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In relation to Does recharging a battery when it is only half-way dead decrease its life span? I read an article about Maximizing the lifespan of a laptop battery where Samsung puts it:

Most battery damage occurs when the cells inside are at or near capacity. That’s because batteries get a bit hot under the collar when they max out their charge at 100 percent. This heat causes the delicate structures within to start to break down, which ultimately makes the battery less capable of holding a charge. Eventually capacity drops to the point where the battery won’t hold a charge at all and it has to be replaced.

They offer a solution which suggests to:

[...][cap] the charge at 80 percent.

To what extent is this true?

And although not directly related, I figure it must matter in this relation that electric cars are charged to 80% when performing quick charging.

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Aske B.
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  • Interesting, AFAIK, the standard claim that Li-Ion batteries should be fully charged to prolong their lifespan. – vartec Oct 08 '12 at 10:33
  • @vartec Can you provide a reference to someone making that claim? – Aske B. Oct 08 '12 at 10:43
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    "electric cars are charged to 80%" - could that be because charging is nonlinear? E.g. charging from 20% to 80% is fast, but 80% to 100% takes longer (per percentage point)? – Piskvor left the building Oct 08 '12 at 12:01
  • @Piskvor Maybe, but about charging electric cars, the method of speed charging to 80% seems to be for safety of the battery. "*A communications link incorporated into the charging cord allows the car's battery management system to control the rate of charge to avoid damage to the battery. The system will, for example, **slow down charging if the battery overheats or will typically stop charging when it reaches 80 percent of capacity**.*" ([source](http://technologyreview.com/news/429283)). I figure that if charging the battery above 80% is bad in cars, it could also be bad on a smaller scale. – Aske B. Oct 08 '12 at 12:54
  • @RoryAlsop You're right. It's entirely possible to charge batteries in cars above 80%, but from the article I just quoted they say "*The system will [...] slow down charging if the battery overheats **or** will typically stop charging when it reaches 80 percent of capacity*", so even if the capacity was 70% before charging, it will stop doing fast charging at 80%. What I'm saying is there must be some reasoning behind this security measure that could be linked to batteries of laptops. – Aske B. Oct 08 '12 at 14:11
  • @RoryAlsop Without knowing where you have that information from, I should emphasize that this question is about laptop batteries, not car batteries. I only linked it because I thought it might be related. If you can prove that it's not, I'll remove it from the discussion as it seems to be taking over. – Aske B. Oct 08 '12 at 15:50
  • @RoryAlsop Well, it sounds plausible, and if you can prove it (or find sources that proves it) I'll remove it from the discussion. The reason I haven't by now, is because I think it could serve as additional research-inspiration for those who are capable of answering this question. I'll only remove it if I'm sure it's not related. – Aske B. Oct 08 '12 at 16:14
  • let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/6056/discussion-between-rory-alsop-and-aske-b) – Rory Alsop Oct 08 '12 at 16:29
  • Although a big one, a Tesla electric car also functions as a telephone, and Tesla recommends charging 85% of battery capacity for daily recharges, reserving 100% recharge for the situations where you prepare for a longer trip. – Morten Engelsmann Dec 22 '17 at 12:37

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Interestingly, despite my comment above (which was based on some info I had on early laptop batteries) I have found this article on batteryuniversity.com on Lithium based batteries. I would definitely recommend reading it, and the other information pages there - a lot of good into on the different types:

This table compares the number of discharge/charge cycles Li-ion can deliver at various DoD levels before the battery capacity drops to 70 percent.

enter image description here

This table summarizes variation due to charge level.

enter image description here

This figure demonstrates cycle count as a function of charge voltage. At 4.35V, the cycle count is cut in half.

enter image description here

Conclusion: 80% may well be the sweet spot, as over-charging causes damage and under-charging reduces effective capacity.

Rory Alsop
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That remembers me that story about apple iPad charging algorithm

As reported in this article, the device showing 100% does not mean that the battery is actually full charged.

So here’s how things work: Apple does, in fact, display the iPad (and iPhone and iPod Touch) as 100 percent charged just before a device reaches a completely charged state. At that point, it will continue charging to 100 percent, then discharge a bit and charge back up to 100 percent, repeating that process until the device is unplugged.

Apparently, "just before" is around 90% charge.