I am involved in a project on gpslogging of free-ranging animals. In this project, we need gpsloggers to be able to run without any maintenance for about one month, whereas those I have found so far have an internal battery to run for about 24 hours. My initial idea was to just use a standard USB battery bank, but they seems to be "too smart" for what I need. Those I have tried so far, turn them self off when the current drawn is too low. So I think I need to build or modify something myself as the GPSs I've tried so far, draws current until their internal battery is charged and then the battery bank will turn it self off and not be available next time it is needed.
As far as I have understood from googling around a bit, the standard way of using LiPO batteries to get 5V is to connect cells in parallel to get the capacity that is needed and then using a step-up circuit to get 5V. As I want as high as possible energy/volume ratio, would it be better to use two cells in series and the regulate the voltage down? I know LiPO are quite sensitive beasts, so then I would also need the appropriate charging circuit. (although, in this scenario, it would not be impossible to reconnect the cells when charging)
Any advices on how to make this battery backup system? Should I look into some other types of chemistry for the cells? I need 10-20Ah total capacity, low current draw over long time and a good power/volume ratio. Also, if I keep using LiPO, are there any specific type of cells that are recommended? Should I use 20Ah cells or would it be better to use several smaller cells in parallel?