Anyone else finding their B2B charger struggles to keep up when the alternator's already warm?

Been running a Renogy 40A DC-DC charger in my Transit for a few months now, feeding a 200Ah lithium leisure battery. Generally happy with it, but I've noticed something a bit frustrating on longer runs — if I've been doing stop-start driving for an hour or so before hitting the motorway, the charger seems to throttle back noticeably once the alternator's been warm for a while. I'm only seeing maybe 20-25A instead of the full 40A, even with the leisure battery sitting at 50% SOC and plenty of headroom to charge.

I've checked my wiring and I'm running 6mm² cable with an 80A inline fuse, so I don't think it's a cabling issue. Voltage at the charger input is sitting around 13.8-14.0V, which seems reasonable. Wondering if the charger itself is just being cautious, or whether there's something about alternator output voltage drooping slightly under sustained load that's causing it to back off.

Has anyone else seen this with the Renogy units specifically, or is it more of a general B2B thing? I'm also curious whether fitting a temperature sensor (if that's even an option on this unit) would make any difference, or whether I should just accept that real-world output is going to be a fair bit below the rated 40A in these conditions.

@JakeLee worth checking whether the Renogy is thermally throttling itself rather than the alternator being the bottleneck. Most B2Bs will derate output when their internal temp climbs — mine (Victron Orion 30A on the narrowboat) drops noticeably on long summer runs until I added a small 12v fan blowing across it.

Where have you mounted yours? Tucked behind a panel with no airflow is a common culprit. Even 50mm clearance each side makes a real difference.

Also worth logging the input voltage during those warm periods — if the alternator itself is sagging under load you'll see it drop below the B2B's minimum threshold and it'll cut in/out rather than running continuously. A cheap Bluetooth battery monitor on the starter side can reveal a lot.

HighlandDweller | Posts: 847 | Location: Scottish Highlands

@JakeLee yes, absolutely seen this with mine. Worth checking whether your alternator's actually hitting its rated output when warm — most drop off significantly once they're up to temperature, it's just physics really. Your B2B charger will only pull what's available without tripping the alternator's thermal protection.

A few things that helped me: make sure your cable run from alternator to charger is properly sized (voltage drop kills efficiency), and check whether you've got decent ventilation around the charger itself. The Renogy units do throttle back when they get warm too.

Also worth logging your battery voltage during charging — if it's sitting consistently below 13.8V at the charger input, your alternator's genuinely struggling rather than the B2B being the culprit. What size alternator are you running?

My Victron Orion-Tr Smart did the exact same sulky act on a hot July run down the M6 — turns out the Transit's alternator was already sweating like a nurse on a 12-hour shift before the B2B even got involved.

Check your alternator's actual output voltage when warm; if it's already drooping below 13.8V the B2B hasn't got much to work with regardless of its rated amps.

LiFePO4_Guy | Posts: 1,203 | Location: Derbyshire

@JakeLee something worth considering that nobody's mentioned yet — Transits are notorious for the alternator being mounted in a particularly poorly-ventilated spot, and Ford's variable voltage charging logic can genuinely confuse B2B chargers. When the ECU decides to drop alternator output to save fuel, your B2B sees what looks like a weak supply and backs off accordingly.

Try logging your starter battery voltage during those long runs if you can. If you're seeing it dip below 13.2V regularly whilst moving, the alternator's simply not got enough headroom left to properly feed the B2B and keep the starter battery happy. A dedicated alternator sense wire on the Renogy (rather than relying on voltage alone) might help it make smarter decisions about when to pull power.

Ran into the exact same pattern in my motorhome last summer — long A-road run on a hot day, Victron Orion-Tr Smart barely pushing 15A when it should've been doing 30A.

Turned out the problem was the input voltage sagging more than I'd expected once the alternator was working hard in heat. The B2B needs a decent input voltage to operate properly; if the alternator's already stressed and output drops, the charger backs off accordingly.

Worth logging your alternator terminal voltage whilst it's happening. If you're seeing anything below 13.5V at the input, that's your culprit — not necessarily the charger misbehaving.

Fitting a heavier input cable made a noticeable difference on mine. Voltage drop across undersized wiring becomes much more significant when currents are high and temperatures are up.

Narrowboat angle here — we see this constantly on the cut. Long summer cruise, engine already cooking, and the B2B just throttles right back.

What helped us was fitting a small 12v fan directly onto the Orion's heatsink, triggered by a simple thermal switch. Cheap fix, dramatic difference. The unit runs noticeably cooler and holds closer to rated output for longer.

Worth also checking your alternator voltage under load — if it's sagging below 13.8v when warm, the B2B has even less to work with before it starts derating. A Victron SmartShunt logging over Bluetooth will show you exactly what's happening in real time rather than guessing.

@LiFePO4_Guy curious what you were about to say — don't leave us hanging!

EcoFlow_Queen | Posts: 2,847 | Location: Array

Worth checking your alternator's actual output voltage under load — not just at idle. I logged mine with a Victron SmartShunt and found it sagging to 13.1V on hot runs, which effectively starved the Renogy's input threshold. The charger wasn't "struggling" so much as throttling correctly.

@RustyTinker the Orion-Tr has that adaptive input current limiting which helps, but even Victron can't conjure watts from a drooping alternator.

Consider fitting a proper alternator temperature sensor if your unit supports one — lets the B2B back off before the alternator thermally limits rather than chasing it reactively.

Good shout from @EcoFlow_Queen on checking actual voltage under load. One thing I'd add — have you looked at where your B2B charger itself is mounted? If it's tucked somewhere with poor airflow near the engine bay, it'll thermal throttle before the alternator even becomes the limiting factor. I moved mine to a cooler spot in the hab area with a small 12v fan blowing across it and saw a noticeable difference on long runs. Also worth logging your input voltage over time with a simple Bluetooth monitor — sometimes what feels like an alternator issue is actually voltage drop across undersized cable runs heating up.

Good point from @T5Solar on cable routing. In my shepherd's hut setup I don't deal with alternators but I did have similar thermal throttling grief with a Victron Orion in the motorhome last summer. Ended up mounting it away from the engine bay heat rather than tucked against the bulkhead — made a noticeable difference. These units have internal temp sensors that will pull back output well before they actually fault. Worth checking where yours is physically sitting before assuming it's an alternator issue.

Really common issue this. What's happening is your B2B's input MOSFET threshold voltage is temperature-sensitive — the charger's internal protection is likely throttling before your alternator actually hits its thermal limit, because the sense circuitry reads ambient temperature inside the unit rather than the alternator directly.

On my garden office setup I use a Victron Orion-Tr Smart and had similar behaviour until I relocated the charger away from any heat soak paths and added a small 12V fan triggered off ignition. Dropped throttling incidents considerably.

@T5Solar raises a good point about cable routing too — voltage drop compounds everything when the alternator's already stressed. Worth logging input voltage with a BM-series monitor during those long runs to see exactly where the drop-off begins.