Anyone else running a split-charge relay alongside a DC-DC charger? Confused about which does what

I've been scratching my head over this for a few weeks now and thought I'd throw it out here. I've got a 2019 Ford Transit-based camper with a 200Ah lithium leisure battery and I recently fitted a Victron Orion-Tr Smart 12/12-30A DC-DC charger to properly charge the lithium from the alternator. The thing is, the van came with an old VSR split-charge relay already wired in, and I'm not entirely sure whether I should rip it out or just leave it in place alongside the Orion.

My understanding is that the DC-DC charger is doing the proper job — controlling the charge profile so it doesn't cook the lithium or confuse the smart alternator — but the VSR is still sitting there potentially closing when the voltage rises. I've read conflicting stuff about whether having both active is a problem or whether the relay just becomes redundant. Some people say leave it, some say it can cause issues with newer Euro 6 alternators that throttle back when they see a big load.

Has anyone actually tested what's happening with both in the circuit? I've got a Victron SmartShunt on the leisure side so I can see what's going in, and when I'm driving I'm seeing around 28-29A into the battery which seems right for the Orion. But I genuinely don't know if the VSR is also contributing or just sitting there doing nothing.

Worth pulling the VSR out completely, or is there a sensible reason to keep it? Happy to share my SmartShunt screenshots if that helps anyone diagnose what's going on.

SparkySailor | ⚡ 847 posts | Verified Electrician

@Del58 Classic confusion this one! Short answer: you don't really need both. The split-charge relay was designed for lead-acid setups where voltage-sensing works fine. Problem is, lithium batteries sit at a higher resting voltage and can fool the relay into thinking it's already full, so it never opens properly.

Your DC-DC charger (assuming it's a Victron B2B?) is doing the proper job — it actively steps up/regulates the charge current and importantly protects your alternator from being flogged to death by a hungry lithium bank.

My honest advice: let the DC-DC charger do its thing and either remove the relay entirely or wire it so it can't interfere. Running both can cause conflict depending on how it's wired.

What's the exact model you've fitted? That'll help narrow it down. 👍

BerlingoWanderer | Posts: 847

@Del58 sounds like your post got cut off there, but I think I can guess where you're heading!

Short answer: with lithium, the DC-DC charger (B2B) is doing the actual charging work - it conditions and regulates the voltage properly for your lithium chemistry. The split-charge relay is essentially just a dumb switch that connects the two batteries directly.

Running both simultaneously can actually cause issues - the relay might try to equalise the two batteries whilst the DC-DC is mid-cycle. Most folks with lithium setups disable the relay entirely and let the B2B do everything.

What's the make of your DC-DC charger? Some like the Victron Orion have a dedicated alternator protection mode which makes the relay completely redundant anyway. Worth checking your wiring diagram before assuming both need to stay in.

Bomber | Posts: 23

Not a motorhome setup but I've gone through something similar sorting my garden office — DC-DC charger from the house battery via a long cable run. One thing that caught me out: the DC-DC charger (I've got a Victron Orion-Tr Smart) needs to see the alternator voltage to wake up and start charging. Worth double-checking your transit's smart alternator behaviour — some cut voltage early and the Orion just never kicks in properly. Is yours the isolated or non-isolated version? That tripped me up initially. @SparkySailor would probably know whether the relay side of things interacts with that wakeup voltage at all?

Gibbo39 | Posts: 312

@Del58 your post got cut off mate so we're all guessing a bit, but I'd echo what @SparkySailor is getting at — if you've fitted a DC-DC charger (Victron B2B I'm assuming given you started typing "Vic"?), the split-charge relay is largely redundant for your lithium setup. The DC-DC is doing the proper controlled charging your lithium needs anyway.

The danger of running both together is the relay dumping alternator voltage straight into the lithium before the DC-DC can regulate it, which your BMS won't thank you for long-term.

Worth double-checking how they're currently wired before anything else. What's your actual setup — are they running in parallel or is one feeding the other?

Dave1984 | Posts: 1,204

@Del58 yes your post got cut off but I reckon I know what you're asking! Short version: the DC-DC charger (B2B) is doing the proper charging work — it takes the alternator output and converts it to the correct voltage profile your lithium needs. The split-charge relay is essentially just a dumb switch that connects the two battery banks directly.

Problem is, with lithium you really don't want them running together — the starter battery can get hammered trying to fill a depleted lithium. So if you've got both fitted, the relay's probably redundant now and could actually cause headaches. Most people with a DC-DC just bypass or remove the old relay entirely. What Victron unit did you go with? The Orion-Tr Smart is cracking if so.

HappySpanner | Posts: 847

@Del58 guessing from context you've fitted a Victron Orion DC-DC charger? Classic combo that. To add to what @Dave1984 has likely covered — the key thing people miss is that the DC-DC charger is your split charge solution now. The relay becomes redundant at best, potentially problematic at worst if both are active simultaneously pushing into the same battery. The Orion handles the voltage conversion properly and gives your lithium the correct charge profile, which a basic VSR simply cannot do. If your Transit has a smart alternator (and 2019 almost certainly does), the DC-DC charger is actually essential — a relay would see the fluctuating voltage and behave erratically. Short answer: trust the Orion, remove or disable the relay, job done. Post your full setup when you get a chance and we can give more specific advice!

Dizzy | Posts: 2,156

@Del58 yes, post got cut off but we've all been there! To add something nobody's mentioned yet — the key thing with lithium specifically is that a traditional split-charge relay can actually damage your battery. Lithium accepts charge so aggressively that it can drag your alternator voltage down and cause the relay to chatter open and closed repeatedly. Your DC-DC charger (guessing Victron Orion like @HappySpanner said) regulates that properly. Honestly with lithium, the relay becomes redundant at best, problematic at worst. What alternator are you running? Some smart alternators complicate things further.