Ahhhh, that could explain it.

The temperature on the day I took the reading was just under 10'C (50'F)
so could have accounted for that higher measurement especially since it
has been very cold (for Wellington NZ) which would have reduced the self
discharge rate. I don't think that the temperature got down to 40'F for
very long but it didn't get much above 50'F at all.

So does anyone have a temperature compensated version of that table?

Thanks for the advise about the recharging. Luckily in the summer the
batteries are recharged at least weekly but the winter has longer
periods of not being used. I have been concerned about the possibility
of self discharge causing damage over these long storage periods but
what you say about self discharge at low temperatures clears up that worry.

I have a question about what the best option would be to increase the
size of the house bank but I will start a new thread for that.

Thanks

Steve


Mark wrote:
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.