Is my Victron 75/15 solar MPPT dead ? Weird behaviour

Had something similar with my 75/15 on the shepherd's hut last spring. The controller was showing absorption voltage but the amps were all over the place — kept dropping to near zero then spiking back up randomly.

Turned out to be a dodgy MC4 connector on the panel side rather than the unit itself. Worth checking those first before writing off the Victron. A loose or corroded MC4 can cause exactly the weird behaviour you're describing.

Few things I'd check:

• PV input voltage — is it within the right range for your panel setup? Too high and the controller protects itself
• Battery voltage readings — does what VictronConnect shows match a multimeter directly on the terminals?
• MC4 connectors and cable joints — wiggle them while watching the app, see if the numbers jump
• Firmware — connect via VictronConnect and check it's up to date, there were some known bugs in older versions

The 75/15 units are pretty robust in my experience. Mine's been running for three years now with zero issues since I sorted the connector. Victron's not infallible but they're not fragile either.

What does VictronConnect actually show when it's misbehaving? Is it dropping into float prematurely, throwing error codes, or just showing daft numbers? And what panels are you running — any chance the open-circuit voltage is pushing close to the 75V limit in cold weather?

More details would help narrow it down. Others on here will have seen most failure modes at this point so worth describing exactly what the display or app is telling you.

@DodgyHermit that erratic amps pattern is a classic giveaway — worth checking your panel connections before condemning the unit itself. Loose MC4 connectors cause exactly that behaviour, especially if the hut gets any wind movement. I've seen it drive people mad for weeks.

Grab a multimeter and measure panel voltage directly at the controller input terminals while it's doing the spiking. If voltage is also bouncing around, it's upstream of the 75/15. If voltage stays rock solid but amps dance, then you're looking at a controller issue.

Also — what firmware version is it running? Victron Connect will tell you. There were some odd MPPT algorithm quirks in older versions that produced similar symptoms on smaller arrays.

Seen this exact drama on my narrowboat — turned out to be a corroded MC4 connector doing its best impression of a faulty controller 🎭

@SmartSolar_Master is right about the connections, but also check your battery temperature — if the 75/15 has a temp sensor fitted and it's reading daft values, the absorption voltage will chase its tail all day long.

Quick checklist before you panic:

• MC4 connectors (pull-test each one)
• Battery terminal voltage at the battery, not just what VictronConnect reports
• Check the protection history tab in VictronConnect for any high voltage events

The 75/15 is a tough little unit — mine's survived three winters on the cut without complaint. Victron's kit rarely dies quietly; it usually tells you exactly what's upset it if you read the logs.

@DodgyHermit this rings very familiar — had identical gremlins on my narrowboat last autumn. Before you go poking at connectors though, pull up the VictronConnect history graphs and look at whether the voltage is also dipping when the amps drop. If voltage holds steady but amps crater, that's almost certainly a connection issue as the others suggest. But if voltage and amps are both collapsing simultaneously, you're potentially looking at partial shading hitting a panel with no bypass diodes, or worse — a cell developing an internal short.

Worth knowing: the 75/15 will throttle output aggressively rather than fault-out cleanly, which makes diagnosis genuinely tricky. Mine fooled me for three weeks before I found a micro-crack in one cell that only showed under thermal load.

@DodgyHermit worth adding — on my motorhome setup I had almost identical symptoms and the culprit turned out to be a loose connection on the battery side rather than the panel side. Everyone jumps straight to the MC4s (fair enough, as @RetiredChef says they're notorious) but the controller can behave erratically if it's not seeing a stable battery voltage either.

Quick checks I'd do:

• Torque the battery terminals on the 75/15 itself — they can vibrate loose
• Check the fuse/isolator between controller and battery
• Look at the VictronConnect history graph — if you're seeing voltage dips alongside those amp spikes, that points battery-side rather than panel-side

The 75/15 is a solid bit of kit, I'd be surprised if it's actually dead. Usually something upstream playing up.

@DodgyHermit my Fogstar-fed van setup threw exactly this tantrum last February — turned out the panel itself had a cracked cell doing a convincing impression of a functioning solar panel in anything other than direct overhead sun, which is basically never in Britain.

Worth ruling out the obvious before you bin the 75/15 — Victron controllers are annoyingly reliable and will happily work around a dodgy panel for months before you notice the battery's quietly starving.

Stick a multimeter on the panel open-circuit voltage and short-circuit current separately and see if the numbers match the spec sheet; a cracked cell will often read plausible Voc but the Isc will be embarrassingly low, like your panels forgot they're supposed to be harvesting energy and not just decorating the roof.