Anyone else running a split-charge relay alongside solar? Getting confused about charge priority

I've been scratching my head over this for a few weeks now and figured someone here would know. I've got a 200W panel on the roof of my Transit-based van feeding into a Victron SmartSolar 100/20 MPPT, with a 100Ah lithium (a Fogstar Drift) as my leisure battery. All works fine when I'm parked up in decent weather. The issue is I've also got a basic 140A split-charge relay wired off the alternator, and I'm not entirely sure what happens when both sources are active at the same time — i.e., driving on a sunny day.

From what I can tell, the MPPT and the alternator are just both dumping into the battery simultaneously, which sounds like it should be fine, but I've noticed the solar MPPT sometimes seems to back off weirdly when I'm driving. Wondering if there's a voltage conflict going on, or whether the relay is somehow confusing the MPPT's sense of the battery state.

Has anyone gone through this and worked it out properly? I'm half-tempted to ditch the relay entirely and just fit a proper DC-DC charger (been looking at the Victron Orion-Tr Smart 30A), but that feels like a lot of money to solve a problem I don't fully understand yet. Would love to know if others have seen the same behaviour or if I'm just overthinking it.

48V_Pro | 847 posts

@Jim1980 Classic setup confusion! The short answer is your MPPT takes priority automatically - it's always "first in" to the battery. Your split-charge relay will only kick in meaningful current when the alternator voltage exceeds your battery's present voltage, so they generally play nicely together without any manual intervention.

One thing worth checking though - if you're using a voltage-sensing relay rather than a proper Battery-to-Battery (B2B) charger, you might find the relay chattering or behaving oddly when solar is already holding the battery at a higher voltage. The alternator can sometimes struggle to "see" a difference to trigger it.

What relay are you running exactly? If it's a basic VSR, I'd honestly consider swapping it for a Sterling or Victron Orion B2B - transforms the whole setup.

@Jim1980 your MPPT and split-charge relay are basically two mates who both fancy the same battery — the solar will always win the priority argument because it's running 24/7, but your relay only kicks in when the engine's above ~13.3V, so they'll mostly take turns like polite British people in a queue rather than fighting over it.

TracyRobinson | 312 posts

@Jim1980 Good question and something I wrestled with on my own Transit conversion last year! One thing worth adding to what @48V_Pro mentioned — keep an eye on your split-charge relay's voltage threshold. If your MPPT has already brought the leisure battery up to say 13.7V during a sunny afternoon, your relay might think the engine's running and activate unnecessarily, potentially causing a bit of a feedback situation depending on your setup. Worth checking what voltage your relay triggers at and comparing that against your MPPT's absorption voltage setting in the Victron app. A simple battery-to-battery (B2B) charger instead of a basic VSR relay would actually give you much cleaner separation and proper charging profiles from both sources. Might be worth considering if you're doing longer trips. What leisure battery are you running — AGM, lithium, or wet cell?

BoxerLife | 1,204 posts

@Jim1980 Worth adding something the others haven't touched on yet — with your Victron SmartSolar you can actually monitor what's happening in real time via the VictronConnect app over Bluetooth. Brilliant for seeing exactly which source is contributing what at any given moment.

One practical thing to watch: if your alternator via the split-charge is pushing in whilst solar's already got the battery near full, your MPPT will simply throttle back into absorption/float anyway. They won't fight each other as such. The battery dictates what it accepts.

Main gotcha with Transit-based setups is making sure your split-charge relay has a decent voltage threshold so it's not draining your starter battery if the leisure side dips overnight. What relay are you running currently? That might be where your confusion is coming from.

Curly63 | 847 posts

@Jim1980 One thing worth knowing specifically about the SmartSolar 100/20 — you can set custom charge profiles in the VictronConnect app, so if your split-charge relay is occasionally interfering with the absorption phase (van running whilst solar's mid-cycle), you can tweak the tail current settings to compensate. Also consider whether your relay is voltage-sensitive or time-delayed, as that affects behaviour quite differently when solar's already holding the voltage up. A VSR can get confused into thinking the battery's fuller than it actually is purely because solar's inflating the reading. Might be worth sticking a battery monitor in the circuit if you haven't already — makes the whole picture much clearer. What leisure battery are you running? That'd help narrow things down further.