Hi Mark and Chris,
Thank you for the information. I guess as Chris points out there are mutiple benefits to lithiums including: less physical size and weight, greater DOD, ability to sit at less than full charge without damage, no to very low losses when charging, better sealed, can be mounted in any position, etc., but what are some of the drawbacks?
1) Price: Costs more upfront. It may be less on a per cycle basis if one can take advantage of the 2000+ cycles during its lifespan, but this would clearly depend on the lifespan.
2) Lifespan: It does appear that the projected lifespan for lithium batteries is 5 years if the press release (copied below) has any merit. Some argue that 5 years might be generous. One article claims: "Lithium ion batteries are particularly susceptible to aging; as soon as one leaves the manufacturing line, its countdown begins. A typical lifespan is two to three years." If the lifespan is 5 years (or less) than it would appear that one would need to more or less discharge the lithium pack every day for it
to make financial sense (365 days x 5 years = 1825 cycles), otherwise the capacity for these extra cycles would not get used. If this is the case, then lithiums might make more sense for a daily commuter car then for a recreational boat that gets used only several dozen times a year. I'm grasping for information here because I obviously don't know. If anyone can give some information about the actual lifespan of the lithiums currently available (e.g., Thundersky) that would be greatly appreciated.
Press Release indicating the lifespan is 5 years -- April 5, 2010 (Bloomberg) -- Hitachi Ltd., Japan's fourth-largest company by revenue, has developed a method to double the lifespan of its lithium-ion batteries to more than 10 years, the Tokyo-based company said in statement today. The technology, which extends the life of manganese cathodes, will cut the use of cobalt, a battery material that is relatively scarce, Toshio Otaguro, general manager at Hitachi's battery-systems unit, told reporters in Tokyo. The batteries are expected to be used for industrial applications such as storing energy from wind power or powering construction gear, he said.
3) Storage: Some online sources, for whatever they're worth, claim that keeping lithiums full on a regular basis leads to a signficant capacity loss over time. Instead, they recommend keeping them stored at only a 40% charge (one article is copied at the bottom of this post). If this is true, it would seem like a huge drawback since one would always want to keep their battery bank topped up to allow the maximum range... otherwise what would be the point of buying these more expensive batteries? Again, I don't know quite what to make of this and would welcome any input.
I suspect that the average recreational sailor might be looking at 200+/- discharge cycles over the course of 5 years. The benefits of lithiums are appealing but do the drawbacks make them impractical?
Regards,
Owen
ARTICLE: How to store batteries from http://www.batteryuniversity.com/print-partone-19.htm
Batteries are perishable products that start deteriorating right from the moment they leave the factory. There are simple preventive measures that battery users can apply to slow the aging process. This paper provides guidelines to reduce age-related capacity losses and how to prime new and stored batteries.
The recommended storage temperature for most batteries is 15°C (59°F). While lead-acid batteries must always be kept at full charge, nickel and lithium-based chemistries should be stored at 40% state-of-charge (SoC). This level minimizes age-related capacity loss, yet keeps the battery in operating condition even with some self-discharge. While the open terminal voltage of nickel-based batteries cannot be used to determine the SoC accurately, voltage fuel gauging works well for lithium ion cells. However, differences in the electrochemistry of the electrodes and electrolyte between manufacturers vary the voltage profile slightly. A SoC of 50% reads about 3.8V; 40% is 3.75V. Store lithium-ion at an open terminal voltage of 3.75-3.80V. Allow the battery to rest 90 minutes after charge before taking the voltage reading.
Figure 1 illustrates the recoverable capacity at various storage temperatures and charge levels over one year. 
Figure 1: Non-recoverable capacity loss on lithium-ion and nickel-based batteries after storage. High charge levels and elevated temperatures hasten the capacity loss.
Among the lithium-ion family, cobalt has a slight advantage over manganese (spinel) in terms of storage at elevated temperatures. nickel-based batteries are also affected by elevated temperature but to a lesser degree than lithium-ion.
Lithium-ion powers most of today's laptop computers. The battery compartment on many laptops rises to about 45°C (113°F) during operation. The combination of high charge level and elevated ambient temperature presents an unfavorable condition for the battery. This explains the short lifespan of many laptop batteries.
Nickel-metal-hydride can be stored for about three years. The capacity drop that occurs during storage is permanent and cannot be reversed. Cool temperatures and a partial charge slows aging. Nickel-cadmium stores reasonably well. Field test reveled that NiCd batteries stored for five years still performed well after priming cycles. Alkaline and lithium batteries (primary) can be stored for up to 10 years. The capacity loss is minimal.
The sealed lead-acid battery can be stored for up to two years. A periodic topping charge, also referred to as 'refresh charge', is required to prevent the open cell voltage from dropping below 2.10V. (Some lead-acid batteries may allow lower voltage levels.) Insufficient charge induces sulfation, an oxidation layer on the negative plate that inhibits the current flow on charge and discharge. Topping charge and/or cycling may restore some of the capacity losses in the early stages.
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