Sorry if this has been covered.  Both Fred Underwood, who flies CLPA locally, and one of my RC club members, John, who flies competitive sailplanes when he's not at my RC field flying things made out of dollar-store foam, use this charging method.

Don't throw away those old battery packs!  Usually when LiPos go soft, it's because they don't like discharging at high rates -- they may still have plenty of capacity if you're gentle with the discharge.  Fred and John both take questionable packs that are of the same cell count, hook them up in parallel, and use them for charging at the field.  It's a good way to get use out of packs that you can't trust in the air any more, and for the energy stored, it's a lot more portable than lead-acid cells.

Note that you only ever want to hook up packs with the same number of cells!!.  Hooking up packs with differing cell count would be a Bad Thing -- at best it would curdle the magic smoke inside the packs, and at worst the magic smoke would come out so vigorously that it caught on fire.

Here's a picture I took at my RC club field of John's setup (note the box built out of dollar-store foam -- that goes into an ammo box for a bit of fire protection).  I didn't get a chance to take a picture of Fred's setup.

I thought this might be cool.