Voltage drop MPPT 100 30

Had something suspiciously similar happen on the narrowboat last spring — brand new Victron SmartSolar 100/30, 400W of panels on the roof, Fogstar 280Ah LiFePO4 sitting nicely underneath, all looked mint on the initial bench test.

Soon as I ran the cable properly through the conduit along the gunwale (about 4m run, 6mm² mind you), the controller started throttling back and throwing low input voltage warnings like it was having a meltdown.

Turns out my "6mm²" cable from a certain well-known online marketplace was, shall we say, aspirationally labelled. Actual copper cross-section was closer to 4mm², which over that run meant enough voltage drop to make the MPPT think the panels were barely awake.

Few things worth checking if you're seeing the same:

• Measure your actual cable — a surprising amount of cheap stuff is undersized
• Check every connection — MC4 connectors that aren't fully clicked home are a classic culprit
• Run the numbers on voltage drop for your cable length, even 4m matters at higher currents
• Victron Connect app — pull the history graph and see if input voltage is sagging under load

Swapped to proper Prysmian flex from a local electrical wholesaler and the difference was night and day — back to stable output, no more phantom warnings.

Anyone else been burned by dodgy cable specs? Feels like half the posts in this forum could be solved by just buying decent wire from the start. 😅

@WheresMeWires87 what were your cable runs like between the panels and the controller? On my motorhome I had a very similar thing — panels looked fine in the VictronConnect app but the MPPT was clearly underperforming. Turned out my PV cable run was longer than I'd accounted for and I'd gone one size down on the cable gauge. Voltage drop on the input side was robbing me of usable watts before it even got to the controller.

Worth checking your actual PV open-circuit voltage at the controller terminals with a multimeter rather than relying solely on what VictronConnect reports. Does the drop happen consistently or only at certain times of day/panel orientation?

@WheresMeWires87 ooh this rings a bell from my garden office build! I ran a 12m cable from roof panels to my SmartSolar and didn't upsize the wire — rookie mistake. Ended up losing nearly 8% to voltage drop before I even got to the controller. Swapped to 6mm² cable and the difference was night and day.

Worth checking your actual input voltage on the Victron Connect app during peak sun — if it's sitting noticeably lower than what your panels should theoretically be producing, that's your culprit right there.

@Squib is on the right track asking about cable runs. On a narrowboat you've often got longer runs than you'd expect once you factor in routing around hatches and bulkheads. Don't forget to account for both the positive and negative runs in your total length calculation — catches people out every time.

@WheresMeWires87 classic narrowboat tax — you think you've done a tidy install then the Severn decides your cable runs are actually 40% longer than you measured because you forgot to account for going around the engine bay 🚢

@ExFarmer90 that's really useful — I've got a similar garden office setup and this is exactly what I'm trying to figure out before I commit to cable lengths. What gauge did you end up going with after you redid it? I'm running about 10m from my roof panels to the controller and keep second-guessing whether 6mm² is going to be adequate or if I should just bite the bullet and go 10mm². The Victron app is great for spotting these issues after the fact but I'd rather get it right first time. Did you notice the voltage drop was worse on cloudy days when the panels were already struggling to get above the absorption threshold?