What do I do if I have a faulty battery and its under warranty?

You can log into www.festool.com.au send an email via https://www.festool.com.au/contact-us/ email your contact details including proof of purchase and a small video clip of the fault of the battery and a photo image of the manufacturing plate (email is the best way to contact us in order to provide the correct information requested).
Once we have all the information and it meets our criteria we will send you a replacement battery.