I found this comment on this very interesting review of the Apple watch by hodinkee.
No one talked about self charging, so these rough calculations will give you an idea why its not possible.
By Eugene Kim G Y on hodinkee
With the amount of charge a smart watch would have to hold, it would require some serious “self charging” to keep it going.
Here is some very rough napkin math that might not even make sense but let’s try since I’m curious.
A normal automatic watch requires about 6 hours of activity to keep going? 6 hours is a lot (for a desk worker), but so is the recommended 3000 steps per day so let’s say they are roughly equal. A watch running on a button cell will last 2 years at roughly 300 mAh, which equals to 730 days of 3000 steps or 2.19 million steps. Again, it’s probably not correct to say that they are equal, but let’s say they are for the sake of napkin math. The newly released Moto 360 also has a 300mAh battery, but goes through the battery in a couple hours but let’s say one day to be fair. In order to keep a 300mAh battery charged daily, it would require 2.19 million steps per day using a normal automatic winding mechanism.
If Apple created a mechanism 10 times as efficient as a normal automatic watch wind, that would still be 219,000 steps per day. Let’s say you still need to charge it, and the self charging mechanism just helps prolong your battery life. A 3 day battery life will require 657,000 steps but 219,000 of those steps will already be accounted for due to the full battery. So over three days, 438,000 steps need to be accounted for. which is a little better, but not by much. In reality, with a 10 times more efficient self-charging mechanism, a full day of activity (6 hours of active motion) will only charge your battery by about 1.5%, or about 7 minutes of battery life.
Maybe I should have gone with solar?
Now go on and comment about how ridiculous my calculations are, it was just for fun anyways 😉
I haven’t asked Eugene Kim G Y permission, but hope he doesn’t mind it. Very interesting…