SmartSolar MPPT 75/10 showers lower voltage than multimeter

Been having something similar crop up on my narrowboat setup and can't quite get my head around it.

My SmartSolar MPPT 100/20 is showing battery voltage in VictronConnect that's consistently about 1.8–2V lower than what I'm reading at the controller terminals with my multimeter. We're talking the app showing ~11.5V whilst the meter is saying 13.3V at the same moment.

A few things I've already ruled out:

• Bluetooth connection dropping mid-read (checked multiple times, live data is stable)
• Dodgy multimeter (cross-checked with a second one)
• Loose connections at the battery terminals

My best guess at the moment is that the controller is sensing voltage at the battery end of a run with some meaningful resistance in the cable — but the drop seems too large for that to fully explain it. The cable run is about 3 metres of 6mm² to a Fogstar 100Ah LiFePO4.

Has anyone seen this on a 75/10 specifically? I wonder if there's a firmware quirk or whether the internal voltage sensing point on these smaller units differs from the larger MPPT range.

Also curious whether anyone has used the VE.Direct cable to pull readings directly rather than via Bluetooth — does that give a more accurate picture, or is it the same data either way?

Would be useful to know if this is affecting anyone else running lithium rather than lead-acid, as the tighter voltage windows matter a lot more when you're trying to protect cells properly.

@SimonJohnson67 I've had almost the exact same head-scratch moment on my narrowboat! Quick question — are you measuring the battery terminals directly with your multimeter, or are you probing somewhere further along the cable run?

On mine the discrepancy was down to voltage drop across the cable between the MPPT and the battery bank. The controller reads voltage at its own terminals, not at the battery itself, so if your wiring has any meaningful resistance (especially with longer runs on a narrowboat), you'll see that difference.

Worth checking:

• Cable gauge — is it properly sized for the current?
• Connection quality at both ends
• Have you enabled the battery voltage sense option in VictronConnect? That can help compensate

What's your cable run length roughly?

Classic one this, @SimonJohnson67. What you're seeing is almost certainly voltage drop across your wiring between the battery bank and the MPPT's battery sense terminals.

The MPPT measures voltage at its own terminals, not at the battery itself. If you've got any length of cable — even decent 6mm² — running between them, resistance adds up, especially under load.

The fix that transformed my van build: enable remote battery sensing via Bluetooth in VictronConnect. If you've got a BMV-712 or SmartShunt on the same VE.Smart network, the MPPT will borrow the battery voltage reading from there instead. Suddenly everything agrees.

Alternatively, run a dedicated sense wire directly to the battery terminals. Night and day difference.

Also worth checking your cable connections for any corrosion — on a narrowboat that damp environment is brutal on terminals.

Something I've always wondered with setups like this — is the multimeter probe actually touching the battery terminal itself, or somewhere nearby on the busbar/cable? Even a centimetre or two away can introduce enough resistance to skew readings if the cable's doing any real work.

Also worth asking: at what point in the charge cycle are you noticing the discrepancy? I'd expect it to be more pronounced under load or during bulk charging than when everything's sitting idle.

On my shepherd's hut setup I found the gap between what the Victron reported and what my meter showed shrank dramatically once I sorted a dodgy negative connection. Sometimes it's less about the controller being wrong and more about the circuit telling you something's loose or undersized somewhere. 🔍

Worth adding something that caught me out when I first set up my garden office system — the SmartSolar samples voltage periodically, not continuously. So if you're catching your multimeter reading at a slightly different moment (say, during a brief absorption pulse or right as a load kicks in), you'll naturally see a discrepancy even with perfect wiring.

On my setup I noticed the VictronConnect graph told a much more honest story than any single snapshot reading. Check the history tab and see if the voltage curve looks smooth — that'll tell you whether you're chasing a genuine wiring issue or just a timing mismatch between your two readings.

@MarinePhil's point about wiring drop is still the most likely culprit mind you, especially on a narrowboat where cable runs can get lengthy and connections go manky with the damp.