DC-DC charger install for VW T6 leisure battery

The Problem

The VW T6 EU6 stop/start engine uses a smart alternator that mostly free-wheels and only charges during braking regeneration, so the stock split charge relay leaves the leisure battery perpetually stuck at around 80% state of charge. Compounding this, a solar-charged leisure battery drains back into the starter battery through the relay the moment the engine starts, wasting the entire solar harvest.

Solution

1. Confirm you need a DC-DC charger if your T6 has EU6 stop/start — the stock split charge relay cannot handle smart alternator voltage swings between 12.6 V and 15.2 V.
2. Size the charger by adding the battery's recommended charge rate (10–20% of its Ah rating) to your worst-case simultaneous load in amps; for a 100 Ah battery and 10 A of loads, a 30 A unit is the minimum. The common UK choices at this power level are the Victron Orion-Tr Smart 12/12-30, the Sterling B2B (e.g. BB1230), and the CTEK D250SE — any of these is a sound starting point for product selection.
   • Power users running large inverters should account for the full inverter draw and consider paralleling two charger units.
3. Run the charger's input cable from the starter battery and its output cable to the leisure battery, using cable rated for the charger's maximum continuous current. For a 30 A charger, use 6 mm² for short runs (up to roughly 2–3 m each way) and step up to 10 mm² for longer runs to keep voltage drop in check.
4. Connect the engine-run signal wire so the charger knows when the alternator is live: the OEM BCM output at connector J519 pin T73a/3 is the best source, an ignition live is second choice, and the charger's built-in auto-detection is a workable fallback that requires threshold tuning.
   • If the van originally had a factory auxiliary battery relay, the small grey wire on that relay can supply the engine-run signal after removing the relay link wire.
5. If using the OEM BCM output, activate the engine-run feed in VCDS by coding Byte 17 Bit 4 (Battery Separating Relay active). This coding lives in the Central Electronics control module (address 09 — the Body Control Module), so open that module's coding block rather than any other unit.
6. Configure the charger's battery profile to match your leisure battery chemistry (AGM, SLA, LiFePO4, or gel) using the manufacturer's app or DIP switches.
7. Confirm the charger cycles through bulk, absorption, and float stages after a drive — the leisure battery should reach 100% state of charge and hold it at float rather than sitting at 80%.