I looked at this same idea. I started with a different premise because my interest was in Electric Vehicles, and since most people don't drive more than an hour at a time to commute, I looked at the comparable 1 hour ratings of the Lead Acid and Lithium choices, and for them LiFePO4 Makes so much more sense, and IS Cheaper to buy - on the first cycle, as I calculated it out.
I am so convinced that they are better than every packaging format of Lead-Acid - I have begun to clarify my thoughts, using specific comparisons of exact battery models - and put them in whitepapers on my website. they can be found at www.myelectricfly.com -> Select Whitepapers in the menu, and then read those that I have posted. There are two so far, in pdf format. the first is a nice single page letter-size, double-sided fit, and the second is four pages, two sheets letter size, each double sided.
Easy reading, and should be of some use in making choices - even in the Renewable Energy Field.
However - with all that said, I have never seen any specs on what a LiFePO4 (Lithium Iron Phosphate) Cell can do in a 20 hour load test similar to how Lead Acid Batteries are done. THAT would be interesting - to see if they can deliver any appreciable larger capacity in the slow drain approach - as is used in Solar Energy and Wind Energy Storage off-grid.
Another However - I also see Renewable Energy/Solar Energy Batteries (Lead Acid) Marketed with a 100 hour rating (0.01C) - and here is where I see that counting: You have no wind, or no appreciable sun for four days, how much energy can you use over that 4 days from your batteries without starting up your backup generator? These type of batteries don't usually look like car batteries, are usually 6V, 4V, or even single cell at 2V types, and very heavy.
So - while I know that for an EV (Electric Vehicle) the LiFePO4 is already a winner on price in terms of direct usable energy on the first cycle, I have no way to know if that is true for Renewable Energy Storage in slow draw applications, where how much they weigh only counts during carrying them into place, and does not affect the daily energy use as it would in a moving vehicle.
However - I do believe the life of the Lead Acid Battery has to be considered, so the usual deep cycle capacity of 300 - 400 cycles alone, could be the primary difference between them and a Lithium Iron Phosphate Cell Set that has a 1500 Cycle Capacity at the same 100% DOD (Depth of Discharge).
Here is the thing - it is hard to find fully supported data for Lead Acid Batteries that cover more than the 20 hour and 5 hour ratings for them. To get a full range down to 1 hour is a long and slow hunt to find spec sheets. I know of just a few mfg'rs that provide such detailed data, and in the LiFePO4 Side - similarly - for the Renewable energy use, most just list the ratings at 1 hour - excellent in the EV world, but not quite as critical for the usual off-grid home, where you want to run for a day or so without input energy, hence the reason for the storage in the first place.
My 9 Cents worth!
Robert
PS Check out www.evsociety.ca for more EV information, and if you want to catch my various thoughts on the fly - join the google groups list there!
