Hi Pat,
It's my understanding that many lead acid batteries (including AGM) die from sulfation.
Sulfation occurs during normal discharge cycles. When discharged "Lead sulfate (sulfation) now coats most of the battery plates. Lead sulfate is a soft material, which can is reconverted back into lead and sulfuric acid, provided the discharged battery is immediately connected to a battery charger. If a lead acid battery is not immediately recharged, the lead sulfate will begin to form hard crystals, which can not be reconverted by a standard fixed voltage (13.6 volts) battery converter/charger."
It's this layer of hard crystals that cause a lasting reduction in battery capacity. That conversion to hard crystals starts almost immediately, so the answer is to re-charge the battery as soon as possible after use and to keep the battery charged while idle.
Flooded batteries discharge at a rate of about 4% per week. This means that within 4 months, a normal FLA will be more than 60% discharged, which is below the normal discharge threshold that many people observe. And the time that the battery has sitting discharged means that the hard sulfation will be well established.
AGM self dicharge at a rate closer to 3% per month. So left unused for 4 months, the battery will still be more than 85% charged. The hard sulfation will be much less prevalent, so the battery can recover more easily. But leave it for a year and you're running the risk of decreasing the overall storage capacity with sulfation.
Lithium self dicharge even slower, but Lithium batteries don't have sulfation issues. They can be left partially discharged with no loss of capacity. Some Lithium experts say that leaving a Lithium battery partially dischared will actually extend the life of the battery.
In theory, all of these battery types can last indefinately if maintained. Some Edison FLA batteries circa 1900 are still usable. AGM take less attention than FLA and I believe that Lithium take less maintenance than AGM.
So your AGM batteries should survive 5 years, maybe more than 10, if kept charged. The Lithiums may last even longer, but since they are a fairly new design, nobody can point to a set of 10 year old LiFePO4 cells and declare them "fine". Time will tell... But I chose them for the light weight and small volume for the same effective range. The long life will just be a bonus (if it turns out)
Good luck and fair winds,
Eric
--- In electricboats@yahoogroups.com, "greenpjs04" <greenpjs@...> wrote:
>
> Eric,
> You are hoping your Lithiums will last 10 years and you know from experience that deep cycle FLA's lasted 3 years. (I understand your point that careful management would have extended that, but I don't want my batteries to be the top thing on my mind). Do you have any feel for where AGM fits between the other two? Mine are still going strong after one year (as expected). I always recharge them after use and usually only use 20% to 40% of the rated capacity during any one outing. I hope they last a long time, but how many years is that likely to be? 5? Maybe I will get Lithiums when the time comes. Weight is not important in my case. (Except when carrying the batteries from the dock to my garage at the end of the season ;-)
>
> Pat
>
>
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