I have a summer cottage running solar panels and a victron mppt. I’m trying to determine if I should just disconnect my lithium ion batteries for the winter leaving them at 50%, or just set the mppt to keep them at 50%, leaving the system running. There would be some drain of the batteries leaving the mppt and the temperature smart sensor on. If I leave the mppt charging throughout the winter, is that considered to be a cycle even though it is only raising the SOC 1% or so? Would that cause the batteries to developer a memory and not want to charge to 100 percent in the future?
The main problem with “winterising” lithium batteries is that they won’t accept charge or discharge at low temperatures. This is due to low electron mobility in the electrolyte.
Lithium batteries also don’t develop a ‘memory’ if only cycled through a small % of SOC. A cycle is determined to be 100% of nominal capacity. This gives a battery a finite amount of energy that once this is cycled through the battery, it is considered to be at ‘end of life’. According to data sheets - and this varies from manufacturer to manufacturer, this can be 2000 cycles at 100%, or 4000 cycles at 80% etc. Thus a period of time where the cycle depth is only 10% is not going to contribute much to the battery degradation.
If in doubt, then charge to between 25 and 50%, then disconnect them. If there is no load e.g. internal BMS, then they can happily sit for 6 months or more.
Some on here are suggesting the SOC during off-season should be maintained at 90% to avoid reducing the maximum SOC when charging for use again. I was considering leaving the system on but lowering the SOC to 50%. Would that make it difficult to get back to 100% during the summer months? Or is that not a thing.
I’ve been reading about calendar aging in batteries, and the key takeaway is to keep them cool—something that’s easy in winter.
The research also recommends maintaining a low state of charge (SOC), though that’s not always practical. As Mike pointed out, there’s a risk that certain loads, like a BMS, could draw the battery below 0% SOC.
One paper I found—Assessment of the Calendar Aging of Lithium-Ion Batteries for Long-Term Space Missions—suggests that LiFePO₄ batteries could experience minimal degradation over 50 years if both temperature and SOC are kept low.
I don’t worry about mine. I just leave them connected and solar on as usual, they just sit there taking charge when it’s warm enough. If it’s really cold the MPPT switches off so no damage done.