Lithium is expensive due to it being a "LIMITED" resource. One development being worked on that we all need to be aware of is UltraCapacitors. I just bumped into it today because I was thinking about using a full bridge and capacitor setup for my lights (avoiding batteries). It seems that this technology is the next efficient cheap thing we are waiting on. Even though it does not have the energy capacity of lithium, it does hold some promise for fast efficient charge cycles (like stopping at a gas station to refill a gasoline car). The other advantage that I am sure many of you will appreciate is that it would have over 500k charge cycles. Currently they're not cheap by any stretch of the imagination. One thing from a post on e-bay. "The 16.2V is intended for 12V applicationa, as a 12V system usually 14V or more when charging. You wouldn't likely use it as a direct replacement for a deep cycle battery as the capacity is much less and the price is more. Farads are amp seconds per volt, so if you discharge a 1F cap 1 volt, you get 1 amp second, or 1/3600 amp hours." $430 for the 16.2V 500F (0.1388A equivalent if the guys post is correct). Who knows what future advances will happen though?