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TPG provides 2 year warranty for the NBN Back up battery from the day the NBN Battery was initially installed. I tried to use your community details to pull up the account unfortunately no match. I'd like to get your account details (Username/Customer ID or Contact number associated).
In case you need a reference: How do I private message (PM) in the community
Thanks for sending us your details. Based on NBN Co System the Back up battery was initially installed 10/07/2015. Although the battery is over two years, the status indicates it might still work. Is everything plugged in properly? Else, replacement is really needed. As mentioned, the battery is out of warranty so you are no longer eligible for the reimbursement.
You can visit this link to get details about NBN Battery Replacement
Let me know should you require further assistance.