Yes, the battery can be changed. It must be soldered/unsoldered. The cost for a single battery is CHF 19.- all inclusive. If you would like to have the work done by our service, the cost is CHF 34.- all inclusive. To do this, you would have to send your device to the following address:
BBI-Züriwest
Waterpik Department
Hardturmstrasse 173
8005 Zurich
With the necessary technical knowledge, you can change the battery, unscrew the cover at the bottom and pull out the inner toothbrush.
The individual batteries can also be obtained and soldered together.
My SR3000, which I bought in 2012, still has a usable battery.
A colleague of mine had to buy a new one after 2 years.
In general, the product is highly recommendable.