OK so much for my "article".... Yes, I promptly got ADHD and totally forgot about the battery in the shed until today. D'oh.......... When I remembered about the lonely battery in the shed I shoveled a path through the snow and took a voltage reading.
As you can see this battery is still resting at 12.72V after two and half months! December 11, 2012 to February 26, 2013 and still reading technically "full"... Ideally I wanted to get bi-weekly readings, but so much for that.
My point here is to illustrate why leaving your batteries on -board your boat, in the winter, does not lead to their instant demise, as many on the net would have you believe. My family and I have done this for more than 40 years and bank longevity has been tremendous. As a professional marine electrician I winterize a lot of boats & batteries and most owner choose the charge to full, EQ if necessary, and isolation from the vessel approach.
A broken back, hip, leg or other mishap can end your season or sailing career. Lugging batteries on and off boats can be very dangerous and is really quite unnecessary, especially if your excuse for doing so is cold weather. Some boat yards like to remove batteries because they can charge you for it. That is their only good reason for ddoing so but they disguise this charge as "healthy for the batteries". The other reason is liability & insurance because with the batteries removed they also know the batteries are in-fact disconnected and it gives them peace of mind.
This particular Wal*Mart battery (actually made by US Battery Inc.) was still performing better than both CCA & MCA specifications based on both Argus and Midtronics analyzers, but these tools only represent short term cranking capability not Ah capacity.
How do those measurements compare to a true 20 hour capacity test? At the end of the batteries 5th year it produced 79.4% of the new Ah rating when capacity tested. The battery had lived every winter on-board the boat uncharged. While technically at end of life, based on Ah capacity results, the battery will still work.
This battery is a cheap Group 31 "deep cycle" Wal*Mart re-stickered, automotive group type, battery but it had lead a rather mild life in a system that was designed for shallow discharges with 70% SOC/30% DOD often being the deepest discharge. It is not an actual deep-cycle battery, but was all that would fit, thus the system had been designed for shallower discharges to increase cycle life and longevity. Based on capacity testing results I often see this same exact battery group type literally murdered in 1-2 years.
Contrary to popular misconception cold weather is actually not a bad thing for the batteries, provided they are fully charged going into it the storage season. A fully charged battery will not freeze until approx -70F. I lived in Alaska, Fairbanks to be exact, and they have been using flooded lead acid batteries up there since they were invented. In most parts of this country we will never see the -60F they see in Fairbanks. In Fairbanks you can literally spit and have it hit the ground frozen, not so in the rest of the country.
Lead acid batteries (AGM, GEL and FLOODED batteries are all lead acid) will lose very little voltage/stored capacity in colder weather due to what is called "self discharge".
If leaving batteries on board I much prefer to see them 100% disconnected from the vessel, AND EACH OTHER, and only occasionally topped back up, if at all. I find this practice safer and more reliable than being left on permanent charge while unattended. There is just too much to go wrong to leave unattended batteries charging.
I see far more problems & ruined batteries when batteries are left charging, without any supervision, than I do when left fully charged, equalized if applicable, disconnected and resting (in cold weather). If you feel compelled hit them once per month, if that, with some good "bubbling" voltage then disconnect them again.
Self discharge rates are directly dependent upon temperature. As the temperature climbs the self discharge rates also climbs. As temps drop the self discharge rates and chemical reactions also drop & slow. When the temps go sub 30F the self discharge rate crawls to a hibernation level or very, very slow pace.
When I say hit them periodically them with a good "bubbling voltage" this applies to flooded batteries. A good gassing voltage of 14.6V to 14.8V will get the electrolyte moving and minimize any stratification going on inside the battery from it sitting idle. If you want to shorten these periodic charging events, use an equalization voltage.
But RC what about stratification?"
Stratification is when the acid and water tend to separate due to sitting idle. The acid sinks to the bottom and the water rises to the top. It is a common misunderstanding that a constant float charge prevents stratification. let be be clear on this IT DOES NOT! A constant float voltage charge does nothing to prevent stratification.
Most so called "smart chargers" will generally not prevent stratification of the electrolyte. The floating voltages are usually far too low to keep the electrolyte moving and in a mixed state. Push the float too high and we suffer plate decay/erosion. This gassing or movement of the electrolyte, at absorption voltages or higher, can help limit stratification.
Smart chargers that revert to absorption voltages every few weeks can help roll the electrolyte and limit stratification, but these are rare. Sadly most "smart" chargers don't do this and are not really as "smart" as they think they are or are marketed as. Most "smart chargers" are really rather dumb and are nothing more than glorified egg-timer voltage limiters.
Still, leaving batteries on-charge and unattended will more often, not less often, result in murdered banks, the exact opposite of the intended goal.. Power outages occur, cords get unplugged, breakers get tripped and not reset and the parasitic loads then go to work killing your bank. I have yet to see more than a hand full of boats that do not have some form of parasitic load. Merely turning the battery switch to OFF is NOT adequate.
Most "smart" chargers, or any smart enough to even consider leaving connected full-time, while unattended, will not restart if the battery voltage gets too low. This is a built in safety feature to prevent charging into an internally shorted battery but can mean the charger will not fire back up if the bank has dropped too low.. The usable capacity of your bank shrinks when it gets cold so even a small parasitic load is essentially amplified in cold temps.
Most boatyards also prohibit leaving batteries connected and charging all winter, and for good reason, safety issues.
As I have seen repeated far too often, power goes out or the solar panel becomes occluded in snow/ice and the parasitic loads of the charger, controller or other "always on" devices, there are many these days, suck the bank down. Power comes back on or the snow melts but now the battery voltage is below the safe turn on threshold and the battery continues to discharge until fully dead. This is not as rare an occurrence as it would seem. Power has been out three or four times in the last month here in Maine due to winter storms. Will your charger automatically re-boot?
With a fully charged and 100% physically disconnected battery bank noting except time & temp can discharge it, and when it is cold, you've got plenty of time.