Something I should have mentioned earlier:
One big caution for anyone else tempted to experiment with these Li-Ion cells. The metal can the cell is sealed in is ferromagnetic which means, as opposed to LiPo cells, even when no current is being drawn, they can interfere with the operation of the compass which is critical to correct operation in loiter mode. Some cells are worse than others. I have been using a powerful degausser designed for bulk erasure of magnetic tape with mixed results. Be sure to check for accurate compass operation before using GPS based modes.
–Brad