22 ms·
Phones and laptops are different beasts. I'm going to make up some numbers here to simplify, but the idea is the important part. Note: I wrote this whole comme
by jws 9y ago
Phones and laptops are different beasts. I'm going to make up some numbers here to simplify, but the idea is the important part.
Note: I wrote this whole comment with C inverted the first time. Too little sleep. It is now fixed thanks to user revelation.
First: Battery people like to talk about discharge and charge rates in a unit called "C". If you drain a battery at 1/10C, you will totally use it up in 10 hours. If you drain it at 1/2C you will totally use it up in 2 hours.
Laptop: 5000 milliamp hours at 10 volts for 50 watt hours of energy. This will last for 10 hours of use if you average 5 watts. You will be discharging at 0.1C on average. Your CPU might be a 15 watt CPU, so you can burst to maybe 20 watts total, or 0.4C.
Phone: 1500 milliamp hours at 4 volts for 6 watt hours of energy. This will last for 6 hours of active use, so that is 1 watt. You are discharging at 0.16C on average. Let's guess the CPU can surge to 5 watts. That takes you to 6 watts total use and a discharge rate of 1C.
So that is a 2.5x safety margin that the laptop has for degraded batteries over the phone.
Summary: Phones operate much closer to the current limits of batteries compared to older laptops. Accumulated wear or damage to the battery will affect them sooner.
- Dylan16807 9y agoMost laptops with dedicated GPUs can drain their battery quite a bit faster than a phone can, but they don't exhibit instability on batteries nearly this young.
- jws 9y agoWithout making the example too complex, it is the instantaneous peaks which get you in trouble. You could have your average power consumption approach 2C (totally discharged in two hours) and be just fine, that is the specified maximum for discharging something like a standard 18650 lithium cell. It is the surge that is going to cause an unacceptable voltage drop.
- Dylan16807 9y agoSo, tiny capacitor? And I don't mean discharged in two hours, I mean discharged in 40 minutes when you keep it at max usage. Faster than a phone.
- brigade 9y agoIf phones had active cooling and could thermally sustain a maximum CPU+GPU load for 40 minutes, they would discharge that fast or faster.
- revelation 9y agoI think you got it backwards, "C" is for capacity (useful since (dis)charge current obviously scales with capacity, ceterus paribus) and a 2C discharge rate is twice the current of a 1C rate. You also need to be careful in your calculations. Battery discharge rates are given in Amps (or C multiples) not Watts because over a cycle, cell voltage will naturally drop. Discharging at 5 Watts puts more strain on an empty than a full cell since the first requires higher current to compensate for the lower voltage. Amps reflect the "demand" better. Your summary is still correct though, there is more margin for degradation in a laptop battery and space for decoupling.
- jws 9y agoYes I did write it all backwards. I'm going to go back and fix the post, so your comment is not going to look insane.