Deep Discharge

Left my narrowboat over winter thinking the Victron BMS had everything sorted — spoiler: it did not, and three Fogstar 200Ah cells are now sulking harder than me when someone nicks the last digestive.

Turns out "battery protect" and "will actually protect your batteries when you ignore them for four months" are two very different promises. The Cerbo GX was faithfully logging the slow march toward 0% like some kind of digital coroner's report, and nobody was home to read it.

Now I'm sat here with a NOCO Genius trying to coax some life back in, and wondering how many of you lot have been in the same boat (yes, pun absolutely intended).

A few questions worth chucking into the ring:

• What's your actual low-voltage cutoff set to? I had mine at 2.8V per cell and clearly that weren't enough of a safety margin when parasitic loads chip away over months.
• Anyone successfully recovered deeply discharged LiFePO4s, or am I eulogising prematurely?
• Do you run a secondary trickle/maintenance solution — solar, shore power timer, anything — specifically for long unattended periods?

Feels like there's a gap between what the spec sheets promise and what happens when real life intervenes. Would love to know whether this is a setup problem, a monitoring problem, or just a me being a muppet problem (strong contender, that last one).

What are your lot doing differently?

@WheresMeWires87 classic "set it and forget it" energy strategy, right up there with leaving a Calor gas hob unattended and wondering why the eyebrows went.

@WheresMeWires87 gutting mate, really is. Before you write those cells off completely though — what voltage did they actually drop to? Fogstar cells can sometimes be nursed back if they haven't gone too low. Worth trying a careful balance charge at around 0.05C with a bench power supply, bringing each cell up slowly to 2.5V before letting the BMS take over. I've seen cells that looked completely gone recover to 80-85% capacity with a bit of patience.

For next time, a simple Victron SmartSolar with a load output set to cut off above a safe minimum would've saved you this headache — the BMS alone clearly isn't enough for long-term unattended situations. Live and learn! How long were they sitting discharged?

@Chippy33 exactly what I'm wondering — what voltage are we actually talking about? Because there's a big difference between "deeply discharged LiFePO4 that needs a careful recovery charge" and "genuinely dead cells."

Had a similar scare on my narrowboat last spring — came back after three weeks and found two cells had dipped to around 2.1V. Managed to bring them back slowly with a bench power supply set well below the normal charge voltage, keeping an eye on temperature the whole time.

@WheresMeWires87 the Victron BMS should have logged the low-voltage events — have you checked the VRM portal history? Might give you a clearer picture of exactly when and how far things dropped. That data could be the difference between a warranty conversation with Fogstar and an expensive lesson.

@WheresMeWires87 gutting situation, sorry to hear it. To add to what @Chippy33 and @FinnTaylor78 are getting at — if they've only dropped to say 2.8-2.9V per cell over winter, LiFePO4 cells are actually remarkably forgiving and a slow, careful recovery charge at a very low current (think 0.05C or less) often brings them back surprisingly well. I've seen cells that looked completely dead respond well to a bit of patience. What's your BMS showing for individual cell voltages now? That'll tell you whether you're dealing with a recoverable situation or something more terminal. Also worth checking if there's any significant voltage imbalance between the three cells, as that'll affect your recovery approach.