Mppt charging dead 48v

Had something eerily similar happen on my garden office setup last autumn — Victron SmartSolar 100/30 on a 48V bank, and it just... dithered. Kept cycling between bulk and float, never actually doing anything. Power in and out was essentially noise — 0W, 1W, back to float, repeat.

Turned out the culprit was a combination of things worth checking if you're in the same boat (no pun intended, though I've chased the same gremlin on the narrowboat too):

Things that fixed it for me or others I've spoken to:

• Battery voltage mismatch — the MPPT was seeing the resting voltage of a nearly-full bank and deciding there was nothing to do. Check your absorption and float setpoints.
• BMS communication — if you're running a Fogstar or similar lithium bank with a BMS that talks to the MPPT over VE.Direct or Bluetooth, sometimes the BMS tells the charger to back off and it gets stuck in that loop.
• Shading or panel voltage — on a dull UK autumn morning, panels barely generating enough voltage above battery voltage to actually push current.
• Firmware — sounds daft but updating the Victron firmware via VictronConnect sorted a weird cycling issue for a mate running a similar setup.

What controller and battery chemistry are you running? And is this a new issue or has it never worked properly? There's a big difference between "worked fine then broke" and "never right from day one" — narrows down where to look considerably.

Curious whether anyone else has seen this on non-Victron kit too — Renogy controllers seem to have their own quirks in this department.

Classic Victron "am I bovvered?" mode — mine did exactly this in the shepherd's hut until I realised the battery temp sensor had come loose and it was essentially guessing like a drunk bloke at a quiz night.

Check your BMS comms cable if you're running a smart lithium bank — the SmartSolar goes into a right sulk if it loses the VE.Direct handshake and just half-heartedly bulk charges forever like it's waiting for someone else to finish the job.

Also worth checking your absorption voltage is actually set for 48V nominal and hasn't somehow defaulted back to 24V — Fogstar cells in particular have a narrow window and the controller will just yo-yo if the target's wrong.

@FormerMariner36 worth checking your absorption voltage setting first — on a 48V bank it's easy to accidentally have it set for 24V profile if someone's been poking the app. Victron Connect will show you immediately.

Also, what batteries? LiFePO4 with a BMS that's occasionally tripping under load will cause exactly this cycling behaviour. The MPPT sees the BMS cut out, resets, tries again — looks like dithering but it's actually the battery refusing charge momentarily.

Mine did something similar with Fogstar Drift cells last winter. Turned out one cell group was slightly out of balance and the BMS was being twitchy about it.

Quick checks:

• Correct battery profile selected?
• Any BMS fault codes?
• Check MPPT history tab for max absorption time reached

@FormerMariner36 this is ringing massive bells for me — had almost identical behaviour on my narrowboat last winter with a 100/50. Turned out my BMS was sending a signal that the Victron was interpreting oddly via VE.Smart networking, so it kept resetting its charge state mid-cycle.

Worth checking a few things:

• Are you running any VE.Smart battery sensing or Bluetooth networking?
• What does the history tab in VictronConnect actually show for cycle counts vs absorbed kWh?
• Any chance your battery voltage is genuinely hitting absorption threshold briefly, triggering the float drop-off?

The Victron logs are brilliant for diagnosing this — far more useful than just watching the LED states. What battery chemistry are you running? Fogstar lithium behaves quite differently to AGM on a 48V setup and the default charge profiles really need tweaking.

@RustySkipper same narrowboat setup here, been through this twice now. One thing nobody's mentioned yet — check your wiring for voltage drop between the MPPT and the battery terminals. If the controller thinks it's hitting absorption voltage but the actual battery terminal voltage is lower due to resistance in the cabling, it'll do exactly this cycling nonsense. Victron's remote battery sense feature exists precisely for this reason. Run the sense wires directly to the battery posts, not the busbar. Made a significant difference on mine. Also worth pulling the VE.Direct logs in the app — the history tab will show you whether absorption is timing out normally or being cut short.

@BodgeItAndScarper raises a good point on wiring, but I'd add — check your battery temperature sensor if you're running one. Mine was tucked against the wrong cell on my shepherd's hut 48V bank (Fogstar Drift lithiums), and the SmartSolar was compensating for a temperature reading that bore no relation to actual bank conditions. It was essentially throttling charge current based on phantom heat. Moved the sensor, recalibrated in VictronConnect, and the dithering stopped dead. Easy thing to overlook when you're chasing voltage settings and wiring runs.