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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:
"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:
"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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My experience is nowhere near as rigorous, but each winter I've topped-up and charged my Wal-Mart Deep Cycle and then store it unloaded in my unheated New Hampshire shed. I've take a voltmeter to it around Valentines Day and it is still 12.7 +\- volts. I'm with the put 'em away fully charged, store 'em cold crowd and I've gotten great longevity from cheap deep cycle Group 27s.
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