eli5: why can iphones be so quick to charge to 80% and then slow down, whilst apple watch charges quickly and doesn’t havw to slow down
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Say you have an empty jug, and you go to fill it up in your sink. You turn on the faucet full blast, and that fills it up pretty quickly up until you hit 80% or so. But now if you keep the water going at its maximum pressure, the water from the faucet is causing the water already in the jug to slosh around and spill out. You need to slow things down in order to fill it up the rest of the way.
Similarly, the chemistry of a rechargeable battery is generally fine charging at a high current (subject to other factors like heat), but once the battery cells are mostly charged, cramming power into them at the same rate damages them. This would result in a significant drop off in number of recharge cycles. So charging tapers off until the battery reports its at peak charge.
Software. The iPhone and the Apple watch have battery management systems. These are what actually controls how the battery is charged. What you plug into the wall is just a power converter.
Apple has programmed that behavior. If the iPhone and watch use lithium batteries with different chemistry this changes their charging profile and best practices to maintain battery health. Normally with lithium batteries they don’t like being fully charged or fully discharged. Chargers will slow down the charge rate as they fill up to reduce heat and maintain battery health. I believe Apple does have some restrictions on charging the watch and EarPods as I get notifications saying that charging is scheduled to complete some time in the morning. The device only charges to ~80% and then waits until just before you need the device to finish charging.
You can see this with EVs. Basic rule of thumb is 20-80. Don’t let the battery fall below 20% and don’t charge above 80% for day to day use. For road trips you can safely charge to 100% and use fast chargers as the battery is designed to handle the extra stress in moderation.
However again specific battery chemistries can change this behavior. In new base Model 3’s from Tesla they have changed the battery chemistry. Now those batteries are fine being constantly charged to 100%. The trade-off was less energy density which is overcome by adding a few more cells. It also might charge a bit slower or something.
To be fair, I am not sure as to whether the iPhone charges to 80% capacity or 100% capacity.
Lithium batteries have two primary methods of charging–constant current or constant voltage (there is actually a third for deeply discharged batteries that we will ignore).
Up to about 80% capacity, lithium batteries are charged by constant current. The battery will be charged at its or the chargers maximum current (which ever is lower).
Once you reach 80% capacity the charging circuitry will change to a constant voltage–the voltage across the batteries terminals is held at the terminal voltage (typically 4.2 or 4.3V for single cell lithium batteries). The voltage of the actual cell is less than this voltage so a current flows into the battery. This current increases the cell voltage until the cell voltage equals the applied voltage. The battery has an certain internal resistance, so thanks to ohms law the current must decrease as the cell voltage approaches the applied voltage. This is why it takes longer.
As a general rule, the first 80% will take about as long as the remaining 20%.
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