Main point... Bats can 'leak' down from about 1% to 15% per month.
Good general read on bats.
http://www.windsun.com/Batteries/Battery_FAQ.htm
some snips:
The lifespan of a battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors. In extreme cases, it can vary to extremes - we have seen L-16's killed in less than a year by severe overcharging, and we have a large set of surplus telephone batteries that sees only occasional (5-10 times per year) heavy service that are now over 25 years old. We have seen gelled cells destroyed in one day when overcharged with a large automotive charger. We have seen golf cart batteries destroyed without ever being used in less than a year because they were left sitting in a hot garage without being charged. Even the so-called "dry charged" (where you add acid when you need them) have a shelf life of at most 18 months, as they are not totally dry (actually, a few are, but hard to find, the vast majority are shipped with damp plates).
These are some general (minimum - maximum) typical expectations for batteries if used in deep cycle service:
Starting: 3-12 months
Marine: 1-6 years
Golf cart: 2-6 years
AGM deep cycle: 4-7 years
Gelled deep cycle: 2-5 years
Deep cycle (L-16 type etc): 4-8 years
Rolls-Surrette premium deep cycle: 7-15 years
Industrial deep cycle (Crown and Rolls 4KS series): 10-20+ years
Telephone (float): 1-20 years. These are usually special purpose "float service", but often appear on the surplus market as "deep cycle". They can vary considerably, depending on age, usage, care, and type.
NiFe (alkaline): 3-25 years
NiCad: 1-20 years
Battery Voltages
All Lead-Acid batteries supply about 2.14 volts per cell (12.6 to 12.8 for a 12 volt battery) when fully charged. Batteries that are stored for long periods will eventually lose all their charge. This "leakage" or self discharge varies considerably with battery type, age, & temperature.
It can range from about 1% to 15% per month. Generally, new AGM batteries have the lowest, and old industrial (Lead-Antimony plates) are the highest. In systems that are continually connected to some type charging source, whether it is solar, wind, or an AC powered charger this is seldom a problem. However, one of the biggest killers of batteries is sitting stored in a partly discharged state for a few months. A "float" charge should be maintained on the batteries even if they are not used (or, especially if they are not used). Even most "dry charged" batteries (those sold without electrolyte so they can be shipped more easily, with acid added later) will deteriorate over time. Max storage life on those is about 2-3 years.
Batteries self-discharge faster at higher temperatures. Lifespan can also be seriously reduced at higher temperatures - most manufacturers state this as a 50% loss in life for every 15 degrees F over a 77 degree cell temperature. Lifespan is increased at the same rate if below 77 degrees, but capacity is reduced. This tends to even out in most systems - they will spend part of their life at higher temperatures, and part at lower.
Myth: The old myth about not storing batteries on concrete floors is just that - a myth. This old story has been around for 100 years, and originated back when battery cases were made up of wood and asphalt. The acid would leak from them, and form a slow-discharging circuit through the now acid-soaked and conductive floor.
State of Charge
State of charge, or conversely, the depth of discharge (DOD) can be determined by measuring the voltage and/or the specific gravity of the acid with a hydrometer. This will NOT tell you how good (capacity in AH) the battery condition is - only a sustained load test can do that. Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less. Specific gravity will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell. This can vary with battery types and brands somewhat - when you buy new batteries you should charge them up and let them sit for a while, then take a reference measurement. Many batteries are sealed, and hydrometer reading cannot be taken, so you must rely on voltage. Hydrometer readings may not tell the whole story, as it takes a while for the acid to get mixed up in wet cells. If measured right after charging, you might see 1.27 at the top of the cell, even though it is much less at the bottom. This does not apply to gelled or AGM batteries.
"False" Capacity
A battery can meet all the tests for being at full charge, yet be much lower than it's original capacity. If plates are damaged, sulfated, or partially gone from long use, the battery may give the appearance of being fully charged, but in reality acts like a battery of much smaller size. This same thing can occur in gelled cells if they are overcharged and gaps or bubbles occur in the gel. What is left of the plates may be fully functional, but with only 20% of the plates left... Batteries usually go bad for other reasons before reaching this point, but it is something to be aware of if your batteries seem to test OK but lack capacity and go dead very quickly under load.
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Note: the word "sulfated" when researching a charger for the Uhaul project I ended up purchasing a IOTA charge with a built in float charge system that 'de-sulfates' the cells, basically it jiggles the voltage to release the sulfer that can build up on the plates, preventing full charge.. I've been able to slap at charger on a few bats (for a few days) that seemed 'dead' and make them useful again...
A few more links... basically it all boils down to... Do your homework and take care of your bats. and they will take care of you...
http://www.repairfaq.org/ELE/F_Car_Battery.html
http://www.repairfaq.org/ELE/F_Car_Battery.html
