Victron (and Others) MPPT failure Powering up down

Been running a Victron SmartSolar 100/30 for about three years now and this caught my attention because I've definitely had moments of uncertainty standing in front of my battery bank wondering which switch goes first.

My setup runs through a Fogstar 12V 200Ah LiFePO4 with its own BMS, and early on I had a couple of occasions where the MPPT threw a fault on startup — nothing catastrophic, just needed a reset. Got me thinking about sequencing.

The general rule I've settled on:

1. Battery first — always bring the battery online before anything else
2. MPPT connects automatically via the battery voltage
3. Solar input follows — the panels then feed into an already-initialised controller

Reverse it on shutdown: solar first (or cover the panels), then disconnect the battery.

The logic being that the controller needs a stable reference voltage from the battery before it sees the PV input. If the solar comes up first into an unpowered MPPT, you can get voltage spikes or the unit hunting around trying to figure out what it's connected to. Some units handle this gracefully, others less so.

That said — does this actually vary between manufacturers? I've only ever run Victron kit so I genuinely don't know if something like a Renogy Wanderer or EPever behaves differently in this regard. Some of those cheaper units don't seem particularly fussed about sequencing from what I've read.

Anyone had actual failures they can attribute to wrong power-up order? Or is this mostly precautionary? Would be good to hear from people running different controllers — especially if you're using a BMS with a pre-charge relay in the mix.

Had this exact thing with my garden office setup. The MPPT "powering up/down" loop is usually one of three things:

• Battery voltage too low to initialise (BMS has tripped)
• Wiring issue — dodgy connection causing voltage drop at startup
• Config mismatch — absorption/float set wrong for your chemistry

Mine turned out to be a loose negative terminal on the Fogstar battery. Looked fine visually but was dropping voltage the moment the Victron drew any current. Classic.

Worth checking in VictronConnect — the history tab shows bulk/absorption cycles and you can usually spot where it's falling over.

Also @HarbourKev — what battery chemistry are you running? If someone's previously had AGM settings loaded and you've switched to LiFePO4, that'll cause weird startup behaviour too. Victron's defaults aren't always reset between installs.

Really relate to this one — spent a baffling afternoon on my narrowboat last summer chasing a similar ghost.

What nobody mentioned yet: check your battery temperature sensor if you're running one. Mine was giving the Victron false readings, causing it to throttle absorption voltage right down, which looked exactly like a power cycling issue. Pulled the sensor lead, ran without it temporarily, and the loop stopped immediately.

Also worth checking in VictronConnect — the history tab will show you whether absorption is actually completing or just bailing out early. That graph tells a proper story once you know how to read it.

@QuietTrekker's three causes are solid, but the temp sensor one gets overlooked constantly because it looks fine visually.

Seen this on my cabin setup — turned out to be a dodgy Anderson connector between the panels and the MPPT. Resistance was just high enough to cause voltage collapse the moment it tried to draw current.

Worth checking every connection point before assuming the controller's faulty. A cheap clamp meter won't show it, you need to do a proper voltage drop test under load.

@QuietTrekker is probably right about the usual suspects, but connectors are massively overlooked. Especially if your array's been outside through a UK winter — moisture gets in, corrosion builds up, and suddenly you've got an intermittent fault that drives you mad.

Swapped mine for MC4s and never looked back. Victron kit itself is solid in my experience, rarely the controller that's the issue.

Good point from @Chopper on the connector resistance — worth adding that this can also manifest intermittently depending on temperature. Cold morning, connectors contract slightly, resistance spikes, MPPT trips. Same panels, same cable, completely different behaviour by midday.

One thing I haven't seen mentioned: check your battery voltage sense wiring if you're using remote sense terminals. On my SmartSolar setup I had a loose sense wire causing the MPPT to see phantom voltage collapse and triggering exactly this loop. Took me ages to find because the main cable connections were all solid.

Also worth pulling the VictronConnect history — the "min battery voltage" field will tell you if it's genuinely hitting a low threshold or if something upstream is causing a false reading. That data point alone narrowed my fault-finding considerably.

@Chopper and @MistyTinker have got the connectors covered, so I'll add — check your battery voltage sense wires if your Victron has them connected separately, because a loose sense wire will have the MPPT convinced your fully-charged Fogstar lithium is a flat AA battery and it'll cycle itself into a panic accordingly.