I think it can be helpful to share my experience with the "Low Battery Transmiter 1" error.
I bought a Vantage Vue weather station in 2011.
Since 2012 I noticed The Low Battery error showing sometimes mainly for no apparent reason, (some 2 or 3 times per month).
Almost all the times the error went off by its own and no data was lost during the nigths just after the error shows.
That can be checked in outside temperature/humidity graphs.
If data is lost for some hours during the night that seems to be a confirmation of real ISS low battery.
But this year begining in March I got the Low Battery warning and lost data during the nights.
Then I replaced the old battery with a new one (May 16).
But the error keept showing and data was lost during the nights with the new battery.
Even without the Low Battery error during the day my weather station was missing data for some hours during the nights.
Because of that I started thinking of replacing the battery, the transmitter, the supercap or the complete ISS.
Then as last DIY fix I decided to replace the battery with a specialy prepaired one.
At the positive battery terminal I soldered a small pointed golden pin so the contact with the positive battery hold terminal would
have a very small surface contact with a higher pressure and so a better contact quality would be achieved. ( the replaced battery
was showing a slight oxidation at the positive terminal).
Besides that I used a very fine used sandpaper, isopropyl alcohol, and a cotton swab to clean the ISS battery holder contacts.
It has been more than two months now since that fix. The weather station has been running flawlessly since.
My conclusion is: the most fragile elements in the Vantage Vue weather station are the contacts between parts and or cables not soldered.
It seems dust, moisture, grease, polution or whatever gets in between the metalic parts and spoil the conductivity need for the proper circuit perfomance.
( I had a similar malfunction with Low Console Batteries warning: the console printed circuit golden tabs were green because of oxidation,
the solution was wire soldering )