Its fine setting it higher, good quality packs often say maximum capacity = 4000mah for example, but will often take up to say 4400mah of charge or something. the 120% option is just taking into account that capacity ratings are often over or underrated or whatever.
GM360: I use a converted computer psu at home (doesnt allow max power output, but when I'm playing at home 15min lifepo4 charges arent really necessary) and a 12V car battery in the field. The update cables are available from a lot of places, you need two bits,
programmer, and
adaptor