Lead-acid batteries do not tolerate being left in a semi-discharged state. The lead sulfate crystals will increase their bonding strength, growing larger, becoming immune to normal charging voltages/current, reducing capacity in the process.
The problem is amplified when sulfated batteries are subsequently cycled as it leads to permanent damage.
Desulfators can eliminate lead sulfate crystals and prevent permanent damage if used prior to the next charge/discharge cycle.
In the situation you cite of batteries being left in a 40% SOC, no harm will come to them if pulse conditioned even at extremely low charge rates.
The critical difference between lithium and lead-acid chemistry is that flooded LA batteries need to be gassed at the end of the charge cycle to remix the electrolyte to avoid stratification.
It has been my experience that while AGM batts do not require this treatment (gassing), they normally do not have the life expectancy of flooded cell due to the pressure relief valves leaking, in time leading to dehydration and cell death.
With proper maintenance, LA batts can rival lithium at a substantial cost savings, although they do carry a weight/size penalty.
Another consideration is the life span of lithium based batteries. I am given to understand they have a set life span, no matter how well they are maintained. LA batteries do not seem to have this problem.
--- In electricboats@yahoogroups.com, Chris Baker <chris@...> wrote:
>
> Interesting thread...
>
> There's another issue not mentioned here and that could be of relevance to life cycle. And that is the tolerance of the battery to remain in a partly discharged state.
>
> My understanding is that LiFePo4 batteries can be kept at pretty much any state of discharge without causing deterioration of the battery chemistry. On the other hand, from my experience, and from anecdotes of others, it seems that time in a discharged state is critical for lead-acid batteries.
>
> I wonder if the figures that Concorde-LifeLine quote are based on the battery being immediately recharged after it reaches its its discharged state? And what would happen to the life-cycle calculation if the battery was left at the discharged state for hours, or even days?
>
> For boating use this can be important, depending on your circumstances.
>
> Consider the weekend use of an electric boat. The boat leaves the dock with batteries fully charged, and uses say 30% of capacity to get to a an anchorage, where it remains for the weekend. So the battery is sitting at 70% state of charge for maybe two days. At the end of the weekend the boat goes back to the dock, using another 30% of capacity. At the dock the recharge can begin.
>
> I have a notion that once recharge is commenced the battery is no longer deteriorating, and there is no longer any penalty for remaining in a state of discharge.
>
> The situation is worse for a boat on a mooring where recharge comes from solar panels. The boat would return to the mooring with batteries at 40% SOC and no recharge will commence until the next sunny day - and it may take a week, or longer, to recharge the pack.
>
> For use where the cycles include significant time at the bottom of the cycle, it seems that LiFePo4 batteries could offer this further advantage.
>
> Cheers
>
> Chris
>
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