I’ve always associated QRP operations with overcoming challenges. Because of this, I don’t really consider the nominal 12V maximum voltage limit to be insurmountable. Of course, being able to plug into a 13.8V DC battery or other power source would be handy in some circumstances, but I also take into account the typical size and weight of such power sources. What I’ve done is purchase two AA 8-cell battery holders with 9V snap connectors. These are inexpensive and they mate well with the 9V optional power connector of my MountainTopper rig. I use 8 AA NiMh rechargeable cells in series, which produces a fully charged voltage of 11.2V that tapers down to a long and steady 9.6V until replacement with a fresh set of batteries is needed. At 2600 mAh, this is a very good and comparatively light weight power source for the MTR-3B that keeps voltage under the 12V recommended maximum.
73,
K7SFA