Anyone else running a split-charge relay alongside solar? Struggling to get the balance right

I've got a 200W panel on the roof of my Transit-based van feeding a 100Ah lithium (a Fogstar Drift, if anyone's curious) through a Victron 75/15 MPPT. Cracking setup most of the time, but I've also got a Sterling Power split-charge relay on the alternator for those grey weeks when the sun just doesn't show up. The problem is I'm not entirely sure how aggressively to let the alternator top things up without causing grief — lithium wants a proper CC/CV charge and I'm a bit nervous the relay alone isn't doing that cleanly.

Had it running through Wales last month, three days of solid drizzle, and the relay was doing most of the heavy lifting. Van ran for maybe four hours across the trip and the battery got to around 80% — fine for the trip, but I kept second-guessing whether I was stressing the cells. The Fogstar BMS should protect things, but I'd rather not rely on it as the only safety net.

Has anyone swapped out a basic relay for a DC-DC charger (B2B) instead? I've been looking at the Sterling Pro Charge Ultra and the Victron Orion, but can't decide if the extra spend is actually worth it on a relatively modest 100Ah bank. What are people running?

Hey @MoorDweller, nice setup! One thing worth checking is whether your split-charge relay is voltage-sensing or a basic on/off type. With lithium, the resting voltage can fool a standard VSR into thinking the bank is full when it's not, or worse, cut the relay out too early during charging.

If you can share what relay you're running and roughly what the issue is (overcharging, not charging enough, both systems fighting each other?), that'd help narrow it down.

Worth noting that the Fogstar Drift has a BMS that'll protect against most disasters, but getting the two charge sources properly coordinated is still worthwhile for longevity. A DC-DC charger instead of a straight relay is often the cleaner solution with lithium, though obviously adds cost.

@MoorDweller classic problem — your van's basically got two parents fighting over who gets to feed the battery 😄

Seriously though, the issue is usually the relay kicking in and confusing the MPPT about actual state of charge. Victron's quite clever but it's not that clever.

Worth digging into VictronConnect and checking

@MoorDweller the thing that catches people out here is alternator output voltage — modern smart alternators (common in Transits from around 2012 onwards) drop to ~12.8V when the ECU decides the battery's "full," which confuses a voltage-sensing relay and can mess with your MPPT's charge profile simultaneously.

Worth fitting a DC-DC charger (Victron Orion-Tr Smart is the obvious choice) in place of or alongside the relay — it isolates the two charging sources properly and doesn't care what the alternator's doing voltage-wise.

On my narrowboat I run Victron kit throughout and having the BMS, MPPT, and DC-DC all talking via VE.Direct/VE.Bus makes diagnosing conflicts considerably easier. Might be overkill for a van but the visibility alone is worth it when things get weird.

What year is the Transit? That'd help narrow down whether the smart alternator issue applies.

@MoorDweller worth flagging something specific to the Fogstar Drift — being lithium, it'll accept charge so readily that your split-charge relay might be dumping alternator current in at the same time your MPPT is pushing solar in. The Drift has a 50A max charge rate, so in theory you're fine, but it's worth knowing your combined input at any given moment.

One thing I'd add to what @PanelEwan mentioned about smart alternators — if you haven't already, consider a DC-DC charger (like a Victron Orion-Tr Smart) rather than a basic relay. It'll properly regulate what's coming from the alternator side and play nicely with your MPPT without either of them getting confused. Makes the whole system much more predictable, especially on shorter drives when solar's already doing the heavy lifting.

@MoorDweller ran almost this exact setup on my narrowboat for a season before I sorted it properly. The issue nobody's mentioned yet is charge source priority — if your MPPT and the split-charge relay are both active simultaneously, you can end up with the alternator essentially "fighting" the solar controller's absorption voltage calculations. The MPPT sees a higher voltage coming from the alternator side and backs off prematurely, so you're actually getting less from your solar than you should be.

Worth looking at a Victron Battery Protect or a simple voltage-sensing relay with a slightly higher engage threshold so the alternator only cuts in when solar genuinely can't keep up. On my garden office setup I just time it manually, but obviously that's not practical in a moving van.

What's your typical daily consumption? That'd help figure out whether you even need both sources running together.

@MoorDweller one thing worth adding to what's already been said — have you looked at a DC-DC B2B charger instead of a straight split-charge relay? Something like the Victron Orion-Tr Smart. It isolates the van's electrics properly and you can set the charge profile to suit the Fogstar's lithium chemistry. The relay approach can work but you're essentially at the mercy of whatever voltage the alternator throws out.

Running a similar arrangement myself — 300W panels into a Victron SmartSolar and an Orion handling the vehicle side. Once I ditched the relay the whole system just... behaved. Everything talks to each other through VictronConnect too which is handy.

The B2Bs aren't cheap but on a lithium setup the protection is worth it.