In reply to LA1KHA:
This is how I have been operating ATS3B though I use 8 x AA alkaline or lithium cells. I do change the batteries after the voltage becomes too low. Typically the first activation with fresh cells has power 4 - 5 W and at the end I am approaching 1 - 2 W. I did not dear to try QRPp though. Here is my incomplete list.
ATS3B POWER OUTPUT DURING SOTA ACTIVATIONS BY F5VGL
The 8 AA alkaline cells were already used a little as spare batteries before
starting to use them for the SOTA activations. The total number of activations
is 7 with total number of 187 QSOs.
The power was measured before the activation to DL1 dummy load. Thus the
actual power during the activation was less than the value shown in the table (not so easy to read).
DATE SUMMIT BAT[V] PWRW QSOs(40m+30m+20m)
Aug 3 F/AB-324 11.5 3.77 43
Aug 24 F/AB-297 11.2 3.62 26
Oct 7 F/JU-002 10.4 2.86 18
Oct 26 F/AB-293 10.2 2.5 30
Nov 9 F/AB-321 10.2 2.5 13
Jan 11 F/AB-401 9.8 2.0 37
new alkaline cells
Jan 17 F/AB-409 12.5 4.5 20
In fact you can operate even without batteries using a solar cell. It is quite funny to hear the RX from ATS3B just connecting it to the panel. The TX output power is very low however unless you are in direct sun light.
73, Jaakko OH7BF/F5VGL