View Full Version : add a battery

06-15-2019, 11:28 PM
i would like to add a battery to my sprinter

can i just hook it to the battery under the hood with a fuse and run wires and be good

i have a 2005 2500 sprinter

it would be nice if someone had pictures of what they did to get extra batterys hooked up

i want to hook uo one agm deep cycle battery

06-16-2019, 01:06 AM
Putting two different (one flooded lead acid, the other AGM) batteries together like that will not make them happy.
The AGM will probably end up not being fully charged (it will reach at least 90%, though), and you stand the change of draining both of them (accidentally), which would leave you unable to start the Sprinter.

Most (if not all) people put an "isolator" (or "separator" or "isolation relay" or "automatic charge relay" (ACR) or "voltage sensitive relay" (VSR)) between the two systems ("Sprinter" and "House").

Some people use "battery to battery chargers" (BtoB or B2B) ... which is an expensive device that boosts the Sprinter voltage higher, then controls what the AGM battery receives in order to give it the best charging environment.

All of those separation devices keep the House from draining the starter battery.

The simplest system is something like a BlueSea ACR .... it joins the batteries when the Sprinter's alternator is running, and separates them when it is not. It will NOT attempt to adjust the charging to optimize the AGM's charging.

One step up (or sideways) from that is to use an Isolation Relay, whose coil is driven by the "D+" signal available on a terminal strip under the driver's seat in 99% of US-delivered Sprinters.

Here is the basic wiring:


... and here is the EK1 terminal strip under a T1N's driver's seat. It's on the side close to the parking brake handle:


Here is an 85-amp rated Cole Hersee relay that would work:



06-16-2019, 01:19 AM
Somewhere on the forum is a pdf of the upfitters guide to installing an aux battery. It’s ridiculously straightforward and I recommend following it, at least as a guide. Dick’s information is good and mounting under the Passenger seat allows easy access for running connections and plenty of space to install a fuse/relay box. There are channels stamped in the floor to run the wiring, the factory made things easy on us here.
If you’re looking to have a house battery then AGM with a battery isolator is the way to go. If you’re looking for an extra starter battery, no isolator is needed, but if you drain the battery bank you’ll need a jump start and possibly two fresh batteries if it’s too much too often.

06-16-2019, 01:22 AM
I suggest you search through the RV section of the forum where this topic has been covered many times in detail.

The short answer is that you should avoid connecting the new battery directly to the starter battery, as you will then have created one big battery and risk running both down together. Instead, connect it using a battery isolation relay (switch) triggered when the engine is running. This signal (D+) is available on the optional “EK1” electrical strip under the driver’s seat, or from the instrument cluster. Look for the “Body Manufacturer D+” fuse listed on the seat pedestal cover.

There is a tunnel through the floor between the driver and passenger pedestals for running your battery cables, and room for a 100 amp hour agm battery and tray under the passenger seat. If your van does not have rear A/C there is likely an open fuse position on the “power distribution centre” on the front of the starter battery that is a good place to run the necessary fused positive cable from, and you can ground your new battery to the van’s chassis.

Keep researching until you thoroughly understand what you need to do, and good luck!


edit: clearly Dick types faster than I do, or has this stuff ready to cut and paste! :thumbup: