Quick update: battery has been replaced with other, more reliable and I was able to test it today.
Long story short - the unit didn't work as well as I'd expected: it was perfectly fine during the climb, showing both ground speed and distance, but during the jump when altitude drops quickly it just stops processing all the readings. Under the canopy it works again, but the moment you do any kind of dive it stops reading again for a second.
Well, it wasn't designed for skydiving after all, so nothing to complain about.
At least I was able to measure my ground speed under the canopy, so it could be used for some basic canopy piloting drills, but nothing more than that.
P.S. Turns out the plane travels in circles roughly 70 km at ~190km/h to climb to a 4k