Optimal time to recharge a Kindle PW4?

[NullNix]

Originally Posted by Aleron Ives

Interesting. I knew electric cars would lie about the current charge level, but I didn't know a Kindle would.

More or less everything does, because if you don't you get users reasonably asking "why does my battery only charge to 85%? Is it faulty?" and "why does it go off when the battery still has 15% left?". (In both cases the answer is "to avoid whiskering", plus, for discharging, "to make sure the battery circuit still has enough power for a long time", because recharging when the battery circuit has no power is *really* slow).

[j.p.s]

NiCd and lead acid each have a point where charge is maximum. A lithium battery charge can still be increasing at the point it explodes. So 100% charge for lithium is arbitrary. There is no natural definition of 100% charge fo lithium that is also safe. Battery manufacturers pick different points, device manufacturers maybe others, and users others still. So it is not really a matter of lieing.

[Aleron Ives]

Originally Posted by j.p.s

100% charge for lithium is arbitrary. There is no natural definition of 100% charge fo lithium that is also safe.

I don't think that's true. The battery is at 100% when each cell is at 4.2V. You can charge beyond that, but any properly designed charging mechanism will prevent it, since you'd probably start a fire. If a battery meter tells you that you're at 100% when the charge is < 4.2V, then it's lying. Some devices do this in order to trick you into treating your batteries better, and others do not. It's hard to know if your device is lying unless you can measure the battery voltage and compare it to what the charge indicator says.

[Quoth]

Also 100% charged sometimes doesn't mean 100% of new capacity. Capacity decreases with age, (elevated temp ages faster) even if not used, and with number of equivalent to total cycles.
Laptops may allow you to see percentage capacity and some devices may immediately show % capacity rather than 100% charged when charger disconnected.

The 4.2V is only approximate. It varies with model and temperature. A proper charger entirely stops charging based on a number of criterion. Hence many devices have a charger circuit (or part of one) in a two wire pack. Other devices use a battery with a 3rd temperature sensor connection. A multicell pack ideally has monitoring of voltage and charge state of each cell and serial data from on board CPU to the host.

[NullNix]

Originally Posted by Aleron Ives

I don't think that's true. The battery is at 100% when each cell is at 4.2V. You can charge beyond that, but any properly designed charging mechanism will prevent it, since you'd probably start a fire. If a battery meter tells you that you're at 100% when the charge is < 4.2V, then it's lying. Some devices do this in order to trick you into treating your batteries better, and others do not.

I'm not aware of any made in the last decade that don't lie in this sense, at both ends. I believe it's a requirement of the design of battery circuits these days; at the high end just to avoid user confusion about why they can't fully charge their battery, at the low end because the battery circuit itself needs power, and because discharging too far, particularly fast, can cause excessive whiskering, which goes beyond capacity reduction and up to shorts and oh look now we're venting with flame again!

[tm3]

Not debating the "best practice" for a Kindle battery, but, just wondering how it jives with information about "best practice" for a laptop battery. I recently started using my laptop daily, and the majority of opinions in my research were that it would be best to plug the laptop in when using it, as the battery degrades faster by cycling down then recharging. Plugging it in for use, then unplugging it, then again the next day, etc. means that the battery stays up around 100% charged.

[Aleron Ives]

The best way to handle a laptop battery is to set a charging threshold of ~60% and leave it plugged in. Unlike mobile devices, a laptop can run directly off AC without using the battery, so you can just leave the battery half charged without cycling it at all. You'll probably want to exercise the battery once every few months. Leaving the laptop plugged in at 100% will degrade the battery faster, even though you're not using it.

[NullNix]

Originally Posted by Aleron Ives

The best way to handle a laptop battery is to set a charging threshold of ~60% and leave it plugged in. Unlike mobile devices, a laptop can run directly off AC without using the battery, so you can just leave the battery half charged without cycling it at all. You'll probably want to exercise the battery once every few months. Leaving the laptop plugged in at 100% will degrade the battery faster, even though you're not using it.

... which is why a lot of laptops have a mode that charges the battery to 70% or so and leaves it there. (You can turn that off if you plan to take it away for a while and actually *run* on battery.)

[tm3]

Originally Posted by Aleron Ives

The best way to handle a laptop battery is to set a charging threshold of ~60% and leave it plugged in. Unlike mobile devices, a laptop can run directly off AC without using the battery, so you can just leave the battery half charged without cycling it at all. You'll probably want to exercise the battery once every few months. Leaving the laptop plugged in at 100% will degrade the battery faster, even though you're not using it.

Wow that is pretty interesting. I'll have to see if I can figure out how to set that on my Macbook.