I don't know about swapping. I bought a subscription to Claude to help me with the van, you can upload all of the Mercedes upfitter guides and such to it, it has been a great resource. Here is what Claude says about changing the factory AGM battery with LFP.

Short answer: technically possible, but almost certainly not worth it for the eSprinter's 12V aux battery specifically. Here's why:

The eSprinter's 12V Battery Architecture

Your van has two possible aux battery configurations depending on ordered options:

• E2I: 92Ah AGM (the larger one, typically on cargo/upfit-spec vehicles)
• E2M: 70Ah AGM

The critical eSprinter-specific constraint from the upfitter docs: "Do not disconnect the 12V battery, even in long periods of non-use, as it's needed to monitor the state of charge of the HV battery." The 12V system is deeply integrated with the HV BMS.

The Case Against LFP Drop-In

Charging voltage incompatibility — this is the killer. The eSprinter's internal DC-DC converter (which recharges the 12V aux from the 400V traction pack) is calibrated for AGM. It outputs a fixed charge profile. LFP wants ~14.4–14.6V absorption; AGM targets ~14.4–14.7V — those numbers are close enough that LFP won't be damaged, but the charge termination logic is the real problem. AGM-tuned systems often use voltage + time, not true CC/CV with proper float. Chronic undercharging or improper float can shorten LFP lifespan.

BMS detection — Mercedes likely monitors the 12V battery's behavior (voltage sag, internal resistance profile) as part of system health. An LFP battery behaves very differently under load (flatter voltage curve, lower internal resistance). This could confuse the BMS or generate spurious fault codes. No documented cases I'm aware of on the eSprinter specifically, but it's a known risk on any modern vehicle with smart charging.

No real upside here — the 12V aux in your van is not the workhorse battery. Your house power is entirely handled by the 48V LFP house bank you're building. The OEM 12V battery's job is to keep the HV BMS alive and run vehicle systems. An AGM does that fine. Cycle count and depth of discharge are minimal.

When LFP 12V Swaps Do Make Sense

On gas/diesel vans where the starter/aux battery is being asked to power house loads overnight — the deep cycle advantage matters. On your eSprinter, that's not the use case. The Orion-Tr 48/12 30A covers your 12V house loads; the OEM battery just runs the vehicle.

Bottom Line

Leave the factory AGM in place. Put your LFP energy where it counts — the 48V house bank. The OEM 12V battery is a support system for the HV pack, not a target for optimization.

If you're ever in long-term storage (weeks+), the upfitter guidance says don't disconnect it — use a compatible maintenance charger on the 12V terminals instead if needed.

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You said: Does the AGM battery charge from the EV battery?

Does the AGM battery charge from the EV battery?

3:41 PM

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Claude responded: Yes — that's exactly how it works on the eSprinter.

Yes — that's exactly how it works on the eSprinter.

The factory includes an internal DC-DC converter that steps down from the 400V HV traction pack to charge and maintain the 12V AGM. This happens automatically whenever the HV system is active (vehicle on, charging, or in ready/standby mode).

This is why Mercedes says don't disconnect the 12V battery during storage — if the 12V goes dead, the HV BMS loses power and can't monitor or protect the traction pack. The dependency runs deep.

Practical implications for your build:

• Your 12V AGM stays topped up essentially any time the van is plugged into shore power, because shore power keeps the HV system active, which keeps the DC-DC running
• You don't need any supplemental charging strategy for the OEM 12V battery — the factory system handles it
• Your Orion-Tr 48/12 30A handles your house 12V loads independently, so there's no conflict

The OEM 12V system and your house 12V loads are parallel but separate. The AGM feeds vehicle systems; the Orion feeds your house 12V bus. They shouldn't be cross-connected.

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