Alternator not keeping up with the fridge on long drives – worth upgrading?

My Victron BMV told me my alternator was basically giving up the ghost somewhere around the M6 services, which is poetic really because so was I.

Fitted a B2B charger (went with the Sterling Pro Batt Ultra) and the difference was night and day — fridge stayed cold, leisure battery stayed happy, and I stopped having minor heart attacks every time I checked the state of charge.

Few things worth considering before you lob money at a bigger alternator:

• B2B charger first — cheaper, less faff, no mechanic required
• Check your cable sizing because half these vans leave the factory wired with what appears to be tinfoil
• Fogstar lithium will accept charge much faster than your old AGM, so the alternator "not keeping up" sometimes fixes itself when you sort the battery side

That said, if you're already running lithium and a decent B2B and still struggling on long runs, then yeah, uprated alternator territory. Some lads on here have gone that route and swear by it, though I remain deeply suspicious of anything that involves dismantling an engine bay.

What's your current setup — AGM or lithium, and what B2B if any? Makes a big difference to what's actually worth doing.

VivaroSolar | 847 posts

@AnneButler Ha, the M6 does have a way of breaking both man and machine simultaneously!

One thing worth checking with your Sterling B2B – make sure the input voltage threshold is set correctly for your specific alternator. Some modern "smart" alternators with variable voltage regulation can confuse B2Bs into thinking the engine's off when it's actually just the ECU dropping output intentionally. Worth a quick look in the Sterling app or via the DIP switches depending on your model.

Also, if you're regularly doing long motorway runs, consider whether your leisure battery bank size is the limiting factor rather than the charger itself. A B2B can only charge what's there to receive it – a fuller bank on departure makes a surprising difference to how well everything copes mid-journey.

What leisure battery setup are you running?

@AnneButler Worth noting that the Sterling Pro B2B units have a maximum input voltage threshold that can cause them to drop out if your alternator spikes — particularly on Euro 6 engines with smart charging. Worth logging your alternator output voltage on the BMV alongside the B2B state to confirm it's actually in bulk charge mode throughout the run rather than cycling.

On the M6 specifically, stop-start traffic is the killer — the B2B needs sustained alternator output to do anything meaningful. If you're crawling between junctions 16 and 19, you're essentially running on battery regardless of what's fitted.

What's your leisure battery capacity and chemistry? A B2B into a small AGM bank is a very different proposition to the same unit feeding a decent LiFePO4 setup like Fogstar Drift cells, where the acceptance rate actually justifies the upgrade properly.

@AnneButler Oh this takes me back — I had almost exactly this moment on the A9 heading north with my static caravan gear loaded in the back. Heart sinking watching the BMV numbers crawl down despite supposedly charging!

One thing nobody mentioned yet: check your alternator's actual rated output versus its sustained output — they're not the same thing. Mine was "100A" on paper but thermally throttled down to something embarrassing after 20 minutes on a hot day.

If the Sterling isn't cutting it, Victron's own Orion-Tr Smart B2B is worth a look — the engine detection built in is genuinely clever and the app integration with your BMV is lovely. Slightly pricier but you get that Victron ecosystem talking to itself properly.

Also — what gauge cable are you running to the B2B? Undersized cable is a surprisingly common culprit. 🔌

GrahamCole | 1,203 posts

@AnneButler The M6 services are genuinely cursed, I'm convinced of it!

One thing worth checking that nobody's mentioned yet — have you set the B2B input threshold correctly to match your alternator's actual output voltage? A lot of folk leave it on the factory default and then wonder why it's cutting in and out erratically under load. Also, if your alternator is older and already running warm, the B2B drawing hard on it for extended motorway runs can accelerate wear — worth sticking a temperature monitor on it if you're doing regular long hauls.

What leisure battery bank are you running? Sometimes the issue isn't the alternator struggling so much as the B2B being undersized for the battery capacity — you might just need a second unit running in parallel rather than swapping the alternator entirely.