Fitted a 40A DC-DC charger yesterday — why is my leisure battery barely getting above 14V?

So I finally got round to installing a Victron Orion-Tr Smart 12/12-30A (yes I know the thread title says 40A, I mislabelled it — it's the 30A version) in the Transit-based van. Wired it straight from the starter battery via a 40A fuse close to the battery, earth back to the chassis, and the output going to a 100Ah lithium leisure battery from Fogstar. Seemed fairly straightforward.

Problem is, when I'm driving I'm only seeing around 13.8–14.0V on the leisure battery. The Victron app is showing the charger in "bulk" mode and claiming it's pushing out 28–29A, which sounds about right, but I'd expect the voltage to climb higher than that with a lithium. The Fogstar battery wants something like 14.4–14.6V to properly top off. I've double-checked the Orion's profile in the app and it is set to lithium, not AGM.

Has anyone else seen this with the Orion-Tr Smart? I'm wondering if it could be a voltage drop somewhere in the cabling — I used 6mm² twin throughout which might be on the light side for a 30A charger over a roughly 3-metre run. Or could it be something daft I've overlooked in the Victron settings? Open to any suggestions before I start pulling the dash apart again.

KenGraham | Posts: 847

@Trevor1967 the Orion-Tr Smart has a default absorption voltage of around 14.4V, so if you're only seeing 14V you're probably still in bulk phase — perfectly normal behaviour. How depleted was the leisure battery when you started driving? If it was fairly flat, bulk can take a good while.

Also worth checking — have you configured it via the VictronConnect app? Out of the box the charge algorithm might not match your battery chemistry. If you've got a lithium or AGM bank rather than standard flooded lead-acid, the default profile won't be optimised.

One more thing: what gauge cable did you run, and how long is the run? Voltage drop on undersized cable can absolutely rob you of those final few tenths.

CraigLamb | 847 posts

@Trevor1967 Worth checking whether you've got the Orion set to standalone or engine detection mode — if it's reading a false "engine off" signal it'll throttle right back or cut out entirely. Also, what cable run length have you got between the alternator/starter battery and the Orion input? Even a short section of undersized cable will cause enough voltage drop to prevent it hitting the absorption threshold properly. The Orion wants to see a solid 13.2V+ on the input before it'll push hard. Pop into the VictronConnect app and check the input voltage while it's running — that'll tell you straight away if you've got a supply problem rather than a unit problem.

FrankPalmer | Posts: 312

@Trevor1967 One thing nobody's mentioned yet — how long are the cable runs from your starter battery to the Orion, and what gauge cable did you use? If the wire's undersized you'll be losing voltage in the run itself, which means the charger never sees a proper input voltage to work with. The Orion-Tr Smart has a low voltage disconnect around 11.5-12V on the input side, so it might be throttling back its output if there's significant voltage drop under load. Worth sticking a multimeter directly on the Orion's input terminals while it's running and comparing that reading to what's actually at your starter battery. If there's more than about 0.3-0.4V difference, your cabling's the culprit. Victron recommend pretty beefy cable for the 30A version.

DODQueen | Posts: 1,204

@Trevor1967 what battery chemistry have you got in the leisure bank? If it's LiFePO4 and you haven't told the Orion that, it'll be running lead-acid profiles and possibly cutting off absorption early. Connect it up via Bluetooth in the Victron Connect app and double-check the charge algorithm — it's a common gotcha.

Also worth noting: the 30A Orion will only output properly if the input voltage is healthy. If your alternator is a smart/variable voltage type (pretty common on modern Transits), the Orion might be seeing low input and throttling back. There's a setting in the app for that too.

@FrankPalmer makes a good point on cable runs — voltage drop on the input side will absolutely strangle output.

MidlandsCamper | Posts: 2,156

@Trevor1967 The 30A Orion in absorption is only targeting ~14.4V by default — sounds like it's actually working correctly. The question is whether it's reaching that 14.4V and sitting there, or topping out early.

Check your VictronConnect app — look at the charger state. If it's flicking between bulk and absorption repeatedly, you've likely got a dodgy connection somewhere causing a voltage drop under load. The Orion senses output voltage at its terminals, not at the battery, so any resistance in the cabling gives you a false reading.

On my cabin build I had exactly this — measured 14.2V at the Orion output terminals but only 13.6V actually reaching the battery. Turned out to be an undersized inline fuse holder with corroded contacts. Classic hidden resistance problem. Check every connection in that positive run with a multimeter under load.

SussexVanLifer | Posts: 847

@Trevor1967 Worth checking your Victron Connect app — the Orion has a "charger only" mode and an "engine running detection" mode, and if it hasn't properly detected alternator voltage it may be throttling back or not entering absorption at all. I had exactly this on my Sprinter build; the unit was sitting in float prematurely because the engine detection threshold wasn't matched to my Ford's alternator output. Took me an embarrassingly long time to spot it. Pull up the charger history in the app and see what phase it's actually reporting.

DazBarker93 | Posts: 847

@Trevor1967 Worth checking your Victron Connect app settings — the Orion-Tr Smart lets you manually adjust the absorption and float voltage targets. Factory defaults are fairly conservative. Also, have a look at whether you've accidentally got it set to "power supply" mode rather than "charger" mode, which I've seen catch people out before. That'll cap the output voltage much lower than you'd expect. If you're running AGM or gel, the profile matters quite a bit. What does the app show as the actual mode it's currently operating in?

BlownFuse | Posts: 847

@Trevor1967 One thing worth checking — is the Orion actually seeing accurate input voltage at its terminals, or is it throttling output because it thinks the alternator is struggling? If your cable run from the starter battery is undersized or has a poor connection, the Orion will see voltage sag under load and back off its output current, which drags the charge voltage down with it.

Also, have you connected the remote on/off wire to a switched ignition live? If it's running in engine-off mode intermittently, the Orion may be deliberately limiting output to protect the starter battery.

Worth pulling up the live data in the Victron Connect app — the input voltage and current figures will tell you immediately whether it's a supply-side problem rather than anything wrong with the unit itself.

Stu1991 | Posts: 312

@Trevor1967 Also worth checking whether the Orion's in standalone mode or engine-detection mode — if it's using alternator sense and the threshold isn't set right, it'll throttle back or shut off entirely and you'll be scratching your head wondering why your Fogstar's barely warm. Had exactly this faff on the narrowboat before I realised the Victron Connect app had defaulted to some baffling input voltage threshold that my tired old alternator couldn't reliably hit. 📱