Batterie plomb tension qui baisse sur MPPT 75/15

Seen a few threads lately about dodgy voltage behaviour on lead-acid setups and thought I'd share something from my own experience, because it catches a lot of people out.

Running a Victron SmartSolar MPPT 75/15 with a 100Ah AGM (Fogstar do decent ones but mine's an older Yuasa) and I had a similar headache — voltage looked fine on the controller display but the battery was clearly struggling under any decent load. Turned out the MPPT was reading battery voltage at the controller terminals rather than at the battery itself, so there was enough voltage drop across the cable run to confuse the charge profile.

A few things worth checking if you're seeing voltage sag under a pump load or similar:

• Cable cross-section — undersized cable between controller and battery will cause misleading readings. I'd go no smaller than 6mm² for even a modest setup
• Charge profile settings — the 75/15 defaults to a fairly generic flooded lead-acid profile. If you've got a sealed or AGM unit, the absorption and float voltages will be wrong out of the box
• Battery condition — a sulphated lead-acid cell will tank voltage under load even if it looks charged at rest. Worth doing a proper load test
• 75/15 input limit — 210W panel on a 75/15 is right at the edge of what it'll handle. Worth checking the PV open-circuit voltage doesn't exceed 75V in cold conditions

What's the actual resting voltage vs. under-load voltage you're seeing? And how old is the battery? That's usually where the answer lives.

@FormerMechanic68 lead-acid and absorption voltage is basically that mate who says "yeah I'm nearly ready" then disappears for another 45 minutes — totally normal behaviour, just deeply inconvenient.

@FellGraham that mate also somehow takes three hours to "nearly ready" then sulks if you unplug him early — lead-acid in a nutshell 🐌

@FogstarGal that analogy is painfully accurate 😄

Worth adding a technical note for anyone landing on this thread from search: the absorption phase duration on a Victron MPPT is adaptive by default — it scales based on how depleted the bank was. So a deeply discharged lead-acid can sit at absorption voltage for 4–8 hours before the controller drops to float. During that whole window, voltage looks "high" and steady, which people mistake for full charge or a fault.

In my static caravan setup I noticed this constantly until I dug into VictronConnect and actually watched the state-of-charge curve. The battery was genuinely still accepting current at 14.4V.

If anyone's concerned their MPPT is misbehaving, connect via Bluetooth and watch the charge current — if it's still flowing, the controller is doing exactly what it should.

@FogstarGal @FellGraham that analogy is entertaining but I'd like to pin down the actual numbers before anyone adjusts their settings based on this thread.

On the MPPT 75/15 specifically — does the "Absorption Voltage" parameter in VictronConnect refer to the voltage the controller targets at the battery terminals, or at the controller's output? Because with any meaningful cable run there'll be a voltage drop, and a 0.2V discrepancy on a 12V flooded lead-acid isn't trivial if you're trying to hit 14.4V accurately.

Has anyone actually measured terminal voltage at the battery simultaneously with what VictronConnect reports? I ran a separate Victron BMV-712 on my van build and found a consistent 0.15V gap versus what the MPPT was claiming — which matters when your absorption threshold is only a 0.3V window above bulk.

Is this a known calibration limitation of the 75/15 specifically?