Steve wrote:
Today I took my digital multimeter down and checked the voltage. The
boat has been sat for 3 weeks with no power drain or charging. They will
have been left fully charged when I last used the boat. The voltage on
the house bank was 12.8V (still isolated and not touched for 3 weeks).
Cabin temperature was around 12'C.

This seems odd to me after 3 weeks of coldish weather (3-10'C).

Self-discharge rate is a function of temperature; the colder the
battery, the more slowly it self-discharges. At room temperature,
typical wet-cell lead acid batteries self discharge at a rate of about
10% per month. The self-discharge rate at 40 degrees Farenheit will be
more than halved because of the lower temperature. So after 3 weeks at
40 degrees F. your batteries probably lost about 4% of their capacity,
not detectable by your voltmeter measurement.

Also, the resting state voltage rises as the temperature drops; at -40
degrees centigrade (brr!) the resting state voltage of a fully charged
battery approaches 15 volts. Your 12.8 volts reading is a bit higher
than what you'd read at room temperature because of this.

Concerning long term storage, at room temperature a fully charged
battery would approach 50% discharged after about three months and
should be recharged, drop the temperature 15 degrees (F.) and it would
take 6 months for the battery to discharge to 50%. So, depending on
temperature, it would be wise to recharge resting batteries every
couple of months or so, if long life is your goal.