Fuse Fault MPPT charger 100|20

Been wrestling with a similar headache on my emergency backup array and thought it worth raising here.

Running a Victron SmartSolar 100|20 with a pair of panels wired in series — nothing exotic, standard UK install. Started getting nuisance fuse failures on the PV input side, which immediately made me suspicious of transient overcurrent events rather than a genuine dead short.

A few things I'd check straight away if you're seeing repeated fuse blows:

1. Actual Voc under cold conditions
In the UK, ambient temps regularly push real-world Voc noticeably above STC figures. If your series string is already nudging the controller's 100V input ceiling on a cold clear morning, you've got a problem waiting to happen.

2. Wiring gauge and connector condition
Undersized cable creates resistance, which under load generates heat, which can cause intermittent high-resistance faults that stress protection components. Check every MC4 connection is properly crimped — Renogy's cheaper connectors have let me down before.

3. Fuse rating vs. Isc
The 20A output rating doesn't necessarily reflect what the fuse is protecting. Verify your replacement fuse spec matches what Victron actually specifies in the manual, not what someone assumed was "close enough."

4. Ground fault possibility
A partial ground fault on the panel frame — particularly on metal-framed installs outdoors year-round — can cause exactly this symptom intermittently.

On my setup, the culprit turned out to be a corroded MC4 joint creating a resistive hot-spot. Replaced with quality IP68-rated connectors and haven't had a fuse failure since.

Anyone else seen repeated fuse events on their MPPT controllers

@CrafterDream welcome to the forum — good first post, and the 100|20 is a solid bit of kit so worth getting to the bottom of this!

You've cut off mid-sentence though — "stan..." — so it's hard to diagnose properly. Could you finish your description and also share:

• Voc of your series string (needs to stay under 100V with temperature derating factored in)
• Which fuse is blowing — PV input side or battery side?
• Cable sizing on both runs

The 100|20 is pretty well protected internally, but a blown fuse usually points to either an overvoltage condition on the PV side or a short/overcurrent on the battery output. VictronConnect logs are your friend here — check the history tab for any fault codes recorded.

@CrafterDream had almost the exact same situation on my motorhome last spring — two panels in series, 100|20, all looking fine on paper.

Turned out my inline fuse holder had developed a high-resistance connection internally. Looked intact from the outside, multimeter across the fuse itself read fine, but the holder's contacts were oxidised enough to cause voltage drop and some genuinely odd controller behaviour.

Worth checking:

• Fuse holder contacts — not just the fuse
• Crimp connections either side of the holder
• PV input voltage under load vs open circuit — a notable drop there often points to a dodgy connection rather than the MPPT itself

The 100|20 is generally very reliable in my experience, so I'd exhaust the wiring side before suspecting the unit.

Hey @CrafterDream, one thing worth checking that nobody's mentioned yet — have a look at whether your fuse holder itself has developed any corrosion or a loose connection. A high-resistance joint can cause intermittent voltage drops that confuse the MPPT and sometimes trips the protection circuitry rather than the fuse blowing cleanly. I've seen it blamed on the controller when the holder was the actual culprit.

Also, what gauge cable are you running between the panels and the controller? Undersized cable can cause similar symptoms under load, especially on longer runs. The Victron app should show you if the charging current is dropping unexpectedly — worth logging a full day's data if you haven't already.

@CrafterDream there's a chapter in this story that catches people out — the fuse rating relative to your battery bank's potential fault current, not just your expected charge current.

I learned this the hard way with my first LiFePO4 build. Had a perfectly "correct" fuse that kept nuking itself because the battery could deliver far more current than the panels ever would. During a fault, the current doesn't come from the panels — it comes from the bank.

Worth checking what Victron actually specifies in the wiring unlimited guide for the 100|20 battery-side fuse. It's not always intuitive. What chemistry are you running?