the fan if all your other wiring was correct.

      Hopefully that doesn’t sound condescending to you – I am just worried that you did something like wiring the fuse panel in line with the aux connector, or removed the copper strip between the two bolts in the aux connector.

      If you can’t figure it out, send me an email with a couple of photos – dieselfumes [at] this site’s domain.

  10. Thanks, so if Im understanding you correctly, the original factory tap in point shouldn’t be removed. What you’re saying is you connect your fuse box to that positive lead, and then ground the negative side to the floor?

    1. Yes, that’s correct. The copper bar across the tap-in point connects the starter battery to the aux battery, so removing it means your aux battery won’t get a charge!

      The circuit is [Starter battery] -> [Relay] -> [Connector (with copper bar)] -> [Aux battery]

      What you are doing is tapping in to that circuit at the connector point. It’s a great location because when the engine is on, you get the power straight from the starter battery. When the engine is off the power comes from the aux battery. (the reality is slightly more complex than that, but it’s a good enough concept for this conversation).

      Your new fuse box positive wire connects to the aux connector without removing the copper bar. Your new fuse box negative wire connects to one of the existing negative posts on the floor. There are two under the driver’s seat. Just find the bundles of brown wires that are bolted to the floor, and add your connector to one of those bundles.

      Let me know if that works for you.

  11. Hey Diesel, Thanks for the hours and hours of info you have given us , incredibly generous. My question , alittle off topic do you have a diagram for your wiring of the entire van somewhere online?

    1. Hey Mike, thanks for the comments. We did’t put our wiring diagram up online. We might do in the future, but the big issue is nobody should ever blindly copy what we did without understanding why we did it. If we put the diagram up, other people could get themselves into trouble.

  12. Hello Diesel…..This is Teresa, again! We are trying to connect two MaxAir fans to the auxiliary battery connection. When we disconnected the Auxiliary battery (by the gas pedal)…..the black cable going to the Auxiliary connector still has 12V running and the Aux. side had zero. Does this mean that the black cable is from the starter battery?
    Also, is there a reason that we should NOT connect anything to the aux. bolt on the relay?
    I am trying to figure out what we are doing wrong. Please help!

    1. Hi Teresa,

      The disconnect by the gas pedal is for the starter battery. Like you found out though, even with that disconnected the Aux battery is still attached to its side of the isolator circuit.

      When you are looking at the auxiliary connector, the black cable is the Auxiliary battery side, the red cable is the isolator/starter side. That’s why you still had voltage on the black cable. If you trace the black cable, it goes all the way to the engine compartment and it’s attached to the positive terminal of the Aux battery.

      The easiest way to kill power from the Aux battery is to undo the brown cable on the negative post of the battery (or undo the other end where it bolts onto the metal of the engine compartment).

      Mercedes says to use the Aux connector, not the relay bolt. There is no reason to use the relay bolt. It’s not the solution to the problem you thought you were having.

      Remember that you’ll need that piece of copper installed between the red and the black wires in the Aux connector because otherwise current won’t flow from the starter battery to your Aux battery and so the Aux battery won’t get charged.

      Hope that helps!

  13. Hello Diesel….THANK YOU, THANK YOU, THANK YOU!!!
    We actually took our van to an automotive electrician because we did not feel confident enough to tackle the electrical connections ourselves—-Well…..after having read so much about how the connections SHOULD be done by reading your posts….I was disappointed by the work done by this so called specialist!
    But….after reading and re-reading your posts—-and after watching what the electrician had done—-I felt confident that I could undo the connections that were made and do it the right way (Your way)—-MYSELF! And, thanks to you—and only you—we were successful! I would like to send you a personal email rather than post on this site. Can you please send me a personal email at: [xxxx]
    Again, Thank you , Teresa and Jose Luis Rauda

  14. Great website. Thanks. I have a 2015 2500 sprinter crew with the factory auxiliary battery under the hood. I have been powering an Engel refrigerator/freezer with it 24/7 for the past 30 months and I think I may need to replace it soon. Do you know if I can replace it with a 12 Volt, 125AH AGM Deep Cycle SLA Group 31 battery such as the one I have linked to below? Thanks.

    https://smile.amazon.com/GOAL-ZERO-Yeti-BATTERY-UPGRADE/dp/B00ISDL05K/ref=sr_1_fkmr0_2?ie=UTF8&qid=1493241383&sr=8-2-fkmr0&keywords=goal+zero+1250+replacement+battery

      1. Martin,
        As you’re probably aware, the reason you need to replace that battery after 2-1/2 years is because you’re working it really hard. It’s not really designed for deep discharge like the usage pattern you’ve got. The AGM deep cycle batteries would be better for your needs. But there’s a catch.

        Mercedes recommends using the same make and model of battery for the starter and the aux. That’s because they are on the same charging circuit. If you used different batteries, they’d have different charging profiles. One might overcharge or the other might never completely charge.

        Some people do replace their aux with a different type of battery. I wouldn’t recommend it without also adding a DC-to-DC battery charger like the Sterling brand ones. Those are not cheap.

        Additionally, I’m not sure I’d buy a Goal Zero battery unless you’re sure you’re getting a good price. That company seems to have a premium pricing strategy, and it’s not clear that their batteries are necessarily very high quality. There are several well-known battery brands to choose from instead. I have no specific recommendations.

  15. I may be late to the game… new to owning a sprinter… I have a 2015 with the e28… and maybe I am over thinking this… I see where to attached the red wite for the inverter in the click off the top box… were do you put the negative terminal to return to the aux battery? can i use the cluster of brown ground wires? Am i slow on the up take? Did I miss something?

    1. Just undo the bolt that holds the brown ground wires to the chassis, add your wire to the set, and tighten the bolt back down again. The Aux battery uses the same common negative feed as the rest of the vehicle, and that negative path is through the chassis.

      If you look at your aux battery under the hood you’ll see a fat brown wire from the aux battery negative terminal that bolts on to the metal of the van body just above the driver’s side front wheel inner fender. That’s where the connection “exits” the chassis metal back into the battery.

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