[8xeh]

LFPs are indestructible and last forever.

For general advice, they like it cooler[1] than other kinds of lithium batteries. Try to keep them between 15-25°C. Other than that, charge LFP batteries to whatever percentage you want whenever you want.

They are simply better than every other kind of lithium chemistry. Their practical downside is lower cell voltage (3.2 vs 3.8), and slightly lower energy density. I've spent the last 7 years building LFP-powered lawnmower-sized robots, and in practice, the lower voltage and doesn't matter. And, unlike other chemistries, they don't catch fire when you stab them.

For other kinds of lithium batteries, to maximize lifetime, keep them between 40-60%, avoid big charge swings (10-90%), and keep them warm (above 25°C).

[1]: At 35°C, tests show they lose about 5% more capacity after 2,000 cycles compared with 25°C. But even that isn't really a problem, because they don't get to 20% capacity loss until 4,000 cycles. See: https://iopscience.iop.org/article/10.1149/1945-7111/abae37, fig 3.

[Filligree]

A practical concern is that the batteries in question will be powering our cottage, which is entirely off-grid.

It’s also unused for half the year, and for much of that half the solar input is effectively zero; for part it’s literally zero. Not enough to even run the Victron gear, never mind a heater. Spring temperatures often drop below -5C.

So we’ll winterise it by shutting everything off, BMS included to the degree it’ll let us, and the question is what SoC they sold be at for that to work.

[8xeh]

Cold batteries do struggle a bit to deliver power and charge when they're cold, but they're just fine being stored in low temperatures. LFPs are really good at maintaining their charge. Even below -5°C, they should be fine, particularly if they're not being used.

Also, I wouldn't worry about the BMS too much. Unless it's really dumb, when there's no charging or discharging happening it will put itself to sleep for most of that time. Technically it will use power, but likely only a few hundred micro watts to a few milliwatts (nothing that will make a difference on a 5 kWh+ battery). Then when the sunlight comes back in the spring (or whenever the snow melts off the solar panels), it'll see some input voltage and start feeding the battery again.