Wouldn't charging a lithium battery to 80% only defeat the purpose of putting the battery into longer use in the first place?

Question

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.

Answer 1

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*:

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).

Answer 2

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.

---

Year 14 update: The site was visited and maintenance was done. The daily discharge profiles are same as year 1. The solar panels have lost 5% efficiency. Batteries were cycled to 80% SoC using a petrol generator and a battery analyzer, then back down to 25%. No measurable differences from year 1. No news is great news :)

In spite of trouble-free experience, I must stress that nothing lasts forever. Even perfectly good batteries may degrade internally just from "shelf life" even at a mostly static SoC.

Mean in-box temperature over last 10 years was about 11C, 95th percentile annual temperature change amplitude was 2C peak-to-peak. The box seals etc. let in about 0.5kg water vapor/year into the box averaged over 10 years. That's based on the weight change of the desiccant pack. There is about 50kg of desiccant in the box, it got 5kg heavier between 2014 and today. The desiccant was replaced with 50kg of dry desiccant.

Answer 3

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.

Answer 4

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.

Answer 5

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.