Or
they live long enough. One thing that lithium battery vendors conveniently ignore in their cost comparisons is physical aging, which along with cycle life determines the ultimate longevity of the battery. If you are in an application (off-grid, frequent boondocking, etc.) where you cycle the battery daily then lithium far outperforms lead-acid, but more typical RV use involves long periods of float and relatively few discharges per unit of time and in this mode it's likely even lead-acid batteries would age-out before they cycle-out. It's difficult to determine lithium battery life because it is so dependent on charge level and operating temperature, but floating at full charge (particularly at warm temperatures) is severe duty for them, in fact
capacity loss can be 20% per year at 100% charge at 77F, so if you used them relatively infrequently they wouldn't last much longer than lead-acid.
That's not mean to denigrate lithium batteries, let's face it, they're seriously cool. But it would take a somewhat unusual use case to ever get to SmartBattery's '5,000 cycles.' In my case I might fully cycle my batteries dozens of times per year, not hundreds or thousands, and the rest of the time (on the road or when connected to external power) they just float. Under those circumstances the additional capacity is nice (additional capacity is always nice), but it sure wouldn't be very cost effective. At half the current price or less the equation might change and I think we might be there sooner than we think, but in the meantime I will continue to lust after lithium but in practical terms I'll have to stick with lead-acid. YMMV of course.
As an aside it seems that it would be very beneficial for lithium battery management systems to have a mode where batteries are held at partial charge when not in heavy use to obtain a longer service life. My laptop does this, i.e. I can select the battery to be held at 60% charge while on AC power. This is a good idea because laptop batteries often sit in a warm docking station with batteries constantly fully charged (and one might easily see similar conditions in an RV), and this is a terrible operating environment for lithium cells. If such protection is worthwhile for a $100 battery it seems like it would certainly be a good idea for a $1,000 battery.
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