I did an experiment last year when I put my camper in storage last year. I disconnected my fully charged battery November 1st. I left it sit, never charged it, but measured the battery voltage with a multimeter every month. Come May 1st, I took the last voltage measurement, 12.45 Volts, which is a state of charge of about 80%. So in six months it only self-discharged 20%. The freeze point of a 12V lead acid battery at 12.45 volts, 2.075 volts per cell, specific gravity 1.225, freeze point -40F (-40C).
My findings correlate to information found on the web that a lead acid battery has a self-discharge rate of 40% per year, and low storage temps actually increase the battery life and slows the self discharge rate.
If your battery is self-discharging faster than that, then the battery is probably due for a replacement.
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