Originally Posted by Barquito
I decided to leave my batteries on the boat this winter. Soon after putting the boat up on the hard, the batteries were both at about 12.6 - 12.7 volts. I charged both of them. Now after sitting for a few weeks, I read about 13.5 volts in both. Is this normal? They are clean and dry, with the cables removed. My volt meter was also left on the boat. Would it get wacky at 35 F?
Batteries really make me feel dim.
While 13.5V is high, and I suspect your volt meter is not quite as accurate as it should be, it is not at all uncommon for batteries in cold weather to take a loooooooong time to self discharge or reach a "resting voltage". The 24 hour "rest" period often talked about applies to 77F..... The colder it gets the longer it takes for the batteries to dissipate the "surface charge".. Batteries can "hang out" in cold temps at over 13V for quite a while and it may take 4-8 weeks+ depending upon temperature for them to even get to a "resting voltage"... When cold the chemical reactions come to a near complete halt thus the battery self discharge in cold temps does too.
I have a battery in my shed right now that I have been monitoring for over three weeks and as of a few minutes ago it was still at 12.96V as measured with a calibrated Fluke 179.. This battery is full at 12.71V to 12.72V.. So even after three weeks, in Maine, in the cold, this battery is still a full 0.25V over "resting full" voltage. Three weeks is a long way from a 24 hour rest to determine SOC..
This is why I have been saying for a very, very long time that leaving batteries on-board, fully charged and 100% disconnected can actually be better for them than storing them in a heated basement.