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 30 years and my bank longevity is tremendous, and always has been very, very good.
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 100% unnecessary, if your excuse for doing so is cold weather. Some boat yards like to remove batteries because they can charge you for it! When doing so 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.) recently ended its sixth year of use and still performs at better than both CCA & MCA specifications based on both Argus and Midtronics analyzers.
How do these measurements compare to a true 20 hour capacity test? At the end of the batteries 6th year she produced 89.5% of the new Ah rating when capacity tested. This means it still produced 112 Ah's (125Ah rating) at the end of year 6. This battery had lived every winter on-board our boat uncharged except for the occasional "top up" charge a few times per winter. Now granted, this battery had lead a rather mild life in a system designed for shallow discharges with 80% SOC often being the deepest discharge. It is not a true deep cycle battery thus the system was designed for shallower discharges to increase cycle life. it apparently worked because I often see this same exact battery 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. 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.. I Fairbanks you can 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/charge 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 only occasionally topped back up. I find this 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 when batteries are left charging, without any supervision, than I do when left fully charged, equalized if applicable, disconnected and resting (in cold weather). Perhaps once per month, if that, hit them with some good "bubbling" voltage then disconnect them again. If your temps moderate, or warm up into the 60's, hit them again.
Self discharge rates are directly dependent upon temperature. As the temperature climbs the self discharge rates also climb. 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 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.
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 will revents stratification. Most so called "smart chargers" will generally not prevent stratification.. The floating voltages are often too low to keep the electrolyte moving and in a mixed state.. This gassing or movement of the electrolyte, at absorption voltages or higher, is will helps limit stratification. Smart chargers that revert to absorption voltages every few weeks can help roll the electrolyte and limit stratification. Sadly most "smart" chargers don't do this and are not really as "smart" as they think they are. Most "smart chargers" are really rather dumb and are nothing more than glorified egg-timer voltage limiters.
Still leaving batteries on-charge and unattended can often result in dead banks, which is 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.
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.. So, 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 suck the bank down. Power comes 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....