I think my battery is starting to fail?

Hi @susan44228

Here's the ifixit Sostituzione schermo Kobo Aura HD guide that will help you to gain access to the battery so it can be removed/replaced.

The battery is glued in place as can be seen in this video for a same brand, different model tablet. (go to 5:00 minutes → into the video to see what they did to remove it).

A better option is to get a syringe and fill it with Isopropyl Alcohol 99%+ (available from electronics parts stores) and run a line of alcohol along the edges of the battery to help soften the glue then gently prise the battery up a bit with a pry tool (examples only) and then add some more alcohol lifting the battery more each time etc, until it can finally be removed. Clean off any residual glue and then perhaps use appropriate thickness i.e. thin, double sided adhesive sheet or tape strips (you cut to size) to hold the new battery in place

Replacement batteries are available online. Just search for Kobo Aura HD battery to find suppliers that suit you best.

Remember try not to puncture the battery when removing it and to dispose of the old battery responsibly ;-)