For this project I want to avoid any form of on-board computer. I'm using a tiny lite-weight GPS transponder I ordered from China.
Battery life seems incredible. After more than 2 days, both units I am testing still have 75% battery. The Li-Ion batteries are not good at low temperature according to some folks, but with good insulation I think they will be ok, especially for a "short" up and down flight.
My only concern is that one of the units seem less capable at picking up GPS than the other. This is evident by looking at the time stamp returned. I don't want false reporting. More testing 'outside' required.
Another issue to worry about in this type of flight is if the balloon lands in a mobile signal dead zone. To counteract this I will fit two different SIMs for two different mobile networks. Will this be enough to guarantee a signal?