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The nominal voltage of the type of battery that A123 produces are LiFePO4 batteries. The nominal voltage is 3.2 volts, not 3.7(lithium polymer and lithium cobalt have the higher 3.7 and 3.6v nominal, which is a bit higher than what we can use for automotive retrofit). They are fully charged when you bring them to 3.6 volts. They seem to tolerate anything up to about 3.85v but there is no reason to bring them that high. When charged most LiFePO4 settles to 3.4v or a little lower so you will be at about 13.6 when parked. Draw some load for awhile and they are basically around 3.2v for almost the entire discharge, lower is discharging at a heavy amperage. After they are below 3v without much of a load their voltage drops off a cliff quickly. Just for the basic understanding of these cells.
A123 may be a decent option but IMHO, expensive and hard to get.
A better bet would be to get Headway cells, cheaper and 4 of them in series will start pretty much any car or motorcycle. 10Ah each, just parallel them to get the capacity you need. The last I checked I think you can get them for about $18 each so each paralleled 12.8v 10Ah set would be $72 plus shipping. Most of the people selling headway cells have little plastic frames you can buy to hold the cells together along with busbars that can be bought to connect them in the series/parallel configuration you need. Your Focus probably has a 40Ah or 50Ah battery in it.
The tough part about this is to figure out the capacity you need. Do you ever drive across the country or even the state? If so, do you have a plan for that?
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