Oster Artisan Model 76997-310 Cord/cordless Charger & Shaver

The unit came in a box of misc items that I purchased.  I tried to charge it up, but it won't hold a charge.  Once it's been placed on the charger, the motor will run, but only briefly.  I believe the unit will likely need to have it's battery replaced, but I'm not sure how to do that.  The charger and shaver both have some spots, debris and scratches, but appear to be in good structual condtion. The items are being sold for parts/repair only, and ship with USPS.  Thanks for looking!