It used to be that LiPO4 batteries was for racing nuts only who shaved every ounce off their vehicles. However now it seems like it is coming down in price to not much more than an AGM:

12V, 30Ah, 2000 cycles at 70% DOD $265
Weight 4kg !!
http://www.evcomponents.com/ProductDetails.asp?ProductCode=H1230

at 70% this gives 21 usable Ah
by comparison a Lead Acid should not be taken beyond 50%, giving the
typical group 41 50Ah battery about 25 usable Ah in deep cycle service. 

Comparing the 2000 cycles to the typical 200-300 of a Lead Acid
the LiPO4 battery is actually cheaper over the long run. 

Now I'd wouldn't run out and buy one right away. While the cell technology is mature by now, there's electronics inside these batteries to control charging, discharging currents and maintain cell voltage balance. I'd like to see how this electronics hold up in automobile service first.

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