I have been tracking self discharge in the winters with known healthy batteries here in Maine for many years. If properly charged in the fall, then 100% disconnected, they loose very, very little capacity over a North East winter..

This is one experiment I did that went awry as I got ADHD on the battery still even after eight months the battery was still technically at 90% SOC according to both SOC and SG readings... Cold weather & temps below 80F drastically minimize the effects of self discharge.