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I see most smart devices (laptops/tablets/phones) now have the option to optimize your battery by charging it to 80% only.

But wouldn't this reduce your capacity i.e. your brand new battery now only has 80% of its rated capacity, which means more frequent recharge and all that. Even if you do charge it to 100% for the occasional road trips or whatever reason that you need to have the battery to last longer, I still don't see the real benefit of capping your battery charge at 80%.

This basically comes down to, I'm trying to make my battery last longer, but then I have to lose 20% of its capacity first, so what the heck, I'd rather just charge it to full every time and let the battery deteriorate annually.

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    \$\begingroup\$ You're probably confusing what "last longer" means. You will only get 80% of energy per charge cycle, but that cycle will "damage" your battery 5x less than charging it to 100%. So in far future, you get 5x 80% = 400%, instead of 1x 100% = 100% of the power. In other words, you will be able to charge the battery many more times, also getting more power out of it, before it dies. \$\endgroup\$
    – akwky
    Jun 13, 2022 at 12:46
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    \$\begingroup\$ Comes down to "do you want a pound from the few or a penny from the many" mentality. \$\endgroup\$
    – Solar Mike
    Jun 13, 2022 at 13:03
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    \$\begingroup\$ "i'd rather just charge it to full everytime and let the battery deteriorate annually." You're assuming the tradeoff is linear which is not necessairly true. What if charging to 100% let the battery last only a year but charging to 80% let it lasts 1.5 years? I am unsure what the actual relationship is though. Other factors include how fast you charge and of course, total number of charges over the battery lifetime. \$\endgroup\$
    – DKNguyen
    Jun 13, 2022 at 13:31
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    \$\begingroup\$ There is a computer company that has a setting to keep batteries changed between 60 to 65% if you are leaving your notebook plugged in all the time and not using the battery. A major factor in how fast lithium ion batter degrade is how long they stay at the higher charge percentages. Deep depleting the battery below 0% is another degradation factor. \$\endgroup\$
    – Perry Webb
    Jun 13, 2022 at 14:00
  • \$\begingroup\$ Sure, if replacement cost is no object. \$\endgroup\$ Jun 14, 2022 at 1:29

5 Answers 5

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When you run, you cover less distance by sprinting for a short time compared to running at marathon pace. Same goes for batteries.

The longevity of Li-ion batteries is greatly affected by a) how deep you discharge them and b) how full you charge them. Not in a linear way. The quantitative benefits or discharging the battery are out of the scope of this question (although if that is of interest, you can check out this other question [disclaimer: I wrote the accepted answer]), but they are universally recognized and it seems like the benefits of under-charging your battery are actually at least as important in the long run:

Using a reputable battery management application I use on my phone, I have observed how "cycles wear" varies vs "charging setpoint" and calculated the relative energy gain at the end of the battery life compared to 100% charging and discharging*: enter image description here

As you can see, charging to 80% instead of 100% multiplies by 4 the amount of energy the battery will have transferred to you over its life - the only tradeoff being to compromise on how much energy you can get out of a full charge (big slices, small cake VS small slices, large cake). This also means you can use your battery for 4 times longer before it gets to the end of its rated life. Note the use of "rated life", because in practice batteries lose capacity as they age and get used, to a point they become unusable.

If the above plot was flat, you'd be right, charging at 80% the battery would be useless because we would charge it more often (less convenient) and it would not live longer. But it's not, not even close.

So, just like in a race, you have a choice: either run smart, or run fast. In practice "smart" would actually mean here that at times, you would have to charge at 100% to survive an intense day, but that in general you should charge it at a lower level.

It is generally recognized that 80% is a good compromise between convenience and life span. I think I remember that in satellites, the battery is generally designed to charge/discharge between a third and 2 thirds of its rated capacity to maximize its lifetime to 15+ years.

My general advice for maximizing the battery is only use your battery as much as you need, i.e. charge it as often as possible, and as little as possible. If you need a rule of thumb, stay within 30%-80%.

*: This particular calculation assumes deep discharging, but although the benefits may be altered by (safer) more shallower discharging, the qualitative takeaway is the same. Note, also, that obviously this chart is not at all accurate below 50% (my app does not even give estimates below that) and probably not that accurate close to 50%. I believe it above 75% though.

Example of scientific proof from the Battery University (e.g. Table 3).

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    \$\begingroup\$ +1, also keeping it charged at 100% ages it faster even if the battery is not actually used and discharged. \$\endgroup\$
    – bobflux
    Jun 13, 2022 at 18:27
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    \$\begingroup\$ "It is generally recognized that 80% is a good compromise between convenience and life span." thanks this is the answer i was looking for. also a followup question since you mentioned keeping battery at 30%~80%, between being on super low charge and fully charged, which condition causes more harm to the battery? because when you are charging the battery to 80% the battery is more likely to suffer from being on low charge before you get to charge it again. \$\endgroup\$ Jun 13, 2022 at 19:58
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    \$\begingroup\$ @GuitlyofNtAsking A truly 0% empty battery is usually not recoverable while you know how well a 100% battery works. \$\endgroup\$
    – DKNguyen
    Jun 13, 2022 at 20:58
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    \$\begingroup\$ @DKNguyen: Well put. It's worth noting that "truly 0% empty" is generally not accessible to the average user thanks to the battery controller that has undervoltage protection; but it confirms that the fully charged extreme is preferable. \$\endgroup\$ Jun 13, 2022 at 21:01
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    \$\begingroup\$ It is also better to store it for longer at a lower charge. Electrons are reactive little gerbils, they tend to do least damage in stabler arrangements (as in, "discharged"). The fully charged battery is brimming with electrochemical potential and is not healthy in the long term for itself, for its anodes and cathodes, for wires and solid state electronic gizmos in the high voltage loop. Charging it to 100% once in a while just before you use it is not a big deal. Charging it to 100% to let it sit in a garage (in case of a car) is not at all preferential if longevity is remotely interesting. \$\endgroup\$
    – Stian
    Jun 13, 2022 at 21:14
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I have a device in a remote location, in temperate climate, that uses a rechargeable lithium battery and solar panels. The device itself is underground where the temperature varies only a little bit, so that's ideal for the cells.

The battery packs have 20x the capacity the device would ordinarily need, with worst case weather. The packs are kept around 25% charge state, and charge and discharge only a few percent each day/night cycle.

In those conditions, the batteries last a very long time. Going on 12 years now! I never tested how much capacity is left by doing a full discharge, as that system must stay on, but given the cell voltages, it doesn't look like much has changed since day 1.

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    \$\begingroup\$ Fascinating! What does the device do? Some type of weather station? \$\endgroup\$ Nov 21, 2023 at 10:54
  • \$\begingroup\$ @nathancahill Close. It’s environmental monitoring. \$\endgroup\$ Feb 1 at 2:54
  • \$\begingroup\$ Do you have a writeup with information? We're building something similar for bird monitoring in Guatemala. \$\endgroup\$ Feb 1 at 9:36
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It's down to how many years, or charge cycles, you want the battery to last for.

If you limit the charge to 80%, then you will have to charge more often. But the increase in battery life will be considerable and it may live for many more years.

A LiIon cell that's treated gently can last 10 years or more.

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  • \$\begingroup\$ 10 years is wishful thinking. In an embedded device sure, but consumer devices (phones and laptops) will lose significant capacity in 2-3 years of light use. Not only will the battery discharge sooner but the device will also become slower when the VRMs start power throttling. \$\endgroup\$
    – Navin
    Jun 27, 2022 at 22:12
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    \$\begingroup\$ Anecdotally, my phone was new in 2017 and six years later my battery app (AccuBattery) estimates the remaining capacity to be about 80%. During that time I've been careful to keep the charge somewhere between 20-80% usually. I won't be surprised if I get another four years out of it, all being well. \$\endgroup\$ Nov 21, 2023 at 4:50
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My battery has been mainly charged only to 80% while I had it. I don't know its far history, but it's now down to 25% of its rated capacity (still good for more than an hour) and was first in use (according to its electronics) 11 years ago. Charging to 100%, in contrast, tends to make batteries go dead after about 4 years of use in my experience.

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It does not necessarily mean more frequent charging. With a Qi wireless charging puck at the office, at home and in your car you may seldom use more than a fraction of your battery capacity, and may not miss that 20% at all. And maybe you can avoid having the battery serviced over its useful life.

If your usage pattern is different, plugging it into a charger only when nearing 0%, and then charging to 100%, then maybe it's not a net benefit with that mode of use. But having the choice is always nice.

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