You've probably heard of lithium batteries catching fire in small devices like phones...Well, the discovery of a safe, nontoxic lithium-ion solar battery chemistry (lithium iron phosphate or LiFePO4) has brought a new standard to the deep cycle lithium battery market. With this safer (and maintenance-free) LiFePO4 battery chemistry and high-quality manufacturing from companies like SimpliPhi and Relion, lithium batteries are becoming widely adopted for off-grid energy storage. Yes, they're more expensive, but consider their benefits:
Aside from the initial cost, the major disadvantages of lithium solar batteries are the same as other batteries - low performance at freezing temperatures, and damage if operated above maximum or below minimum parameters. SimpliPhi has implemented a real-time equalization circuit - the weaker cells are dynamically cared for so they're not over-stressed. This in combination with the solid-state protection circuit makes for highly reliable and effective off-grid energy storage.
Budget approximately $1.20/Wh for small systems, or as little as $1/Wh for large systems.