Why are(n't) rechargeable batteries damaged by partial charging?

Question

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).

• 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):

• 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. What's the best way to charge a battery, and why is that that best way?

Answer 1

The three statements you quote are all about correct. What's good for a battery heavily depends on the type (chemistry) of the battery.

In the old days, earlier than 10-15 years ago, the majority of rechargeable batteries were the NiMh (or NiCd before that) type. This type is more or less susceptible to the so-called "memory effect", which basically says that the battery ages sooner (starts losing capacity) if it is recharged before being more or less completely discharged.

With current Lithium-based batteries (LiPo, LiIon, LiFePo,...) which have largely replaced NiMh (replaced them completely in most areas, esp. in mobile computing), the situation is different:

Lithium batteries take some damage from deep discharges (0% or less, 'under discharge') and also suffer a bit from high charge levels (100%) especially if held there for extended periods and/or at elevated temperatures. That's why for almost all current rechargeable batteries (Lithium) the optimum useful lifespan in terms of total energy stored/provided before becoming more or less unusable is achieved by maintaining neither too low nor too high charge states. A rule of thumb is to try and keep them between 30-70% as much as you can. The actual optimum varies by battery type, manufacturer etc.; some may yield more energy in total if cycled between 20 and 80% for example, but the difference should not be much, i.e. a given battery always cycled between 20% and 80% may live twice as long as the same type of battery cycled between 2% and 98%, but going to 30-70% instead of 20-80% will probably not make much of a difference in either direction.

It may be noteworthy that the relation "lower cycles" = "longer lifespan" is not always true due to non-linear effects. That is, permanently cycling between 49% and 51% will give many more charge-discharge cycles than, say, 20%-80%, but the total usable energy will likely be less. (Notice that a "cycle" comprises only 2% of the total capacity in the former vs. 60% in the latter case. If you get 10x as many cycles for the shallow cycle, you still only get 10*2%/60%=1/3 of the total usable energy over the life of the battery.)

There are some articles at battery university, including at least one where they tested different strategies and plotted a graph of cycling range vs. total energy stored. I'll try and come up with a link.

Edit:

This should cover all your questions about what's best for Lithium-based batteries: https://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

Some quotes:

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.

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Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life.

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On the negative side, a lower peak charge voltage reduces the capacity the battery stores. As a simple guideline, every 70mV reduction in charge voltage lowers the overall capacity by 10 percent. Applying the peak charge voltage on a subsequent charge will restore the full capacity.

The last quote can be interpreted as: Charging only up to ~85% (which should be at about 4.1V) will double the cycle life compared to charging to 100%.

So the key points to be aware of are:

• Practically all rechargeable batteries you'll find around you are Lithium-based nowadays and these should not be treated like the old Nickel-based batteries
• Rechargeable batteries gradually and permanently lose more and more of their initial capacity over time
• Deep discharges (like to below 5-10%) permanently reduce a Lithium battery's capacity
• High charges (like to above 90-95%) also take their toll on their lifetime
• Every charge "cycle" eats a small amount of capacity, even if it's only a 1% charge cycle