Anyone else finding their B2B charger struggles to keep up when alternator output drops at idle?

Been running a Victron Orion-Tr Smart 12/12-30 in my Transit conversion for about eight months now and generally it's been brilliant. Charges my 200Ah lithium leisure battery nicely on the move. The issue I keep running into is on longer slow drives or when I'm sat in traffic for a while — the engine's ticking over at around 600–700 RPM and the alternator output drops off noticeably. The Orion seems to back off its charge current and sometimes just cuts out entirely for a bit.

I've got the input voltage threshold set to 13.2V to try and protect the starter battery, which feels about right. But I'm wondering whether I've got that set too conservatively and it's triggering a cutoff when the alternator's just struggling a bit rather than the starter battery actually being low. The cable run from the starter battery to the Orion is about 2.5 metres of 10mm² which should be plenty for 30A.

Has anyone tweaked their input voltage settings to deal with this, or is this just a fundamental limitation of running a high-output B2B on a smaller van alternator? My alternator is rated at 90A so in theory there should be headroom, but I'm starting to wonder if it's a bit marginal at idle. Would be curious whether dropping to a 20A unit would actually solve anything or if the real fix is sorting the voltage threshold settings.

@Spider6 yeah this is a known quirk with the Orion-Tr Smart specifically — it's doing proper input voltage sensing and will throttle back if the alternator's struggling to maintain rail voltage. Modern Transits with their smart alternators make this worse because the ECU is actively managing output anyway.

Worth checking your actual alternator terminal voltage at idle with a multimeter rather than relying on the Victron app figures. Wouldn't be surprised if you're seeing sub-13V at a long red light.

Ran into similar on my static caravan setup using a B2B from a tow vehicle — solved it partly by bumping the input voltage threshold settings slightly lower in VictronConnect. You can tweak the "charge disconnect" voltage — just don't go too aggressive or you'll be robbing the starter battery.

@Spider6 worth checking what your actual alternator output voltage is at idle with a multimeter — some modern Fords have smart alternators that drop voltage quite aggressively when the ECU decides it's not needed, sometimes down to 12.8-13V which isn't really enough to trigger the Orion properly.

Had similar grief with my Sprinter until I realised the issue wasn't the B2B at all, it was the variable voltage management on the alternator itself. A fixed voltage relay override sorted mine, though obviously that's not for everyone depending on how comfortable you are poking around the charging system.

What's your input voltage showing in the Victron Connect app when it drops off? That'd confirm pretty quickly whether it's an alternator voltage issue rather than anything wrong with the Orion itself.

Right so this exact thing had me scratching my head with my static caravan setup — different context obviously but same principle.

What sorted it for me was understanding that the Orion essentially protects the alternator by throttling back when input voltage sags. It's not a bug, it's doing its job.

One thing nobody's mentioned yet — have you checked your cable run length and gauge from the alternator feed? A dodgy voltage drop across undersized cable can fool the Orion into thinking the alternator is struggling even when it isn't. I was losing nearly 0.4V across a run that was borderline spec.

Swap to proper marine-grade tinned cable if you haven't already. Made a noticeable difference to how aggressively my Victron would pull current before it started backing off.

Had this same issue with my Sprinter last winter. What sorted it for me was enabling the engine running detection via the ignition wire rather than relying on voltage sensing alone — means the Orion knows the engine is definitely running and behaves more predictably at low RPM.

Also worth experimenting with the input voltage lockout settings in the Victron Connect app. I bumped my "restart" threshold down slightly so it wasn't constantly cutting in and out when the alternator voltage dipped at traffic lights.

@JakeLee makes a fair point about Ford specifically — my mate's Transit had a notoriously lazy alternator that barely hit 13.8V at idle. Sometimes just giving the engine a brief rev clears the hesitation temporarily while you sort a proper fix.

@Spider6 had something similar with my cabin setup — different context to a van but the underlying issue's the same. Worth looking at your input current limit setting in the Victron Connect app. If it's set too high, the Orion is drawing hard and the alternator voltage sags, which then triggers the low-voltage cutoff. Try dropping the input current limit down a notch and see if it stabilises at idle. Also, some people run a small capacitor bank on the input side to smooth out the sag, though that's getting into more involved territory. The app tweaks are the obvious first port of call before anything else.

Been through exactly this on my narrowboat — different setup to a van but same principle applies. The alternator on a marine diesel will dip noticeably at tick-over, and my Orion-Tr Smart would throttle back or disconnect entirely.

What made the real difference for me was dropping the input voltage threshold in the Victron Connect app. Factory default is often conservative. Wind it down a touch and the charger stays connected through those low-idle periods rather than cutting out repeatedly.

Worth checking your alternator health too — a tired unit struggles to maintain voltage under load even at cruising revs. Had a worn belt contributing to mine before I realised what was happening.

@WildWanderer's point about engine detection is solid, but sorting the threshold first means you're not fighting the charger's logic before you've even addressed the root cause.

Something worth checking that I haven't seen mentioned yet — have you looked at your cable runs and connections between the alternator and the B2B input? Even a small voltage drop across poorly crimped terminals or undersized cable can cause the Orion to back off its output significantly, especially when alternator voltage is already marginal at idle. Grabbed my multimeter last year for exactly this diagnosis on my own setup and found nearly 0.4V drop I hadn't expected. Sorting that made a noticeable difference before I even touched any settings.