Hi there. I want to chip in my own two cents.
I'll be linking articles from Battery University.
From their own frontpage:
The information (from Battery University) is compiled from the manufacturer's specifications, test labs and crowdsourcing. The tutorials differ from scientific papers by taking average readings because lab tests cannot always be repeated in real life.
Battery University is sponsored by Cadex Electronics Inc., a technology provider in batteries, chargers and diagnostics systems.
Charging
Lithium ion batteries charge up to a certain peak voltage. At that peak voltage, current drops and the battery remains at that peak voltage till full charge. This stage of constant voltage is called the saturation charge. I will use the term saturation charge more below. (Charging Lithium Ion)
For my Pixel 6, this peak voltage is around 4.381V. This was measured during a typical charge cycle using the app Charge Monitor. The graph of the real-time voltage from the app clearly shows the moment my battery reaches the peak voltage and current falls, thus entering the saturation charge. This moment happens right at 85% of charge which is seen in the graph measured by Battery Monitor. I recommend downloading the app for your own learning.
From Battery University:
Li-ion does not need to be fully charged as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge because a high voltage stresses the battery. Choosing a lower voltage threshold or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime.
(Charging Lithium Ion)
Battery University states that the biggest factor that degrades battery life in such a scenario is voltage. A higher voltage stresses the battery more. Which is why the saturation charge stage (when the battery is at the peak voltage constantly) is the most stressful charging stage for the battery and should be avoided.
I assume this is why sources recommend unplugging at 85% as to avoid the saturation charge altogether. This percentage is backed up by the measurements of my own phone by the Battery Monitor app showing the peak voltage being reached at 85%.
About overcharging, is is practically impossible that overcharge a phone's battery as the hardware would cut off the charge once the battery is full.
Li-ion cannot absorb overcharge. When fully charged, the charge current must be cut off.
...
Once the charge is terminated, the battery voltage begins to drop. This eases the voltage stress.
(Charging Lithium Ion)
Once fully charged, the battery voltages lowers, lowering the stress on it.
Because phones continuously drain battery, a topping charge is applied to keep the phone at max capacity when plugged in. Once the phone's battery voltage drops below a certain level, a topping charge is applied till the battery reaches its peak voltage again. So although the voltage of the battery is lowered after reaching full charge, the occasional topping charge still pushes the battery to peak voltage, stressing the battery. (Charging Lithium Ion)
Therefore is it recommended by Battery University to unplug once fully charged to avoid putting the phone's battery at peak voltage occasionally.
It is also advised to keep your phone cool when charging (or even typical operation).
Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage
(How to Prolong Lithium Based Batteries)
Here are my recommendations regarding charging:
- Avoid high temperatures.
- Avoid scenerios that put the battery at peak voltage such as saturation charge and topping charge. The easy way to do this is to unplug at 85% to avoid the saturation charge and topping charge altogether.
- Charge more often instead of charging from near empty to full. This will be explained further under Discharging.
Discharging
Take your time to read this quote from Battery University and it's okay if you don't understand immediately. I will give examples below.
Similar to a mechanical device that wears out faster with heavy use, the depth of discharge (DoD) determines the cycle count of the battery. The smaller the discharge (low DoD), the longer the battery will last. If at all possible, avoid full discharges and charge the battery more often between uses. Partial discharge on Li-ion is fine. There is no memory and the battery does not need periodic full discharge cycles to prolong life.
(How to Prolong Lithium Based Batteries)
The depth of discharge (DoD) is the difference between two level of charges. For example, discharging from 100% to 50% would be a DoD of 50%. Whereas a discharge from 75% to 50% would be a DoD of 25%. Keep in mind a discharge from 100% to 75% is also a DoD of 25% but affect battery longevity differently than 75% to 50%. This will be explained below.
In the Battery University test, batteries that start out at the same level but with a higher DoD degrade quicker than batteries with a lower DoD.
Case 1: 75–65% SoC offers longest cycle life ...
Case 2: 75–25% SoC has 3,000 cycles ...
So draining from 75% to 65% (10% DoD) degrades battery life less than going from 75% to 25% (50% DoD).
However, discharging from 100% to 50% (50% DoD) degrades the battery more than discharging from 75% to 25% (same 50% DoD). (How to Prolong Lithium Based Batteries)
From Battery University's testing it appears that the same DoD but with a higher starting voltage (linked to higher charge percentage) degrades the battery more than the same DoD at a lower starting voltage.
So my takeway on how to improve battery health when discharging is:
- Charge my phone's battery to a lower percentage.
- Discharge my phone less between charges and charge more often to lower the DoD between charges.
This fits well with what I've learnt about charging. By charging up to 85% and unplugging, I am keeping my phone's battery away from its peak voltage during saturation charge and topping charge. By starting my discharge at 85% instead of 100%, it preserves my battery more at the same DoD.
Final Recap
- Voltage and temperature degrade battery life.
- Charge up till 85% to avoid saturation charge and topping charge that push the battery to its peak voltage.
- Don't charge overnight to avoid topping charge occasionally pushing your battery to peak voltage.
- A higher DoD degrades battery life more than a lower DoD. So charge more often instead of charging from empty to full.