Anyone else running a split-charge relay alongside solar? Seeing weird voltage readings when both are active

I've got a fairly standard 200W panel setup on the roof of my Bessacarr (two 100W mono panels in parallel, going into a Victron SmartSolar MPPT 75/15), feeding a 100Ah AGM leisure battery. Been running it for a couple of seasons without much bother. Recently though I've added a Sterling Power BB1230 split-charge relay so the alternator tops things up on longer drives, and since fitting it I've been getting some odd readings on the Victron app.

Specifically, when I'm driving on a sunny day with both the alternator and the MPPT actively charging at the same time, the battery voltage shown in the app jumps around quite a bit — sometimes reading 14.6V, then dropping back to 13.8V within a few seconds, even though I'm moving and the engine's definitely running. The actual charging seems fine and the battery feels healthy, but the erratic numbers are making me wonder if something's interfering with something else.

Has anyone else seen this with a similar dual-input setup? I'm wondering whether it's just normal voltage "competition" between the two sources, or whether I need to look at grounding, the MPPT settings, or something else entirely. I did read somewhere that AGMs can be a bit twitchy when two chargers are pushing into them simultaneously, but I couldn't find anything definitive.

Hey @LakelandSolar, classic situation this! The weird readings are almost certainly down to your split-charge relay and MPPT "fighting" over the battery voltage. When the engine's running, the alternator pushes voltage up to around 14.4V which your MPPT sees and thinks the battery's nearly full, so it backs off charging. Totally normal behaviour really.

Worth checking whether you've got a VSR (voltage sensitive relay) or a proper isolator relay - VSRs can chatter a bit when solar's holding the leisure battery just at the threshold voltage, which causes all sorts of odd readings.

One thing that helped me enormously was fitting a proper DC-DC charger (B2B) instead of the split-charge setup altogether - the Victron Orion is brilliant for this. Bit of an outlay but it plays nicely with your SmartSolar and you'll get much cleaner charging from both sources.

@LakelandSolar what does the voltage look like in the Victron Connect app when you isolate each charging source separately? Worth logging both states so you've got something concrete to compare rather than guessing at the cause.

I had something similar during my van build — running a Victron SmartSolar alongside the alternator feed confused the MPPT's absorption thresholds because the relay was effectively "topping up" the battery mid-cycle and making the controller think it was further along than it actually was.

A few questions that might help diagnose:

• Is your split-charge relay a basic voltage-sensing type, or a proper DC-DC charger like a Victron Orion-Tr?
• What voltage does the relay cut in/out at?
• Are the weird readings appearing in bulk, absorption, or float stage?

The relay type makes a significant difference to how the MPPT behaves alongside it.

Great thread this. @LakelandSolar one thing worth checking that nobody's mentioned yet - when both sources are active simultaneously, your MPPT is essentially "seeing" the alternator voltage through the battery and may throttle its output accordingly, thinking the battery is fuller than it actually is. This can throw your readings right off.

Try having a look at your battery terminals with a multimeter whilst both are charging - if you're getting voltage drop across connections or undersized cabling, that'll compound the confusion further.

Also worth checking what relay you're using. A basic voltage-sensing relay can sometimes chatter a bit when the MPPT is in absorption phase, as the voltage sits right on the relay's threshold. A proper VSR with a bit of hysteresis built in sorts that right out.

What cable gauge are you running between the relay and leisure battery?

Great points already from @Slim13 and @Tommo10. One thing I'd add @LakelandSolar - worth double-checking whether your split-charge relay is a voltage-sensing type or a proper VSR (voltage sensitive relay). Some of the cheaper units don't play nicely with MPPT controllers because the solar can hold the voltage high enough to keep the relay engaged even when the alternator isn't actually running.

This means you might be seeing the relay chattering or staying open unexpectedly, which could explain the odd readings. The Victron will log this behaviour if you check the history tab in VictorConnect - look for any unusual absorption/float cycling patterns.

A DC-DC charger like the Victron Orion-Tr Smart would solve the issue entirely and give your AGM a much cleaner charge profile from the alternator side too. Bit of an outlay but worth it long term.