I bought one of these recently and the display started to say batt?
So I read the manual and obviously it said change the battery.
Now because my watch is out of warranty I was not going to take it to a local watch shop.
I did however ask my local shop and they sad it would have to be sent to Tissot for a charge of £45 and would take approx 2 weeks?
I did several searches on the web to no avail
So here we go
NOTE! I am not liable for any damage done to anyones watch.
This is just how I did it, please do this at your own risk.
First I put the watch into sleep mode.
This is done by Activating the Crystal and navigating to the opt display.
Then press the T button.
Now tap the centre of the crystal until the word sleep displays and wait approx 10sec.
Both the hour and second hands should move to the 12 o'clock positions and the display will go blank.
The watch can be taken out of this mode by pressing the T button again.
Now remove the 4 flat head screws carefully. I did this in a criss cross manner.
Now carefully lift the backplate to one side.
This is what you should see.
Now again lift this plate off very carefully making sure NOT! to touch the sensor "the circle on this plate"
as you can see on the back of the inside plate there is some sort of circuit board. DON'T TOUCH IT!
Now you should see this.
My watch had what I think was the original CR2032 Renata battery.
Carefully remove the very small flat head screw holding the battery securing plate in place and put somewhere safe.
Now you can carefully prise the battery out from approx the 4 o'clock position where you can see a small recess.
I have replaced it with a Duracell for the time being.
I have ordered a direct replacement and when this arrives i'll repeat this process.
At this point you could very carefully apply some silicone to the black seal to help waterproofing.
I have not done this yet as i will be replacing the battery again very soon.
I suppose you could even go as far as replacing this seal?
I don't use my watch in water so i won't be doing this.
Now carefully place the internal backplate in the correctly aligned position followed by the external backplate.
Now put the 4 screws back in place again being very careful not to cross thread them.
Now press the T Button and your watch should come back on.
You may have to re-set the time, date and possibly sync the analogue hands to the digital display.
I hope this helps.
I am by no means an expert.
If anyone has any comments i'm more than happy to amend what i have written.