Battery function in settings even tells you how long it has been running on the battery since last charge, I think, which makes it easier to determine battery life. (blog.the-ebook-reader.com)
Open account: 5 years from last charge, payment, or admission of debt in writing. (fair-debt-collection.com)
NOTE: Payments made toward the obligation toll the statute and the time period will then run from the date of last payment or last charge by the debtor, whichever occurs later. (fair-debt-collection.com)