I own an iPad Pro 10.5" and a first-generation Apple Pencil for it.
The Pencil was bought in March 2018. In January 2019, i.e. about 9.5 months later, it had a battery failure and had to be replaced under warranty by Apple. This is not an isolated issue. I'm not looking forward to another failure in less than a year again, except it'll be out of warranty this time, so I'll have to pay out of pocket for a repair next time.
It's well known that batteries are consumables, and it's not expected that they'll last forever. On the other hand, to see Pencils failing (such as my own) before the warranty expires suggests that there is a design flaw in the Pencil.
Side note, feel free to skip
As an electrical engineer with experience designing battery-powered devices, I have a theory regarding this design flaw:
- The Pencil's battery is very small (0.332 Wh, or about 90 mAh).
- Point 1 by itself is not a problem, except the Pencil is an always-on device. Its internal accelerometer senses when it's moved, and it pairs to the iPad. If, like me, you throw the iPad and the Pencil in a backpack which you take around with you all day, it's going to be waking up quite often, and draining the battery in the process. I don't care what Jony Ive says, it really needed an on-off switch.
- When a Li-Ion battery is drained beyond a certain threshold, it isn't safe to try to recharge it. Most battery protection ICs include undervoltage protection and turn off the charge MOSFET in series with the battery when this condition is detected.
- When the Pencil reaches a low battery state (say at 5%), I assume it enters a very low power mode, disconnecting from the iPad, to save battery (if it doesn't, and the user doesn't immediately start recharging the battery, it will be irrecoverably dead in a few hours as per point 3).
- However, even in a low power mode, there is some current drain from the battery -- unless you use a relay to cut power to the circuit, which Apple certainly didn't use in the Pencil. A good design might have a leakage current budget of about 10 µA. Unfortunately, going back to point 1, and considering that 5% of 90 mAh is 4.5 mAh (assuming it still holds 90 mAh of charge; over time it will be less, so adjust the figures downward correspondingly), a 10 µA drain would completely drain the battery after 450 hours, or about 19 days (less than 3 weeks).
As a consequence of point 5, if you leave your iPad unattended for a few weeks, you may come back to a dead Pencil. Apple realized this, which is why they made it easier to dock the 2nd generation Pencil to the iPad Pro, which keeps it permanently charged.
Another, completely unrelated issue, but which may also play a part: Apple implemented a fast charging protocol, since it only takes about 15-20 minutes to fully charge the Pencil starting from a discharged state, which suggests a C-rate of about 3C to 4C. This paper shows noticeably reduced battery cycle life at 1.2C and 1.4C compared to 1C, so extrapolating to 3C or 4C rates, I'm frankly surprised the battery lasts more than a few dozen cycles.
It is my theory that these two issues are responsible for most battery-related Apple Pencil failures.
All signs point to the Apple Pencil requiring constant attention to avoid killing the battery. On the other hand, I haven't found any reports of newly-bought Pencils being dead, so clearly they can stay on store shelves for weeks, possibly months, without the battery dying (although it surely helps that they're sitting still on the shelf before being sold).
What measures can I take to ensure the maximum battery longevity of a Pencil?
In particular, can I revert the Pencil to a factory reset state, in which the battery drain is very low?
Can I "slow"-charge the Pencil (say at 1C rate, therefore not slow at all) so as to prevent the hypothesized deterioration of the battery due to the default fast charge protocol?
Are there apps to remind me to do this if I leave the Pencil paired for more than a few minutes without using it?
Note: I will leave an answer describing the measures I took to try to prolong the lifetime of my current Pencil, although clearly they weren't enough, seeing as it died after 9.5 months. I'm looking for other measures to take to make sure the replacement lasts longer.