48v LiFePo4 battery won't charge.

Had something similar on my narrowboat build last spring — turned out to be the BMS going into protection mode and just refusing to wake up without a bit of coaxing.

Few questions that might help narrow it down:

• What's the resting voltage sitting at right now? If it's dropped below around 44v the BMS may have triggered low-voltage lockout
• Is this a Fogstar, Eve cell, or something else? Some BMS units need a "wake-up" charge at a lower current before they'll accept a full charge
• What charger are you using? A lot of 48v Victron kit won't push current if it can't detect a minimum battery voltage at the terminals

On my motorhome I had a Daly BMS that kept doing this after the van sat unused for a few weeks. Fixed it by briefly connecting a small 12v charger across the terminals to nudge the voltage up enough for the main charger to recognise it — obviously do this carefully and check polarity first.

Worth checking the BMS output terminals with a multimeter too — is there actually voltage present on the load/charge side, or just on the cell side? That distinction matters quite a bit for figuring out whether it's a BMS trip or something upstream.

What does your monitoring show, if anything? Victron Cerbo or similar?

@BerlingoConvert classic "sleeping BMS" situation — mine did exactly that until I poked it back to life with a bench supply set just above the low-voltage threshold, like jump-starting a very stubborn Victron-powered toddler.

@BerlingoConvert @MultiPlusFan — worth checking whether it's actually a sleeping BMS or a genuine undervoltage lockout first, because the fix differs quite a bit.

If your cells have dropped below ~2.5V per cell (so ~40V on a 16S pack), most BMS units won't respond to a bench supply poke — you need to trickle charge each cell group individually back above the recovery threshold before the BMS will even negotiate.

On my tiny house build I use a Fogstar Drift 48V with a Victron MultiPlus-II, and I've had to do exactly this twice after leaving the system unattended over winter. A cheap bench PSU set to ~2.8V per cell in current-limit mode sorted it without stressing the cells.

Also — what BMS are you running? Some Chinese units need a physical button press to wake from protection mode regardless of voltage.

@WheresMeWires67 good point on the distinction — easy way to tell is just stick a multimeter across the terminals. If you're seeing anything below about 2.5V per cell (so ~40V on a 16s pack) then you're in genuine undervoltage territory and the BMS is doing its job protecting the cells.

Had this exact thing on my cabin setup with a Fogstar pack last winter. Cells had drifted too low sitting idle. Had to use a small bench PSU limited to about 2A to creep the voltage up before the BMS would even consider waking up.

If the resting voltage looks reasonable though, it's almost certainly the sleeping BMS issue the others mentioned — some BMSs just need a brief charge signal applied directly to the BMS wake pin to kick them out of it.

@SaltyMaker that multimeter check is the right first move — been there myself with the 48v Fogstar bank in my shepherd's hut build last winter.

One thing nobody's mentioned yet: if you're running a Victron charger, check whether the absorption voltage is set correctly for your battery's actual cell count. I had mine undershooting by nearly 2v because someone had templated the settings from a lead-acid profile. The BMS saw the incoming voltage as "wrong" and just sat there sulking, refusing to engage.

Sounds daft, but it kept me scratching my head for two days before I spotted it in VictronConnect. Worth a look before you start poking the BMS with a bench supply — sometimes the fix is embarrassingly simple.

@GaryParker97 @SaltyMaker — the multimeter check is essential but I'd add one caveat: measure at the cell terminals directly if you can access them, not just at the battery's output terminals. A BMS in lockout will show cell voltage at the cells but zero (or near-zero) at the output — that distinction tells you immediately whether you're dealing with a protection trip versus genuine cell undervoltage.

On my narrowboat setup with a Victron Multiplus-II 48/3000, I've had the BMS refuse to wake even after the source voltage was corrected. In that case I used a bench power supply set ~0.5V above resting cell voltage to "tickle" the BMS back into recognising a valid charge source. Most quality BMS units (Daly, JK, Overkill) have a wake threshold that needs to be met before they'll re-enable the output FETs.

What brand/model BMS are you running?