When you Cozmo or Vector battery stops holding a charge it is time to replace them. The typical 503040 600 mAh 3.7v lipo is always recomended but it is not the best choice for both robots.

For Vector get the 503040 600 mAh 3.7V for ~ 45mins of play time
For Cozmo get a 603035 600 mAh 3.7V for ~ 1hr 30mins of play time

I have these for sale on my shop

https://techshop82.com/shop/

You can also get them much cheaper on AliExpress, just have to wait for shipping.