Take what Smead says with a grain of salt. Most of it is great but some of it is not so great.

I have many banks up here that sit on-board every winter, after a fall equalization, (not for GEL or non Eq AGM) that are into their seventh or eighth year. I have one bank of gell batts that just broke year 15 in May. Those batts have been on-board every winter uncharged but fully charged and disconnected. The key to storage on-board in the winter is a full charge (many battery chargers do a horrible job at a true "full charge") and the batts must be fully disconnected. I have another customer with L-16's going on year 11. Been on-board uncharged every winter...

The cold temps slow the chemical process to a crawl thus spring voltages can still be above 12.7V on healthy batteries... You still wind up with some stratification after long sits but it does not seem to affect longevity when the temps are cold (constantly remaining below gassing voltage on float can also result in stratification). For flooded batts I put them to bed with an Eq charge and wake them up with about 30 minutes of Eq.. There are too many HORRIBLE chargers out there, and high charger failure rates, for me to feel comfortable leaving batteries connected, charging and unattended. I have also seen batts destroyed in winter by "small controller-less" solar panels.....

The batteries that came off our boat before I converted to Li are now into their 8th season. They are currently in temporary service on my brothers power boat. They are still testing at 86-89% of rated capacity yet were stored on-board fully charged every winter......

I am still waiting for the failures of battery switches Smead talks about..... Yet to see a single one like his photo and I work on some boats with extremely LARGE engines and some with 400A thrusters too... The Blue Sea switches are excellent quality.. The biggest failure I see with switches is a slight break before make indicating the contacts are getting worn. Just yesterday I was working on a very large Cummins with an air intake heater that alone draws 200A on-top of the starting current. The boat also has a Vetus bow thruster that is rated at 400A. That battery switch is original to the 1982 boat..... We have no current on most sailboats that even approaches what that Cummins can throw at a battery switch...