Mppt 100/20 not charging, (blue light flashing)

Done this exact thing in my motorhome last summer — connected the panels first like a muppet, watched the blue light do its little disco routine, and spent three hours convinced I'd bricked a brand new Victron.

Spoiler: I hadn't.

The 100/20 without Bluetooth is a bit of a pain because you can't just pop open VictronConnect and have a nose at what's going on. But nine times out of ten, the flashing blue is just the controller in a strop because it hasn't properly sensed the battery bank on startup.

What sorted it for me:

• Disconnected everything (panels first this time, then battery)
• Waited a full minute — not ten seconds, a full sixty
• Reconnected battery first, then panels
• Let it sit for five minutes before judging anything

If the light's still misbehaving after that, check your battery voltage at the terminals with a multimeter — if it's reading below about 10V on a 12V setup, the controller might be refusing to recognise it as a valid battery at all. Fogstar lithium cells can sometimes confuse cheaper controllers too, though Victron is usually pretty forgiving.

Has anyone else had the non-Bluetooth version do something weird after a dodgy startup sequence? Wondering if there's a hidden reset procedure I've never found, or whether the correct sequence genuinely fixes it every time — seems almost too simple for how much panic it causes!

@ShesBeRight the flashing blue is just the MPPT's way of telling you it has no battery reference voltage to work from — it's not damaged, it's confused. Victron's protection circuitry is actually quite robust against this particular mistake.

The correct sequence is always:

1. Battery first
2. Panels second

Disconnect the panels, let the controller sit for a minute, then reconnect battery terminals first and confirm you get a solid connection before introducing the panel input. The 100/20 should initialise properly and the bulk/absorption LEDs will behave normally.

Worth checking your battery fuse/isolator is actually closed before assuming the controller is at fault — I've chased this exact "fault" on my narrowboat only to find I'd left the domestic isolator open like a complete numpty.

@DefenderAdventure so essentially the Victron is standing there going "I'd love to charge something but who even are you, battery?" 😂 — reconnect your battery first, panels second, and it'll sort itself out sharpish.

@BenJackson haha exactly that — it's just sat there completely lost 😄

Worth adding for anyone who finds this thread via search: the correct wiring order is always battery first, then panels. Disconnecting is the reverse — panels off first, then battery.

I made the same mistake with my Victron 100/30 when I was setting up my shed system. Panicked completely. Once I disconnected the panels, hooked the battery up properly, and then reconnected the panels, it sorted itself within about 30 seconds — solid blue, then into bulk charge, job done.

No harm done to the controller in my experience, Victrons are pretty robust little units. Just don't leave it in that confused state for ages with the panels pumping voltage in.

Made the same mistake with my Victron 100/30 — the fix is dead simple: disconnect panels, connect battery first, then reconnect panels and it'll sort itself out immediately.