Rod Collins — Marine How To, the same bench that published the 17-year-old LFP bank we led with in June — posted a piece on 18 September that is less a product story than a sizing rule nobody currently applies. It starts with a photograph of a 12V lithium iron phosphate pack from an RV, its metal case split wide open by cells that swelled under overcharge. No fire: the cells vented white smoke and blew their vent discs, the labels melted, and that was the extent of it. Collins is blunt about why that matters — "if these had been solid state marine batteries, it would be game over."
The chain of events is the useful part, because every link is ordinary. An MPPT charge controller is, in most marine and mobile installations, a buck converter: PV-side switching MOSFETs chop the array voltage down to battery voltage. When a MOSFET fails, it overwhelmingly fails drain-to-source — a short. "Basically you're just taking the MOSFET out and replacing it with a piece of wire." Open-circuit failures happen, but they are the rare case. So the failure mode you should design against is not the controller going dark. It is the controller becoming a straight-through connection from array to battery.
What arrives at the battery then is not the array's operating voltage but its open-circuit voltage, VOC. Collins shows a real catamaran-class design sized around 116V operating, with a VOC of 141.8V, feeding a 12V LFP bank. And VOC rises as temperature falls: a 200W panel he measures at 37.44V VOC at standard conditions goes to 43.23V at −30°F, the ASHRAE design temperature for his part of Maine. Two of those in series is 86.46V — for a boat stored on the hard over winter with the solar left running.
86.46V matters because of a number almost nobody publishes. The chip that runs a BMS is its analog front end — it watches cell voltages and temperatures and drives the charge and discharge MOSFET banks. Collins holds up a Texas Instruments BQ76942 and reads its absolute maximum of 85V off the datasheet. Most AFEs sit around there. Exceed it and the AFE stops being able to turn the charge FETs off, which is the precise moment the BMS stops being a protection device. And the FETs themselves are frequently 60V-rated parts, because 80V parts cost more.
This is not a story about cheap controllers. The failed unit in the photographs is a Victron, and Collins says so plainly — "it's not just, you know, cheap Furion controllers." Rare, yes. But the mitigation is nearly free: one controller per panel keeps array voltage under 60V, and Collins notes that work he was involved in with Nigel Calder and Bruce Schwab found per-panel controllers outperform series strings on boats anyway, because of shading. The reason to know the number is that nothing in the standards will tell you it.
