Best way to charge a 200Ah LiFePO4 from a split charge relay on a 2.0 TDI — am I bottlenecking myself?

So I've got a 200Ah LiFePO4 (Fogstar Drift) fitted in my Transit conversion, currently being charged off the alternator via a basic split charge relay — not a B2B, just an old-school voltage-sensing relay I had lying around. It does something, but I've got a sneaking suspicion I'm not getting anywhere near what I should be.

From what I understand, LiFePO4 batteries have such a low internal resistance that they'll basically just hammer the alternator trying to fill themselves up — and a dumb relay does nothing to manage that. My 2.0 TDI alternator is rated at 140A, and I'd rather not cook it on a long motorway run with a half-flat leisure battery sat behind it demanding everything it can get.

Has anyone actually measured what current they're pulling through a setup like this? I'm wondering whether fitting something like a Victron Orion-Tr Smart 30A B2B would actually be a step backwards in terms of charge speed, given it'd limit me to 30A input. Or is the controlled, steady current worth it for alternator longevity even if it slows things down?

Trying to work out if the B2B is genuinely necessary or whether I'm overthinking it and the relay is actually fine in practice.

Yeah, the relay is almost certainly your bottleneck. A voltage-sensing relay just sees "battery connected" and dumps whatever the alternator decides to give — problem is LiFePO4 sits at a low internal resistance so it'll hammer the alternator trying to bulk charge fast, then the alternator gets hot and derate kicks in.

Proper B2B (I run a Victron Orion-Tr Smart 30A in my tiny house setup on a similar TDI) limits current properly and lets you set absorption voltage correctly for LiFePO4 — something a dumb relay fundamentally can't do.

The Fogstar Drift deserves better than a relay tbh. At 200Ah you're leaving serious charge efficiency on the table.

• Victron Orion-Tr Smart — solid but pricey
• Sterling BB1260 — cheaper, well-regarded in van builds
• Renogy DC-DC — budget option, works fine

Worth the upgrade before your alternator decides otherwise.

Hey @Tommo55, just to add to what @LiFePO4Fan was getting at — the other issue with a plain voltage-sensing relay is that your alternator has no idea it's talking to a LiFePO4. Because the Drift accepts charge so aggressively at low state of charge, you're essentially asking your 2.0 TDI's alternator to sustain near-maximum output for extended periods, which these modern "smart" alternators really don't appreciate. You can end up with premature alternator failure.

A decent B2B like the Victron Orion Smart or Sterling BB1260 solves both problems — it regulates the draw on the alternator and delivers a proper charge profile to the LiFePO4. The 60A Orion would be my recommendation for your setup. Pricier upfront, but your alternator will thank you long-term. What's your typical daily drive distance? That'll help work out whether 60A is worth it over 30A.

Great points from @LiFePO4Fan and @NeilSmith already. One thing worth adding — LiFePO4 batteries have a very flat voltage curve, so a voltage-sensing relay struggles to know when they're actually full. You could easily be running around thinking the bank is charged when it's sat at 80-85% because the relay never triggered a proper absorption stage.

A B2B (DC-DC charger) like the Victron Orion Smart or Sterling B2B solves this properly — it'll give your Fogstar a genuine multi-stage charge profile regardless of what the alternator's doing. Your 2.0 TDI should handle a 30A B2B without complaint, and you'd be looking at a realistic 30A into the battery rather than whatever scraps the relay's passing through. Worthwhile investment given what a Fogstar Drift costs.

Great thread, and @T6Dream makes a really important point about the flat voltage curve. Just wanted to add something nobody's touched on yet — your 2.0 TDI's alternator is probably rated around 150-180A, but it's not designed to sustain that continuously, especially with modern "smart" alternators that deliberately reduce output to cut fuel consumption and thermal load. So even if you ditched the relay for a proper B2B charger, you'd want to size it sensibly — something like a Victron Orion-Tr Smart 30A wouldn't hammer your alternator and would give your Fogstar a proper CC/CV charge profile. Worth checking whether your Transit's alternator is externally regulated too, as that affects your B2B options. Don't let the headline 200Ah capacity tempt you into fitting a massive B2B thinking faster is always better!

Really good thread this. To add a practical angle — I run a B2B (Victron Orion-Tr Smart 30A) in my motorhome and the difference vs. a basic relay was night and day. The big thing nobody's mentioned yet: a 2.0 TDI alternator hates having a hungry LiFePO4 hammering it at full load constantly. The B2B actually protects your alternator by letting you limit the charge current, which is worth its weight in gold on a modern variable-voltage alternator that'll throw fault codes if it's overwhelmed.

The Orion also has engine-running detection so it doesn't accidentally drain your starter battery — proper job.

@Tommo55 what cable gauge are you running between the batteries currently? If it's the original relay wiring, that could be a bottleneck regardless of what charger you fit.

Yeah, the relay issue is real. Had the same setup in my static before I knew better — relay just sees "voltage high enough, connect" and your LiFePO4 just sits there half-charged wondering what's going on.

Upgraded to a Victron Orion-Tr Smart 30A and the difference was noticeable straight away. Proper CC/CV profile, isolates correctly, and you can tweak settings in the app.

On a 2.0 TDI you've got a decent alternator but without a B2B you're not extracting it properly anyway. The Orion won't overload the alt either — it ramps up gradually which is handy on older engines.

Fogstar Drift deserves better than a relay tbh — it's a decent battery, give it the charger to match.

Something worth mentioning that nobody's touched on yet — check what your 2.0 TDI's alternator is actually rated at before sizing a B2B. Some of those engines (depending on year and spec) are only running a 90A alternator from factory. A lot of people assume bigger is better with a B2B, but if you're pulling 40A+ for charging on top of your vehicle loads, you can stress the alternator something rotten, especially at lower RPMs in town. I'd say a 30A B2B is probably the sweet spot for that engine. @ExTrucker73's Victron Orion-Tr Smart would be exactly what I'd recommend too.

Great point from @DuctTapeDave62 — worth adding that even if the alternator is rated at, say, 150A, you won't see anything like that available for charging once the vehicle's own loads are accounted for (lighting, ECU, fans, etc.). Realistically you might have 40-60A spare on a good day. A B2B will use whatever's available efficiently and protect the alternator from sustained high draw, whereas your relay setup could be pulling unpredictably depending on how depleted that Fogstar is. Definitely worth the upgrade if you're doing regular wild camps.

The real villain nobody's naming is the voltage drop across that relay — by the time the LiFePO4 actually sees enough volts to charge properly, you're basically tickling it rather than filling it. A Victron Orion-Tr Smart DC-DC sorted my static caravan's leisure bank overnight, and yes I know you're in a Transit but the principle's identical — the B2B actively pushes current rather than waiting for gravity to do the work. Your Fogstar Drift deserves better than a relay that was designed for flooded lead-acid in 1987.

Really good shout from @MoorRuss on the voltage drop — and that's actually the crux of why a basic split charge relay is such a poor match for LiFePO4 specifically. Unlike AGM, a LiFePO4 sits at a fairly flat resting voltage for most of its charge cycle, so the relay's threshold never really "sees" a meaningful difference between the two batteries and can close/open erratically. A proper B2B like the Victron Orion-Tr Smart sorts all of this in one go — regulates input, outputs a proper absorption profile, and your alternator isn't getting hammered cold either.