Apple Pencil battery capacity

The Apple Pencil Battery battery is tiny - as is its capacity (not officially published). The Apple Pencil runs at very lower power; the BTLE radio, being very short range, consumes very little energy.

You should always keep your second generation Apple Pencil docked with the host iPad whenever is it not being actively used - as doing so optimally charges and maintains the Pencil battery - in so doing protecting the tiny battery from damage.

The tiny internal Li-ion battery is susceptible to permanent/irreversible damage through being left discharged for long periods. Even some “new” pencils can exhibit signs of failure out-of-the-box if they are “old stock”. 

It is essential that if you have an Apple Pencil that you charge it regularly - whether used or not - so as to protect the battery from deep-discharge. Do not allow a pencil to remain in low-charge state for any period of time - as the internal battery will fail, rendering the Pencil useless. Setting aside an unused Pencil, for extended periods, is a recipe for premature death of the Pencil battery.