Swapped out my split charge relay for a Victron BMS on the van build — few things I didn't expect

So I've been running a pretty standard leisure setup in my Transit for about two years — 100Ah AGM, split charge relay off the alternator, a basic PWM solar controller. Did the job for weekends but I've just done a full rebuild with a 200Ah lithium (Dakota Lithium 12V) and a Victron SmartSolar 100/20 with a proper BMS. Thought it'd be straightforward but there were a few surprises along the way.

The big one was the alternator charging side. The BMS cuts off charging once the battery hits full, which is great, but my old split charge relay wasn't happy with the sudden disconnect — got a spike that nearly took out my alternator diodes apparently. Ended up fitting a Victron Orion-Tr Smart DC-DC charger (the 12/12-30) instead, which isolates everything properly and the alternator hasn't complained since. Adds another £130 or so to the build but honestly feels like it was non-negotiable.

The other thing that caught me out was the solar figures. On paper I thought 200W of panel through the 100/20 would easily keep up with my fridge (a Dometic CFX3 35, pulls about 45W average) plus lights and the odd phone charge. And it does — but only just in proper overcast British weather. I'm now wondering whether to add a second 100W panel or whether a bigger controller and third panel is the smarter move.

Has anyone else gone down this lithium conversion route in a Transit or similar? Particularly interested in whether people have found the DC-DC charger genuinely keeps up on longer drives, and whether you've hit any issues with the Victron BMS playing nicely with non-Victron kit.

@Trevor1986 interesting timing on this — just went through similar with a Sprinter build last year.

One thing that catches people out with Victron BMS setups is the load disconnect behaviour. When the BMS trips on low cell voltage, it cuts everything simultaneously — no graceful shutdown. If you've got an inverter on that output rail, it'll die mid-draw. Caught me out badly the first time.

Also worth checking: is your alternator temperature-compensated? LiFePO4 charging profiles via a DC-DC (I run a Victron Orion-Tr Smart) are far kinder than direct alternator connection through a relay, but you need the engine battery isolation configured properly or you'll pull your start battery down during long BMS-managed absorption phases.

What cells are you running — Fogstar Drift or something custom? Makes a difference to how aggressive you can set the BMS parameters.

@Trevor1986 mate the moment you go Victron you basically sign up for a second hobby of just staring at the VictronConnect app at 2am wondering why your absorption stage looks "a bit sus" — ask me how I know 🔦

Real talk though, the jump from a dumb relay to proper BMS comms is like going from a sundial to a GPS, your alternator will actually thank you once you sort the charge profile properly.

Did you go the DC-DC charger route for alternator input or are you still feeding direct? Because with lithium that split charge relay setup can cause interesting moments for your alternator on cold starts — learned that the hard way in my Transit conversion before I chucked in an Orion-Tr Smart.

@NaeClue 😂 so true, the VictronConnect rabbit hole is real

@Trevor1986 did you notice the alternator behaviour change at all? When I wired my Victron setup on the boat I had to add a DC-DC charger (went with the Orion-Tr Smart) because the BMS kept dropping load and confusing the alternator. Nearly cooked it before someone on here flagged it.

AGM to lithium is a bigger jump than people realise — the charging profile difference catches everyone out at least once. What battery did you end up pairing with the BMS, Fogstar or something else?