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"Why?"

Because you can't SEE the damage and the odds are you don't have access to the equipment to do any real meaningful SOC testing.
I do have the test equipment, about $5000.00 worth, I am also one of the only marine electrical systems techs offering physical capacity testing. I am one of the few who has ponied up for the equipment to do that.

I also work directly with battery manufacturers and am currently in the middle of some long term PSOC testing for one of them right now. This is a new AGM variant that looks very, very promising. I compile data, run testing and send the data off to the manufacturer.. This testing has been geared specifically at marine use and how to best develop a battery that can survive this type of abuse. This company understands not everyone will move to LiFePO4 or other chemistries so is working hard to build a sulfation resistant lead acid battery. This gives me excellent access to some of the best engineers in the business, and still, I leave my LA batteries on board all winter...... ;)

Yep, I leave my batteries on-board 100% disconnected but charged/equalized before disconnecting. I can do physical capacity tests to see exactly the impact it has. I have done this multiple times. (capacitance, pulsed load and carbon pile testing is a joke compared to a physical 20 hour capacity test) It has not lead to shorter life and in fact the batteries I service last a good long serviceable life nowhere close to 3 years..

Heck even my AGM reserve battery, that winters on-board, is not on a charge all winter and my boat is 4' from my house. I use one of the smartest chargers made yet I still won't leave it unattended because there is simply no need to do that in the winter... I simply won't do that to my battery when there is less than zero need to.....

And yet, they ALL will say what I just said. Thirty days, and you've probably damaged the batteries. 90 days, and you've definitely damaged the batteries.
And because you don't have the equipment you believe them, then connect to a charger and leave it there thinking it will do good. Remember these are the folks trying to sell you lead who are telling you to leave a battery on constant charge.... ;)

Most of these chargers are certified pieces of $hit and are far from anything even resembling smart. They do nothing to prevent stratification during winter storage and all they do is hold the voltage slightly above resting with mA level current that serves no real purpose in cold weather because there is little to no self discharge going on..

My now 8 year old Wal*Mart batteries have outlasted many sets of Trojan's left on charge but yet they were left on-board every winter not on charge.:confused: Can't be? Cold weather slows all the chemical process dramatically including sulfation and self discharge. This is chemistry 101..

You think doing nothing works well enough? OK, do nothing. They'll be happy to sell you new batteries every three years instead of every six or eight years.
I had a set of 12 year old GEL's on my bench two weeks ago for capacity testing. They have been on-board for 12 years and not charged in the off season... I have many customers well beyond 8 years with typical wet cells all stored on-board & disconnected. As long as the batteries are put away properly the only real effect you have is stratification but a constant float charger will not prevent that either. Oh the marketers will have you believe their charger is as smart as Einstein they do still do not prevent stratification.It is is pure hog wash that a constant float voltage prevents stratification. You need to roll the electrolyte to do that and this requires voltages above gassing... So now you have stratification WITH additional charge current. Think about it, I do, and this is one of many reasons I will not leave a battery in cold storage on constant float.....

I would personally rather see an owner hit them with 14.6V - 15.5V once or twice per winter, a good gassing voltage, than risk burning their boat down with a crappy "smart" charger. Most yards disallow any sort of charging during the winter for just this reason..

Not my opinion. Objective fact, repeated and confirmed by every business that's in the business of making batteries.
Your facts are simply opinion when it comes to winter storage.
 

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Maine-
Gels are nothing like wet lead acid when it comes to long term and sulphating. You know that as well as I do.
And obvisouly, all chargers and maintainers aren't built to the same quality.

But what? Do you think the battery companies are all telling the same lie, that the batteries will sulfate and lose capacity? All lying because, what, they get paid back by the battery "tending" makers?

I don't doubt your equipment, your tests, or your veracity, but I also don't think the entire battery industry is engaged in some grand conspiracy to misinform the public about how batteries should be treated in storage. Not that it isn't possible, just that it is more likely there's some other reason that your observations don't match their statements.
Trojan Battery Co. said:
Storage:
"Batteries gradually self-discharge during storage. Monitor
the specific gravity or voltage every 4-6 weeks. Stored
batteries should be given a boost charge when they are at
70% state of charge (SOC) or less.
Refer to Table 7 for specific gravity
and voltage measurements."
Nowhere in this statement does it say to leave the batteries on a charger in storage. Considering I have not once seen a winter stored battery, properly decommissioned in the fall, drop to 70% SOC before spring. This means no charger is necessary. Trojan's words, not mine.... Course every manufacturer of LA batteries differs slightly thus broadening the confusion.

Trojan Battery Co. said:
4.2 Storage in Cold Environments (less than 32°F or 0°C)
"Avoid locations where freezing temperatures are expected, if possible, during storage. Batteries can freeze in cold temperatures if they are not fully charged. If batteries are stored during cold, winter months, it is critical that they are kept fully charged."
This is the don't be a knucklehead bit.. Again they do not specify to leave your battery on a charger in cold just to make sure they are fully charged. This is very, very easy in cold temps because self discharge is nil... Yep if the batteries are not full charged then they can freeze. Lots of batteries freeze each winter due to the knucklehead effect and parasitic loads.. This is why you charge to 100%, equalize, then fully disconnect them from the vessel. I have never once had a battery freeze.

Heck I was born in Fairbanks, we used flooded lead acid batteries, and the temps regularly get to -60F.... A fully charged battery won't freeze until -72F.. If it gets there you have a lot more to worry about like a split engine....;) Read the link posted above in this thread and you will see that even after three months my battery was still at 12.72V...... That is a FULL battery even after three months.... Put them away correctly and this is a non-issue..

Rolls Battery said:
"Storage:
"It is normal to expect 1% self discharge per day when not in use, under normal temperature conditions. Stored batteries should be recharged every 3 months until battery is put in service to avoid sulfation."

Winter Storage:
Prior to placing batteries into winter storage make certain the electrolyte level is approximately 1.2" (13mm) above the top of the separators. The electrolyte level in very cold batteries will be lower than normal, so let batteries warm to a normal temperature before judging electrolyte levels.

Once the electrolyte level is correct ensure that the batteries are fully charged. Ensure that the battery tops are clean and dry.

Now the choice is whether to leave the batteries aboard your boat or remove and store in a cool dry area. If the batteries are stored aboard the boat, disconnect the terminal cables. This will prevent premature discharge of the batteries due to a ground in the electrical circuits or failure to turn a piece of electrical equipment off.

If the batteries become discharged, the electrolyte can freeze when stored below +20° F (70° C). Below shows temperatures at which electrolyte, in various states of charge, starts to freeze.

A 3/4 charged battery is in no danger of freezing. Therefore, batteries should be kept at least 3/4 charged, especially during winter weather. The frequency of checking batteries depends greatly on temperature. The effect of temperature on self discharge for the average fully charged, new, conventional battery in good condition is approximately as follows:

At 100° F (37.8°C) .0025 Sp.Gr. per day
At 80° F (26.7°C) .001 Sp.Gr. per day
At 50° F (10°C) .0003 SG per day

A fully charged battery stored at 80° F (26.7°C) will take 30 days before it self discharges 25 percent. At 50°F (10°C) the time period increases to 100 days. This will give you an idea of how often a battery should be checked.

Some makes of batteries will have a higher and some a lower rate of self discharge. This depends on the method of manufacture and purity of materials used."
It's pretty hard to charge the stored aboard batteries with disconnected cables.... If at "normal temps" you need to charge stored batteries every 3 months, how long during cold...? A LOT longer...
 
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