O, I didn’t measure the two inside pins, only two outer pins. I think the two center ones are used to communicate (send data). My pokit pro (Pokit PRO | All-in-one multimeter, oscilloscope and logger by Pokit Innovations — Kickstarter) is being delayed, otherwise I might perhaps capture the signals.
That’s exactly what my charger does, power the outer pins, so it charges the battery inside the tempest.
If you want to extend the capacity of the battery, perhaps it is better to wait for weatherflow’s powerbooster, or just open the unit up and think of adding the same kind of battery that’s inside and connect it in parallel. There is room inside, not sure how to keep it water proof.
If you want to add an external charging battery, that battery has to be connected through some charging circuit. so the power has to be able to go into the charging unit when the sun is shining and come out of the charging circuit when the battery of the tempest is low. You have only two connections. I don’t know how to, but it might be obvious, just didn’t think about too much.