Differences in Voltages

Running into something similar with my garden office setup and wondering if others have experienced the same.

I've got a Victron SmartSolar MPPT 100/30 paired with a separate battery monitor, and I'm noticing a consistent discrepancy between what the MPPT reports as battery voltage versus what the monitor is actually reading at the battery terminals. We're talking roughly 0.2–0.3V difference, which on a 12V LiFePO4 system feels significant enough to matter.

My thinking is that it's down to voltage drop across the wiring between the charger's sense point and the battery terminals — especially given I've got a fuse, a busbar, and a decent cable run involved. The MPPT is measuring at its own output terminals rather than at the battery itself.

A few questions for the group:

• Does anyone use remote voltage sensing on their Victron MPPT to compensate for this? Is it worth the extra wiring hassle?
• If your charge profile is set to, say, 14.2V absorption, is the charger actually delivering 14.4–14.5V at its terminals to achieve 14.2V at the battery? Or is it just reading high and undercharging?
• For those running AGM rather than lithium — does the discrepancy matter as much, given AGMs are generally more forgiving on charge voltage precision?

I'd rather understand the root cause properly before I start tweaking charge profiles as a workaround. Seems like the kind of thing that could silently cause problems over time, particularly heading into the shorter daylight months when every bit of charging counts.

What's everyone's experience?

@MuddySkipper the voltage offset between the MPPT and a separate battery monitor is almost certainly down to wire resistance and measurement point differences.

My garden office setup had exactly this — the SmartSolar was reading 13.4V whilst my Victron BMV-712 was showing 13.1V. Drove me mad for weeks.

The MPPT measures voltage at its own terminals, not at the battery. Once I properly sized my cabling (moved to 35mm² between controller and battery) and tightened every lug, the gap narrowed considerably.

Worth checking:

• Connection points — are both devices measuring from the same physical location?
• Cable length and gauge between controller and battery
• BMV shunt placement — should be the single negative return point for everything

If you're using a separate monitor, configure the MPPT's "tail current" and absorption settings using the monitor's readings, not the controller's own voltage sense.

Seen this exact issue with my shepherd's hut setup. Quick question for @MuddySkipper — where are you taking the voltage sense from on your battery monitor?

If it's measuring at the shunt rather than directly at the battery terminals, you'll get a slightly different reading depending on your cable run. Worth checking whether your MPPT has remote voltage sense wires connected too — if it doesn't, it's measuring at its own terminals rather than the battery, which compounds the discrepancy.

What's the actual voltage difference you're seeing? A few millivolts is normal, but if you're getting 0.3V+ variance that could genuinely affect your charge profile.

Hey @MuddySkipper, just to add to what @WattNick and @SolarDoug are getting at — it's also worth checking whether your Victron SmartSolar has a remote voltage sense wire connected. If it's just measuring at its own terminals rather than directly at the battery, you'll always see a discrepancy under load. The SmartSolar 100/30 does support remote sense, and once I wired that in on my setup the readings between my MPPT and battery monitor came much closer together. Makes a real difference, especially if your cable runs are on the longer side. Victron's VictronConnect app will also show you whether remote sensing is active. Might be worth having a look before assuming anything else is wrong!

My static caravan had the same gremlin until I realised the Victron app and my separate Fogstar battery monitor were basically having an argument about whose reading was "correct" — turns out they were both right, just measuring at different points in the circuit! 🤦‍♀️