For the cost of one or two packs for your plane, one of these power packs can
recharge you packs by running your DC charger. Yu can carry them on a bike,
or the lighter ones
could be carried in a back-pack with the rest of your field equipment.

They are simple to use. It is just like using your car's cig lighter plug or
clips on the car's battery. You attach a 12 volt DC charger and charge your
batteries at the field. They can also
be used at the house so you don't need a DC and an AC charger. I use one in my
shop for that purpose.

Examples
http://www.nextag.com/jump-starter/search-html

You can also use one of these gel pack batteries that normally go in a field
box. If your charger has clips, you are set. If not, you pick up a car power
outlet extender or doubler that has a wire on it and add clips.
http://www2.towerhobbies.com/cgi-bin...&I=LXL370&P=ML
You will need a charger. This is an example
http://www2.towerhobbies.com/cgi-bin...?&I=LXL353&P=V