Newb needing a new traction battery, help!

Hello, lurking for quite a while, 2007 prius with 70k miles. Purchased from my elderly neighbor who bought new and rarely drove. I have been driving regularly for a year with zero problems. Now that the weather is colder I have been getting p3000 90a80 codes at every drive. Watching pack voltage with the Dr. Prius app it looks like #5 may be dead as there is a voltage difference of .89 volts in this pack. 12v battery is holding strong at 14v. I plan on keeping the car as long as possible and have been looking into the project lithium nexpower upgrade. Does anyone have any coupon codes? Thank you!