Alternator not keeping up with leisure battery on long drives – worth upgrading?

Ran a B2B charger (DC-DC) between my van's alternator and the leisure battery for my garden office setup and honestly it was the single best upgrade I made — the stock alternator was basically just waving at the battery from across the car park.

The issue with modern vehicles (especially anything Euro 6 with smart alternator management) is that the ECU throttles output to save fuel, so your leisure battery is essentially getting the bare minimum whilst the engine battery sits there living its best life.

A few things worth considering before just throwing a bigger alternator at it:

• B2B / DC-DC charger (Victron Orion or Sterling B2B are popular) — forces a proper charge cycle regardless of what the alternator is doing
• Smart alternator? Check first, because a simple split charge relay will be basically useless
• Battery chemistry matters — LiFePO4 will annihilate your charge time compared to AGM, so you'll see improvement regardless

Upgrading the alternator itself is the nuclear option — doable, but pricey and pointless if the ECU is still limiting output anyway.

What van are you running and what leisure battery setup? That'll narrow it down massively — someone on here will have done exactly your configuration already.

@NickBennett interesting timing on this thread — I'm mid-conversion on a Transit and wrestling with exactly this decision right now.

Quick question though: what size B2B did you go with? I've been looking at the Victron Orion-Tr Smart (the 30A isolated version) but wondering if that's overkill for a 110Ah Fogstar Drift LiFePO4.

Also — did you notice any weird behaviour when the van's stop-start system kicked in? That's the bit that's got me nervous. Apparently some of the cheaper units don't handle it well and you end up with the charger hunting around like a confused Labrador.

My alternator is only 150A stock on the Transit, so I'm also wondering whether to just get the B2B sorted first or factor in an alternator upgrade at the same time. Anyone done both simultaneously?

@HollyGaz Transit is a decent platform for this — what alternator does yours have? The stock 180A units are actually reasonable but a quality B2B like the Victron Orion-Tr Smart will make a massive difference regardless. I've got one on my motorhome running into a 200Ah Fogstar Drift LiFePO4 and the charging profile is night and day compared to running direct.

Key thing people miss: wire gauge matters enormously on the input side. Undersized cable will throttle your available current before the B2B even gets a chance to do its job. I went 35mm² on mine after initially bodging it with 16mm² and wondering why I was only seeing 15A output.

What's your leisure battery chemistry? If you're going lithium the B2B is basically non-negotiable anyway since most alternators can't handle the low internal resistance load safely.

@MrBodge65 @HollyGaz — curious what B2B charger you ended up going with, @NickBennett? I'm in a similar situation with my tiny house build where I'm trying to balance EV charging alongside a leisure battery bank (Fogstar Drift LiFePO4s).

Does the B2B make a meaningful difference if you're also running solar? My worry is that on longer motorway runs the alternator input would just overlap with what the panels are already doing, making the upgrade pointless for my use case. Or does the DC-DC still add value as a secondary charge source even when solar is doing most of the heavy lifting?

Also wondering whether upgrading the alternator itself vs just adding a B2B is the smarter spend — has anyone done a proper cost comparison on that?

@CarolThomas79 Victron Orion-Tr Smart is the answer to that question before anyone else types it — mine's been bolted in the motorhome for two years without a single sulk, which is more than I can say for my last alternator.