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Lithium (LiFePO4) conversion for a Sprinter camper van

Thinking about a lithium house battery in a Mercedes-Benz Sprinter? Start with what Mercedes-Benz itself tells upfitters. This guide explains those rules in plain language.

LiFePO4 (lithium iron phosphate) is the most common lithium chemistry for van house batteries. Compared with lead-acid, makers such as Battle Born say you can use much more of its rated capacity, and it is lighter for the same usable energy. Our battery types guide covers the basics.

The Sprinter has a few rules of its own, though. Mercedes-Benz publishes a Body and Equipment Guideline (BEG) for upfitters, the companies that turn bare vans into campers, ambulances and work trucks. The current Sprinter (907) edition is dated 2026-9. Section 8.3.2, "Auxiliary battery," is the part that matters here. Everything below is a summary of that document, not an installation guide.

What the guideline says, in plain words

No big batteries straight onto the van's wiring. The guideline says capacities over 100 Ah "must not be connected directly to the on-board electrical system, because this may damage the basic vehicle."
Use the factory cutoff relay. Extra batteries have to connect through the factory battery cutoff relay, option code E36, and a fuse. The guideline says "It is not permitted to retrofit an aftermarket cutoff relay." It also says E36 must be chosen when the van is ordered, so it is installed at the factory.
Lead-acid matches lead-acid. An auxiliary battery wired through the relay should be the same type (AGM) as the van's own battery.
Different chemistry needs a charger in between. To use "different battery technologies," such as lithium, the guideline requires a current limitation system (a DC/DC converter or charger) between the cutoff relay and the extra batteries.
80 A is the ceiling. That charger must be set to a maximum of 80 A. The total current across the cutoff relay, including the charging current while the engine runs, must also stay under 80 A. On the electric eSprinter the limit is 40 A, and the upfitter must test the system for voltage oscillation.
The alternator has to keep up. The guideline says a "positive total charge balance" must be ensured by choosing a suitable alternator.
Never jump-start from it. Auxiliary batteries connected through the relay must not be used to start the vehicle from an outside power source.

The path the guideline describes for a different battery chemistry. The diagram is simplified; it does not show fuses, shunts, solar or shore charging.

Factory options to know about

Mercedes-Benz recommends using its own factory options when an auxiliary battery is needed. The guideline lists two: code E2I, a 92 Ah battery in the front passenger seat box, and code E2M, a 70 Ah battery in the engine compartment. The E36 relay comes automatically with either. It also lists EK1, an upfitter connector to use when you need to tap vehicle power, and says that if discharge demand is above 25 A, the E2I or E2M battery and E36 relay are used.

If you are buying a new Sprinter for a camper build, it is worth asking the dealer about these codes before you order. They are much harder or impossible to add later.

Why a DC-DC charger fits these rules

A DC-DC charger takes power from the starter side and charges the house battery with a controlled current and a charge profile for its chemistry. That matches what the guideline asks for: a limited charging current between the relay and a battery of a different type. Our alternator charging guide explains how they work.

As one example of what to look for, Victron's Orion XS datasheet lists:

Adjustable charge current in steps as small as 0.1 A, so it can be set under a limit. The 12/12-50A model adjusts from 1 to 50 A; the 12/12-70A from 1 to 70 A.
Compatibility with Euro 5 and Euro 6 "smart" alternators, which can otherwise supply too low a voltage to charge a house battery well.
Automatic shutdown at low temperature to protect lithium batteries, and input undervoltage protection so it does not drain the starter battery.
IP65 protection, except the 70 A model, which is IP20 and meant for a dry location.

This is one product's spec sheet, not a recommendation. Other brands make DC-DC chargers too. Whatever you choose, its current must be set so the total across the relay stays within the guideline's 80 A, and the charger's own manual must be followed.

Solar and shore power

The guideline says that charging an auxiliary battery connected through the E36 relay with an outside charger or a solar controller may only happen while the engine is off (relay open, or terminal 15 or D+ inactive). Many DC-DC chargers include solar inputs, and many lithium systems have separate solar and shore chargers. Make sure your design and wiring meet that rule. Sizing solar is covered in our van solar calculator, and controller types in MPPT vs PWM.

Lithium in a van: the battery side

Battery management system (BMS). LiFePO4 batteries rely on a BMS to protect the cells. Check what protections your battery's datasheet lists.
Cold charging. Lithium batteries should not be charged below freezing unless they have heating or a low-temperature cutoff. Battery University explains why in BU-410. Battle Born's 100 Ah datasheet, for example, lists cold-charging protection and internal heating.
Charge settings. Every charger in the system (DC-DC, solar, shore) should use settings the battery maker approves. See LiFePO4 charging basics.
Size. Work out how much you need with the house battery calculator.

Wiring, fuses and ventilation

Large house batteries can deliver very high currents. Cable size, fuses and connections matter for safety. Victron's free Wiring Unlimited book is a good plain-language reference. The guideline also says that an auxiliary battery inside the passenger compartment needs proper ventilation through a vent hose to the outside. That rule is written for the factory AGM battery; follow your lithium battery maker's instructions on venting and mounting.

Before you start

Get the current guideline. Rules change between editions and model years. Download the latest BEG from the Mercedes-Benz Vans upfitter portal, and check the section for your exact van.
Ask the upfitter portal. The guideline repeatedly points to the upfitter portal for questions, including any design with currents above 80 A, even briefly.
Consider a professional. The guideline warns that unauthorized changes to the vehicle electrics can cause failures in safety-relevant parts. A qualified installer who knows the Sprinter guideline is the safest route. Changes may also affect your warranty; ask your dealer.

Not sure you want a built-in system at all? A portable power station avoids most of this wiring. Compare them in power station vs built-in system, or start with camper van electrical systems, explained.

This page summarizes a manufacturer document for general information. It is not installation advice. Always follow the current Mercedes-Benz guideline for your vehicle and the manuals for every component.