Anyone else's MPPT controller reading wildly different voltages to their battery monitor?

My Victron SmartSolar 100/30 is insisting my Fogstar 100Ah lithium is sitting at 13.8V whilst the BMV-712 is laughing at it from across the van saying 13.2V — half a volt of disagreement and I'm losing my mind.

Both wired to the same 175Ah bank (two Fogstar 100Ah in parallel, yes I know, fused properly before anyone asks). Bluetooth readings from both devices simultaneously, so it's not a timing issue.

Is this just a case of the MPPT measuring at its own terminals rather than the battery and the cable drop is actually that significant, or have I got a calibration gremlin eating my sanity?

RayCross | 847 posts

@OhmsLaw7 Classic one this! Half a volt discrepancy almost always points to voltage drop across your wiring between the two measurement points. Your SmartSolar is reading at the controller terminals, the BMV-712 is reading directly at the battery — if there's any resistance in that cable run (undersized wire, dodgy connections, corroded terminals), you'll see exactly this kind of gap.

Worth checking your battery negative cable connections first, then measure the actual resistance of the run with a multimeter if you can. Also make sure your SmartSolar has the battery voltage sense wires connected if your model supports it — that lets it measure directly at the battery rather than guessing from the controller end.

Which reading do you actually trust more? The BMV is almost certainly closer to the truth given where it's measuring from.

RogerOliver | 312 posts

@OhmsLaw7 Worth checking which terminal your MPPT is measuring from too — the SmartSolar reads voltage at its battery terminals, not at the battery itself. If your cable run is any length at all, or your connections aren't spotless, that reading will be optimistic compared to what the BMV-712 sees at the actual battery posts.

Quick test: with no load and no charging, do both readings converge? If they agree at rest but diverge under load or charge, that's your wiring resistance showing itself. The BMV is almost certainly the more accurate of the two for true battery state of charge purposes anyway — trust that one for day-to-day decisions. Victron's own docs recommend the BMV as the primary reference for exactly this reason.

CoastalWanderer | 1,204 posts

Something @RayCross and @RogerOliver haven't touched on yet — have you got the Victron Connect app open? Check whether your SmartSolar is in float or absorption, because the controller will report the charge voltage it's pushing rather than resting battery voltage if there's any current flowing at all. Even a trickle in float can skew the reading quite noticeably at the controller terminals.

Pop a load on the battery for a few minutes to pull it down, disconnect the panels, then wait ten minutes and compare both readings at rest. If they agree, your wiring's probably fine and it's just a measurement-under-load situation. If they're still diverging, then yeah, worth investigating the wiring path as the lads above suggest. The BMV shunt measurement is generally the more trustworthy figure for actual battery state of charge anyway. 🙂

RussMitchell75 | 203 posts

Good shout from everyone above. One thing worth adding — are both devices on the same VE.Smart Networking via Bluetooth? If not, they're measuring independently with no synchronisation between them. Once I linked my SmartSolar and BMV-712 together through the Victron Connect app, the MPPT started using the battery monitor's voltage sense rather than its own, and my discrepancy basically disappeared overnight. Makes a real difference especially when there's any load running whilst the solar's doing its thing. Takes about two minutes to set up if you've not done it already — just go into VE.Smart Networking in the app on each device and join them to the same network. Worth trying before you start chasing wiring gremlins.