Background
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.
The questions
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.
Best Answer
Just after buying the Pencil, due to my use case detailed in point 2 of the side note in the question, I was getting the low battery message almost daily. Therefore, I searched the web for a way to get as close to a powered-off state in the Pencil as possible while it's not in use.
Removing Bluetooth pairing
Eventually I settled on going to Settings -> Bluetooth and unpairing the Pencil. When I needed to use it again, I just paired it again by taking out the cap and connecting its male Lightning connector to the female port in the iPad, then confirming the pairing when asked by the iPad.
I tested this and it seems to hold charge much better. I could go a day or two without using the Pencil, and when connecting it again, it had lost no charge at all, or at most a couple percent.
Need for periodic charge level checks
The main issue is that you have to remember to unpair it after use every single time. I realized my Pencil had died after a few weeks without using it, and it's not entirely impossible that I just forgot to unpair the Pencil at the time. The only solution I can think of, for now, to this issue is to add a reminder to check the Pencil's battery charge level every so often -- at least weekly, I'd say, and ideally daily if you don't find that too bothersome.
Charging tests
Also, I have performed charging tests with my replacement Apple Pencil, both sticking it directly into the iPad's Lightning port, as well as using the female-to-female Lightning adapter that comes in the Apple Pencil's box, which allows you to charge it from a regular USB-Lightning connector.
I performed two nearly full charges, from 5% back to 100%, using the female-to-female Lightning adapter connected to an authentic Apple USB-A to Lightning cable, connected to an authentic Apple wall charger (not sure if the 5 W or 12 W model though). Between the wall charger and the USB-A to Lightning cable I connected a USB ammeter. I verified the charging rate to be approximately 0.05 A (unfortunately I don't think this is a very accurate ammeter, so I wouldn't be surprised at all if the actual current deviated from this measured value by 10 or even 20 mA). Most importantly, the charging time in both cases was about 1 hour and 20 minutes, indicating a theoretical 0.75C charging rate, possibly closer to 0.8C due to inefficiencies. Either way, this is a charge rate that probably shouldn't degrade the battery and reduce its lifetime.
On the other hand, charging with the Pencil directly connected to the iPad's Lightning connector, also starting at 5% and ending at 100%, took only 20 minutes to charge. This indicates a minimum charging rate of 3C, probably more given that fast charge is even less efficient (I wouldn't be surprised if the actual rate is 4C). For energy-optimized cells such as LiCoO2, this is an extremely high rate that is sure to lead to battery degradation, as explained in the question.
Side note: there exist power-optimized cells with different chemistries such as LiMn2O4, and for these such high rates are not necessarily prejudicial. However, these batteries are heavier and larger than their LiCoO2 counterparts for the same capacity, so they're usually employed in specific applications such as power tools. Apple as expected doesn't disclose what chemistry they employ, but teardowns show that they go for LiCoO2 for most of their lineup, so it'd be slightly unusual for the Apple Pencil to deviate from this.
Recommended battery maintenance procedure
In closing, I would recommend the following procedure as an attempt to maximize the Apple Pencil's battery lifetime: