[SML] portable power pack

[jdunfee12 at yahoo.com]

 There are car jump-staring battery systems that provide a cigarette lighter output.  So, it would be viable to just use a 120V AC inverter with them. That way when one goes bad, you don't have to replace the other.  But, it depends upon how much current he needs, and for how long.  Lead-Acid needs charged or topped-off about every other month at  minimum.  Whereas Lithium's self-discharge rate is much less, so you can go at least 6 months, up to a year without having to top it off.
While it is easy to tap into the voltage available in a tool battery pack, keep in mind that Lithium battery packs require some electronics to monitor them during both charging and being used. Otherwise they are prone to some very bad failure modes.
Also, Lithium batteries should never be fully discharged, or they very quickly will loose their capacity.  Some batteries have internal electronics to cut off the battery when the voltage gets low, but this low-voltage limit is intended as a fail-safe.  It is still low enough to significantly reduce the life of the battery.  
Some battery systems have the battery management system internally, and some expect the tool to have it.  Some even have a two-way digital communications process that make hacking the battery for other uses much more difficult. The reason the Dewalt battery system is popular with the Amish is both because it is cheaper than some of hte other systems, and because it has the battery directly connected to the output.  But, the tool has to monitor voltage. Its internal circuitry is just used for charging.
As for getting 12 volts.  Most power tools have gone for higher voltages, typically 18v (note that the 20v systems are just marketing hype. They are the same batteries as 18v systems).  But, circuit boards are cheap, and readily available that convert to both higher and lower voltages. (search for buck-boost)
-Joe  
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