Anyone else running a Renogy 40A MPPT with lithium on a split-charge setup — getting weird voltage readings?

Right, bit of a head-scratcher this one and hoping someone here's had similar. I've got a Renogy Wanderer 40A MPPT (the newer one with the bluetooth dongle) paired with a 200Ah lithium (LiFePO4) drop-in from Fogstar, fed by two 200W panels on the roof of my Peugeot Boxer conversion. On paper it all looks lovely. In practice, the controller keeps reporting absorption voltage at around 13.8V when I've got it set to the lithium profile at 14.4V target. It's not consistently wrong — sometimes it hits 14.4 just fine.

The split-charge side is a Sterling Power BB1260 B2B charger running off the alternator, and I'm wondering if the two charging sources arguing with each other might be confusing the MPPT's voltage sense. The Fogstar BMS doesn't seem to be throwing any faults — cell balance looks fine on the app — so I don't think it's the battery protecting itself and throttling input.

Has anyone else seen a Renogy MPPT behave oddly when a B2B is also active? I'm trying to work out whether to pull the voltage sense wire and run it directly to the battery terminals rather than the busbar, or whether there's something in the MPPT settings I'm missing. Genuinely stumped on this one.

Gazza99 replied:

@KarenEvans Worth checking whether your MPPT is actually set to lithium profile or if it's defaulted back to sealed lead acid — mine did exactly this after a firmware update and was showing all sorts of daft voltage readings. The Renogy BT app can be a bit temperamental about saving profile changes, so even if you've set it before, double-check it's stuck. Also, on split-charge setups the voltage drop across the relay/VSR can confuse the controller's readings slightly. What voltage is it showing at bulk/absorption stage? If it's pushing above 14.6V you've almost certainly got the wrong battery profile selected. Genuine LiFePO4 wants around 14.2-14.4V for charge voltage. Would help to know what isolator you're running between the banks as well.

@KarenEvans the split-charge relay is almost certainly collapsing the voltage reading when the alternator kicks in — your MPPT sees a "full" battery and backs off like it's done a hard day's work, when really it's just reading the alternator's output voltage rather than your actual LiFePO4 state of charge 🎭

Worth isolating the solar input entirely and watching what voltage the MPPT reports with only the battery connected — if it suddenly makes more sense, you've found your culprit.

Bonus chaos: if you're running a standard relay rather than a VSR or Victron Orion DC-DC, you're probably also partially cooking your lithium with unregulated alternator voltage anyway, which is a whole other thread waiting to happen 😬

ColCrane replied:

@KarenEvans Had almost exactly this with a Victron MPPT and a LiFePO4 setup — the issue for me turned out to be where the voltage sense wire was picking up its reference point. If it's reading from the same bus as your split-charge relay, you'll get all sorts of phantom readings when the relay engages because the alternator voltage and battery voltage are briefly fighting each other on the line.

Worth isolating the solar controller completely from the vehicle circuit temporarily just to confirm whether that's your culprit. Also double-check your battery positive connection is clean and tight — lithiums are quite unforgiving about any resistance in the circuit showing up as voltage oddities.

@NaeClue's point about the relay collapse sounds plausible too and ties into what I'm describing really. What gauge cable are you running between the relay and the leisure battery?

KenMitchell replied:

@KarenEvans A few things worth checking that haven't been mentioned yet — first, make sure your battery negative and the MPPT negative are on the same busbar rather than daisy-chained, as voltage drop across connections can throw readings right off. Also, the Renogy bluetooth app can sometimes display cached readings rather than live data, so don't fully trust what it shows without cross-referencing with a multimeter directly at the battery terminals.

The other thing with LiFePO4 specifically — that flat discharge curve can make the MPPT behave oddly if the absorption setpoint isn't dialled in properly. What have you got yours set to? I'd typically run 14.2V absorption and 13.6V float for LiFePO4, but it varies slightly by cell manufacturer. What brand are your cells?

Slightly different context but I run a Renogy 40A MPPT on my boat with a 200Ah LiFePO4 — not a split-charge setup, but I did get bizarre voltage spikes that turned out to be a grounding issue rather than anything with the controller itself.

Worth checking: are your negative connections all sharing a proper common ground point, or are there separate grounds running back to different points? On my setup I had the MPPT negative going to one place and the battery negative elsewhere — caused all sorts of phantom readings in the Bluetooth app.

What's the actual voltage discrepancy you're seeing? Is it reading high or low versus a multimeter at the battery terminals? That might help narrow down whether it's a sensing issue or something upstream.

DodgyNomad replied:

@KarenEvans Worth checking whether your split-charge relay is actually rated for lithium. A lot of the older voltage-sensing relays get confused by LiFePO4's flatter discharge curve — they're calibrated around lead-acid voltage thresholds, so they can drop out or behave erratically when the lithium sits at 13.2-13.3V under load. The relay thinks the starter battery's not charging properly and starts doing odd things.

Also, is your MPPT sensing voltage at the battery terminals directly, or picking it up somewhere further down the cable run? Any significant voltage drop along the cable will skew what the controller thinks is happening. Renogy's app can be useful here — check what voltage it's actually reporting versus what you're measuring at the battery with a multimeter. If those two numbers don't match closely, your sense wiring's the culprit.

ExPostie82 replied:

@KarenEvans One thing nobody's flagged yet — the Renogy MPPT's voltage sensing is measured at the controller terminals, not at the battery. If you've got any appreciable cable run between controller and battery (even 1-2 metres of undersized cable), you'll see a voltage drop that confuses the charge algorithm. The controller thinks the battery is fuller than it is and backs off early.

Worth measuring actual battery terminal voltage with a decent multimeter mid-charge and comparing it against what the Bluetooth dongle reports. If there's more than 0.2V discrepancy, your cable sizing or connections are the culprit rather than anything exotic with the split-charge arrangement itself.