Disclaimer: I've had to deal with batteries a lot in my career so I'm familiar with their complex nature. I am not an expert though, so I'm sure there are a million more confounding variables that complicate things further.
> What I don't understand is that most other people seem to treat the percentages as meaningful. They certainly aren't based on "time until a recharge is needed", which is what people want. What are we notionally seeing percentages of?
So yes it kind of is based on "time until recharge". The problem is batteries are more complicated than that. I think a lot of people tend to view batteries as like a power supply that has a certain capacity you consume. This isn't an accurate view.
First capacity isn't static, it depends on temperature. Lithium batteries generally have thermal cut-off circuity so temperature compensation is pretty standard to add in.
Second batteries have internal resistances which affects its behavior. Internal resistances naturally rise as batteries age, or more accurately go through recharge cycles. This changes voltage sag behavior as GP noted.
Third, as GP mentioned, batteries don't supply a constant voltage. A lot of people realize battery voltage drops as a battery is used, which is true in a very simple sense. A cell can have a fully charged open circuit voltage for 4.2, but it quickly goes down to 3.7 where it spends most of its cycle around. Note that I specified an open circuit voltage. The second you apply a load the voltage will drop in relation to the load applied. This voltage sag and the subsequent recovery is not instantaneous either, it takes time to reach equilibrium states.
So in essence a battery percentage is simplistically portraying the capacity remaining in a battery, but isn't - nor can't - portray the fact that some of that capacity is unusable depending on loading conditions combined with safety cut-offs to protect the battery. Apple's solution was to change the load so there was less unusable capacity.
I'd like to re-iterate that I'm not an expert here. Batteries are complicated electrochemical devices.
Edit - Let me illustrate a hypothetical analogy. Let's say you have a car with a fuel tank and a poorly designed fuel pump. You fill up the tank with ten gallons, and start driving. The fuel pump heats up as you press on the accelerator, the harder you accelerate the more it heats up. However the fuel acts as a heat sink for the fuel pump. After driving for a while you have two gallons left. At this point you go full throttle and suddenly the engine cuts out because the thermal protection circuitry on the fuel pump kicked in. Your fuel gauge still reads two gallons, the problem is you simply can't consume it at full throttle because the pump won't operate under those conditions. You can still drive at maybe half throttle though.
> What I don't understand is that most other people seem to treat the percentages as meaningful. They certainly aren't based on "time until a recharge is needed", which is what people want. What are we notionally seeing percentages of?
So yes it kind of is based on "time until recharge". The problem is batteries are more complicated than that. I think a lot of people tend to view batteries as like a power supply that has a certain capacity you consume. This isn't an accurate view.
First capacity isn't static, it depends on temperature. Lithium batteries generally have thermal cut-off circuity so temperature compensation is pretty standard to add in.
Second batteries have internal resistances which affects its behavior. Internal resistances naturally rise as batteries age, or more accurately go through recharge cycles. This changes voltage sag behavior as GP noted.
Third, as GP mentioned, batteries don't supply a constant voltage. A lot of people realize battery voltage drops as a battery is used, which is true in a very simple sense. A cell can have a fully charged open circuit voltage for 4.2, but it quickly goes down to 3.7 where it spends most of its cycle around. Note that I specified an open circuit voltage. The second you apply a load the voltage will drop in relation to the load applied. This voltage sag and the subsequent recovery is not instantaneous either, it takes time to reach equilibrium states.
So in essence a battery percentage is simplistically portraying the capacity remaining in a battery, but isn't - nor can't - portray the fact that some of that capacity is unusable depending on loading conditions combined with safety cut-offs to protect the battery. Apple's solution was to change the load so there was less unusable capacity.
I'd like to re-iterate that I'm not an expert here. Batteries are complicated electrochemical devices.
Edit - Let me illustrate a hypothetical analogy. Let's say you have a car with a fuel tank and a poorly designed fuel pump. You fill up the tank with ten gallons, and start driving. The fuel pump heats up as you press on the accelerator, the harder you accelerate the more it heats up. However the fuel acts as a heat sink for the fuel pump. After driving for a while you have two gallons left. At this point you go full throttle and suddenly the engine cuts out because the thermal protection circuitry on the fuel pump kicked in. Your fuel gauge still reads two gallons, the problem is you simply can't consume it at full throttle because the pump won't operate under those conditions. You can still drive at maybe half throttle though.