# Why does charging a phone battery to 100% keep the discharge rate slower than charging to 90 or 80%?

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I have experienced it on multiple mobile phones and my laptop. If i charge the battery to 100% the discharge to 99% is way slower than if i charge the battery to 90% and discharge to 89%. And this makes me charge the battery to 100% even when I know that it is bad for the battery just because the charge lasts much longer than charging to 90 or 80%. Is it related to some kind of chemistry?

I am ok with eli 20 also. Thanks.

In: 2

My understanding is that when you charge to 100%, it is really ~90% (for example), which is the “safe spot” you are thinking of. Thus if you charge up to 100% for a little longer, you really get closer to “100”. And then as you use battery life and it starts to drain, it isn’t really registered as falling until you get down to the point where the software was telling that you were at 100. So effectively, it is not a linear sliding scale yet. By contrast, if you stopped charging at 80 or 90%, say, then you are already in the linear sliding scale, and you see the delta in % immediately.

Similar to how old cars’ gas tank gauges behave. You fill the car up beyond the 100% point, and fuel starts to fill the path to where the cap is screwed on, then as that gas is consumed, the car still sits at 100% Full. The gauge starts to move after you settle into consuming what’s in the actual tank.

I think.

Charging it all the way to 100 and draining it to 0 puts a lot of stress.on the battery. Keeping it between 80 and 20 is a lot less stress. It’s like bending a paper clip back and forth, the more you do it, it eventually breaks. If we don’t bend it as far, it’s less stress, and the stress is highest at the extremes.

Cell phones, laptops and tablets are all made where the battery percentage indicator isn’t a true indicator of how much battery is there. They intentionally make it look like it is discharging slower when it is full, so you think you are getting great battery life. There is no laws that say the battery percentage had to be an accurate representation, so it is really whatever the manufacturer wants it to be. This is also why is discharges so much faster below 50%. Also, for all of them, when the battery says 0%, they aren’t actually empty either.

A battery puts out fewer volts as it is depleted, so in order to get the same amount of energy, the amperage used must increase (watts = amps x volts)

Battery capacity is measured in amp hours (or really milliamp hours) so if you’re using amps at a faster rate, the percent remaining drops faster.

It’s not necessarily intentional, but measuring battery capacity/charge is not easy.

Typically you do it by measuring the voltage over the terminals, but that will also change depending on how much load is out on the battery (i.e. it’s high when the battery is not being used, drops when the phone pulls a lot of current).

Modern smartphones are a little more sophisticated, they might use impedance testing, voltage testing, a coulomb counter that records how much power has been used and also some algorithms that adapt these calculations to the battery’s behaviour over time.

All that’s to say, it’s not an exact process and things can be slightly wrong all the time.