Why are(n’t) rechargeable batteries damaged by partial charging? Originally asked in ElectricalEngineering.SE, but I was told Physics.SE might be a better place to ask.
Over the years, I've come across websites and people with different opinions on the "correct" way to charge rechargeable batteries (I'm more concerned with laptop, tablet and phone batteries than with rechargeable AA or AAA batteries, if that makes a difference).


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*Some (e.g. this one and some of my family members) claim that one should discharge all the way to 0% (or close to that) and charge all the way to 100% to prevent the battery from developing a "memory" (whatever that means; do they mean hysteresis?) and losing charge-storing capacity.

*Others (mainly Apple representatives and some of my family members) claim this was the case with older batteries only and newer batteries are "smarter" (whatever that means) than that and it's battery cycles that count now.

*Still others (e.g. this one, this one and this one) claim that the best way to charge a battery is to let it oscillate only between X% and Y% (with different values of X and Y depending on whom you ask, but all agree that X>0 and Y<100) because that somehow increases battery life by preventing overloading (no mention of what the lower limit is for).


None of those websites or people have ever explained why what they say is true is, in fact, true, so I thought I'd ask here.
I'm aware of how battery cycles work:

I'm also aware that battery life/health decreases over time and it's just a process that can't be helped because entropy.
My own experience with battery charging and health is varied (and I haven't performed nearly enough experiments to come up with a reasonable conclusion because I only have so much money to spend on electronics and so much time to spend draining and recharging batteries):


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*My laptop (which is from mid-2014) spends most of its time permanently plugged in and at 100% battery. If I ask it what its health is, it says it's at 82% health, it's completed 609 cycles and its actual charge is 96%... which probably means health is not a linear measure of the scaling of the 0%–100% range as it goes down compared to the original 0%–100% range, but some more complicated quantity. When I do unplug it, the battery lasts a good while (haven't measured it and certainly haven't compared it to what it originally lasted).

*My old phone (an iPhone 6, so it's as old as whenever that came out, which is more recent than 2014) was charged whenever I thought the battery level was lower than I wanted it to be and I knew I wouldn't be able to charge it for several hours, and it often (but not always) got all the way to 100% and then remained plugged in for hours. If I ask it about its battery health, it says the maximum capacity is 73% and "[the] battery is signicantly degraded". The battery now lasts very little and drains very quickly; the phone will turn off (and claim it's run out of battery) when the indicator is anywhere from 1% to 53% — but this may be due to either age or charging habits, so it's inconclusive without more evidence.

*I can figure out the battery healths and explain the charging habits of my old tablet and my wife's tablet phones if necessary.


I'd like to settle this once and for all, please, preferably with an answer that includes physics. Why is the best way way to charge a battery the best way?
 A: The answer is more about chemistry than physics - but then there are those who will claim that chemistry is just applied physics. There's also a bit of electronics in the mix - which is also a form of applied physics.
What is the memory effect
Typically "memory effect" refers to the phenomenon that a battery that is not fully discharged for multiple charge cycles will "remember" ("become lazy"), and will not be able to deliver its full capacity. This phenomenon is closely related to the structure and chemistry of the battery - older NiCd and NiMH batteries were susceptible to this. If you have a smart load distribution and charging system you may be able to avoid this problem by making sure individual cells don't always get discharged to the same amount.
A form of "memory effect" (not the original meaning) is "voltage depression". When a cell is OVERcharged, you can create certain contamination on the surface of the electrodes - for example crystals of electrolyte. This acts as an impediment to electron flow, and the battery will drop its voltage faster when you draw a lot of current (because of higher internal impedance). This is something that can be fixed by taking individual cells through a full charge/discharge cycle: but when multiple cells are in series, it can be hard to achieve this as different cells will have different capacity. If you have a way to control the charge of individual cells, you can overcome this. You need a smart battery.
What is a smart battery
A smart battery is a battery with a built-in BMS - battery management system. This means that regardless of what power you supply to it for recharging, it will try to direct that charge in an optimal way to (or away from) the cells that need it, with the right amount of current and voltage to best charge the battery without damaging it. It can also control the way individual cells are used to deliver power: when you don't need a lot of power you can "really drain" one of the cells that is already nearly exhausted; when you need to draw more power you can include some of the "healthier" cells to deliver enough power. These BMS systems often contain DC-DC converters as well so you can get more voltage from a particular cell - again ensuring you can get full discharge.
How do batteries get damaged
When you don't have a BMS, the easiest way to damage your batteries is to run to full discharge. Since different cells do not have exactly the same capacity, as you get to zero capacity on some cells, you may still have capacity on others. The "live" cells will continue to push current through the "dead" ones. The dead ones are now getting "charged in reverse" which will damage the electrodes irreversibly.
All batteries will slowly get damaged in normal use: during charging, you build up a layer of material on the electrodes that will react with the electrolyte to generate power during discharge. The structure of this build-up will change over time, depending on the geometry of the battery. Once the spacing between electrodes becomes non-uniform, the field strength will change as a function of position and this affects the rate of chemical reactions. This can further increase the non-uniformity, in extreme cases even causing a short circuit. Temperature plays a big role in this: at higher temperatures batteries have a significantly shorter life. Since charging can raise the temperature, one thing you can do to improve battery life is make sure that your device has sufficient cooling while being charged. I have read that some of the Qi charging devices make the problem worse by adding more heat during charging.
With all these things in mind, the following guidelines are helpful:


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*Don't let the battery get hot during charging or discharging

*When you have individual cells (AA etc), try to use a charger that charges individual batteries (i.e. don't charge them in series). Do not let those cells run "all the way to zero" (especially in things like flashlights which will just keep drawing current until the last cell has pushed its last electron).

*Full cycles of charge / discharge are good for the battery IFF they have a BMS.

*Most modern devices have a BMS that makes them quite tolerant to whatever charging you throw at them - but none of them will protect you from heat.

A: I have included a figure for the Cycle number as a function of Depth of discharge.

As you can see the cycle number decreases dramatically the larger the range of state of charge usage is.
The cycle number for a given cell chemistry is a function of the temperature, C rate (current) and the State of charge (Depth of discharge)
