Anyone else finding their B2B charger underwhelms on a long motorway run?

Fitted a Victron Orion-Tr Smart 12/12-30 in the van about three months ago and overall I'm pretty chuffed with it, but I've noticed something a bit odd. On longer motorway stretches — say 90 minutes or more at a steady 65-70mph — the charger seems to throttle back well before the leisure battery (a 100Ah lithium) is anywhere near full. Talking maybe 60-70% SoC and it's already stepping down to 8-10A. Engine bay temps are reasonable, it's not a heat issue as far as I can tell.

I've got it wired with 6mm² cable from the starter battery, runs about 1.8 metres, and I'm using a 40A inline fuse on the positive. Bluetooth app shows the unit going into absorption prematurely, which makes me wonder if the voltage profile from the alternator is fooling it somehow. Van is a 2019 Ford Transit Custom with the stock 180A alternator if that helps.

Has anyone else seen this with the Orion-Tr Smart, or with B2B chargers generally? I'm wondering whether tweaking the absorption voltage threshold in the app would actually help, or whether I'm barking up the wrong tree entirely. Could it be the Transit's smart alternator doing something weird with voltage that's confusing the unit?

Hey @UWW_Power — I reckon I know where you're going with this! Are you finding the charging tapers off and you're not getting the full 30A sustained?

Worth checking whether your alternator voltage is drooping under load — some modern smart alternators back off when they detect what they think is a fully charged starter battery. The Orion-Tr is clever enough to follow that voltage down, which can make it look like it's underperforming when actually it's doing exactly what it should.

Also worth a look at your cable run and connections — voltage drop across longer cable lengths will genuinely rob you of amps. What gauge are you running and how far is the Orion from the starter battery?

If you've got the VictronConnect app paired up you can check the live data and history graphs, which makes diagnosing this much easier.

Great thread @UWW_Power — I had almost identical behaviour with my Orion-Tr Smart before I sorted the wiring. Worth checking your voltage drop between the starter battery and the B2B input. On longer runs the alternator is happily sitting at 14.2-14.4V at source, but by the time it reaches the Orion you might be seeing significantly less due to undersized cable or a dodgy connection. The Orion will throttle back if input voltage dips. I ran a dedicated 10mm² cable directly from the alternator positive rather than tapping off the existing loom and it made a noticeable difference to sustained output. Also worth checking whether you have a smart alternator — those can cause all sorts of grief with B2B chargers behaving erratically on longer runs. What cable size are you currently running between the two batteries?

Interesting one, and I suspect @Slim13 is already sniffing around the right answer but let me add some precision here.

The Orion-Tr Smart has a fairly well-documented behaviour where, once the absorption stage kicks in, current drops significantly — this isn't a fault, it's intentional. On a 90-minute motorway run your leisure battery might be reaching that threshold sooner than you'd expect, particularly if it's a lithium like a Fogstar Drift where voltage rises quickly under charge.

Worth checking your Victron Connect logs after a run. The absorption voltage target and duration are configurable — I had mine set far too conservatively initially on my motorhome setup.

Also worth confirming: is your alternator voltage actually staying stable at motorway speeds? A sagging starter battery voltage will throttle the Orion regardless of your settings. That tripped me up for an embarrassingly long time.

Good shout from @DebbieTaylor on the wiring — that's often the culprit before anything else. One thing I'd add that hasn't been mentioned: check your alternator type. Newer vehicles with smart/variable voltage alternators (Euro 6 onwards especially) will actively reduce output voltage on a long cruise once the ECU decides the battery is "happy." The Orion sees that drop and throttles back accordingly — working exactly as designed, but frustrating if you're expecting a full 30A the whole run. Worth logging the input voltage over a journey with the Victron Connect app to see if that's what's happening.

Really interesting thread this — @UWW_Power, one thing nobody's mentioned yet is the Orion-Tr Smart's thermal derating. These units will quietly reduce output current when the internal temperature climbs, and on a long motorway run the engine bay gets properly warm. If yours is mounted somewhere with limited airflow, that 30A rating becomes somewhat theoretical after the first hour or so.

Worth downloading the VictronConnect history and having a look at what current it was actually pushing during those motorway stretches — the app logs it and you'll see if output gradually dropped off rather than cutting suddenly. If that's the pattern, repositioning or adding a small ventilation gap around the unit often sorts it completely. Mine made a noticeable difference once I moved it away from the bulk of the engine heat.