Here is a video on manually replacing the batteries.
You will first need to purchase 2 CR2032 batteries, they are commonly found at most stores.
Remove the sensor stick from the bottle lid.
Unscrew the lid from the the sensor stick.
Remove the battery pack.
Remove the rubber ring securing the battery pack.
There will be a small metal clip that is creating a contact between the batteries within the pack. Carefully remove that clip.
Then remove the batteries from the plastic pack using a tweezers and dispose of them appropriately.
Replace with the new batteries.
Replace the small metal clip to create a contact between the batteries.
Then make sure that you insert the battery pack the correct way within the lid. The side that has two metal contacts is the bottom side that will come in contact with the metal rings on the stick portion.
Then screw down the cap keeping the battery pack in place. Make sure to screw it down as tight as possible to ensure a water tight seal.
The sensor stick will glow if you have inserted the battery pack the correct way. If it doesn’t glow then try inserting the battery pack the other way.
Re attach the sensor stick to the lid and you’re good to go.