apple pencil 1

Apple Pencils are not serviceable. If the battery has failed, the Pencil itself must be replaced.

If your Pencil is within its one-year limited warranty, and otherwise undamaged, you should be able to have it replaced under warranty.

Genius Bar Reservation and Apple Support Options - Apple

Otherwise, repair options and costs are outlined here:

iPad Service & Repair - Apple Support

Apple Pencils are completely sealed up devices.

Trying to open up any Apple Pencil to do ANY type of internal repair, basically, completely destroys the Apple Pencil

Your ONLY option is to purchase another new Apple Pencil

OR

If the Pencil is still under its free, one-year warranty, get another free replacement Pencil from Apple.

You can use the “Get Support” link, at the upper right hand corner of every Apple Support Community Webpage to attempt to schedule an appointment to visit your nearest Apple Store location to purchase or replace your Pencil under warranty.

Apple Store employees will be glad to help/assist you.

Best of Luck to You!