Vantage Vue battery issues

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I've had the Davis Vantage Vue for about 5 years now. 

Never had any issues with it, it's been well maintained and survived 2 house moves! 

Noticed a couple of weeks ago one of the wind cups had snapped off. I've brought it off the roof, done the usual maintenance, replaced the cups and added new C123 lithium battery. 

Since changing the battery I've struggled to get any signal. When the  console receives data it's ridiculous values (68c temp etc).

The station is transmitting on 1. Now here's what I cant get my head around...  

The station transmits for a few seconds AFTER removing the backup battery, then it dies. 

I left the backup battery out of the transmitter last night, and put the station out again, no data received overnight. This morning its receiving data again, obviously the solar element is working fine. 

It all seems very backwards to my simple brain! Is it possible to have failure of the backup battery component? And I've used 3 different batteries so no issues with them! 

 

Any advice would be massively appreciated!