Anyone else had grief with a 100Ah LiFePO4 going out of balance after sitting unused for months?

Left my Fogstar Drift 100Ah on the static caravan all winter — nothing connected, no trickle charge, isolation switch off. Came back in March and the BMS had tripped. Checked cell voltages via the Bluetooth app and one cell was sitting at 2.9V while the others were around 3.2V. Took a few slow charge cycles to bring it back into balance but it was a nerve-wracking few hours.

I've seen people say LiFePO4 self-discharge is so low this basically never happens, but clearly it does if you leave them long enough. The caravan sits unused from November to March — that's four months with zero top-up. Wondering if a simple Victron IP65 on a timer or even a cheap solar trickle panel on the roof would have kept it healthy.

Has anyone got a reliable winter storage routine sorted? Keen to know if others are doing anything specific — float voltage setting, periodic top-up charge interval, anything like that. Feels like there's not much written about long-term LiFePO4 storage in a static/cabin context specifically.

DaiCole | 847 posts

@OldSailor79 Classic winter storage issue, this. LiFePO4 cells self-discharge at slightly different rates, so over several months those small differences compound until the weakest cell drops below the BMS low-voltage cutoff and trips the whole pack.

The fix is usually straightforward — connect a charger and let it do a full charge cycle. The BMS should balance the cells on the way up, assuming the passive balancing kicks in above around 3.4V per cell.

Going forward, I'd suggest leaving it at roughly 50-60% SOC before storage rather than full or empty, and ideally a brief top-up charge every 6-8 weeks. Some people leave a small parasitic load like a voltage monitor connected just to keep tabs remotely.

What were the individual cell voltages showing on the Bluetooth app? That'd confirm whether it's a balancing issue or something worse.

JohnDixon | 312 posts

@OldSailor79 Exact same thing happened to me with the boat last winter. Came back in April thinking I'd just fire everything up, spent the first afternoon watching the Victron app have a minor breakdown instead.

What nobody tells you is that even with the isolation switch off, there's often something quietly sipping away — BMS quiescent current being the main culprit. Tiny draw, but over four months it adds up and the weakest cell hits the floor first.

My fix now: before I button everything up for winter, I top the pack up to about 90%, then physically disconnect the BMS comms cable if I can. Bit faffy but saved me the grief this year.

Also worth checking whether your Fogstar BMS has a storage mode — some of the newer Drift firmware does, though I'll be honest I haven't dug into mine properly yet.

RogerOliver | 203 posts

@OldSailor79 One thing worth trying before you do anything else — connect a small load (a 12v bulb or similar) briefly across any cell that's showing significantly higher voltage than the others. Sometimes after long storage you get a rogue cell that's drifted high and is blocking the BMS from accepting a charge, rather than a genuinely weak low cell. Once you've nudged it down slightly the BMS often resets itself quite happily.

Also worth checking whether the Fogstar app gives you individual cell readings — if one cell is sitting noticeably below ~2.5v you'll want to bring it up slowly with a bench power supply rather than hammering it with the main charger.

Long-term, even a tiny parasitic load like a Bluetooth BMS module will actually help keep cells balanced during storage, funnily enough.

Gibbo53 | 67 posts

Had almost identical grief with my tiny house build last autumn — left a 100Ah sat for about 10 weeks with nothing connected and came back to a tripped BMS.

What I found helped was using a Victron IP65 charger set to the storage/float profile before I leave for any extended period. Even just getting the cells balanced and sitting at around 50-55% SOC before disconnecting seems to make a big difference compared to leaving them wherever they happened to be.

@RogerOliver raises a fair point about the small load trick — did that work for you @OldSailor79? Curious whether the Fogstar BMS resets cleanly once you've brought the low cell back up slowly, or whether it needs a manual reset via the Bluetooth app.

MandyGrant | 89 posts

Boat life here too — this is pretty common with LiFePO4 left completely disconnected over winter. One cell drifts low while the others sit fine, BMS sees it go under threshold and cuts out.

What worked for me was a slow top-up via a proper LiFePO4 charger on a low amp setting — I used my Victron IP65 on the 5A profile. Gets that weak cell nudged back up enough for the BMS to reset, then you can do a proper balance charge.

Worth checking if your Fogstar app shows which specific cell is the culprit. If it's always the same one drifting, that cell may be weakening and worth keeping an eye on long-term.

CrafterWanderer | 341 posts

Worth noting that self-discharge rates between individual cells are never perfectly matched — over several months that tiny variance compounds until the weakest cell hits the BMS low-voltage cutoff whilst the others are still relatively healthy. It's not a fault as such, just physics.

On my static caravan setup I leave a Victron SmartSolar running a small 20W panel through winter specifically to maintain resting voltage around 3.30–3.35V per cell. Costs almost nothing and the BMS never trips.

@OldSailor79 if your Fogstar Drift connects via Bluetooth, check whether the BMS app shows one cell significantly lower than the rest — if it's consistently the same cell across charge cycles you may have genuine capacity degradation on that cell rather than a simple balance issue, which is a different conversation entirely.