mobile devices - screens light up when charge cords plugged in

Why does this happen? It seems obviously counter productive. The reason to plug in the charge cord is, well, to charge the device. Lighting up the screen, which seems to be the universal default action, sucks power away from and prolongs the charging process.

Is this a design flaw, oversight, or is there some reason it “has to” happen?

My iPhone lights the screen for a few seconds showing the battery state of charge. This does 2 things

  1. Allows me to verify that it is charging
  2. Gives me a clue how long it will take for a full charge

Actually, it’s probably a feature intended for most peoples convenience. The logic being that having the screen go into sleep mode frequently is an unnecessary annoyance when you’re connected to power.

I’ll bet there’s a setting you can change to stop that behavior. On my device it’s “stay awake: screen will never sleep while charging”

How much longer do you think it takes to charge with the screen lit up? An electrical outlet is something that can power huge, power-hungry appliances like air-conditioners, washers, dryers, etc. Do you think the LCD on a little cell phone is going to make a big difference? If the phone is designed properly, there should be no reason for it not to be able to light up the screen and charge the phone just about as quickly as possible.

The limiting factor in charging time is the battery chemistry, not power supply. I’m more concerned with wearing out the screen’s LEDs.

Concur - on devices that don’t do this, I’ve often returned to them later to find that although I plugged in the cable, the charger is turned off at the wall (or someone has unplugged it from the power strip) - chiming and/or lighting the screen when plugged in confirms that the thing is actually charging as intended.


you’re not cramming 120VAC into your phone :rolleyes: a generic micro USB charger delivers up to 0.5 amps at 5 volts DC (i.e. one watt) to the device, same as a standards-compliant USB port on your PC. any power used to light the display is deducted from that one watt and is not used to recharge the battery.

device specific chargers (e.g. ones for tablets) can supply more current but still at only 5VDC. and as above, power used to light the display is power not used to recharge. and given the display is one of if not the most power hungry components of a modern smartphone or tablet, keeping the display on can significantly lengthen recharge time.

Sure, but the default behaviour for most phones I’ve seen is for the screen to light when plugged in, then time out and switch off again after 30 seconds or a minute, or whatever.

So the phone’s charging rate is possibly diminished for maybe a minute, then it’s charging as normal.

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So why doesn’t my DustBuster turn on when I plug in its charger, just to let me know the juice is flowing? Oh yeah, because there’s that tiny red light that tells me the same thing. I suppose that sucks power away from charging too, but not as much as turning on the whole unit.

It doesn’t turn on the whole unit just the screen for a few seconds.
You are bound and determined to be outraged over this so I guess no reason no matter how logical will change your mind.

I just timed it. The screen was on for 8 seconds.

Yup, same on my HTC 8x. Eight whole seconds of possibly-suboptimal charging! Oh noes!

An LED operated under normal conditions will vastly outlive the device it is used in; I have a digital alarm clock which has been running for around a couple decades (170,000 hours) yet shows no signs at all of wearing out (that is, the 10s of hours digit would gradually become brighter relative to the other digits since it is only on 1/4 of the time; if it has, it isn’t noticeable yet). Even the lifetime estimates of 50-100,000 hours are just estimates; nobody has actually tested them for that long (high power LEDs also run far hotter, which greatly accelerates wearout, roughly halving life every 5-10°C). LEDs also don’t care about cycling on and off; they are probably already brightness controlled using PWM as well (hundreds to thousands of on/off cycles per second).

As far as power drawn by the phone affecting the charging process, the actual charging circuitry is inside the phone; the power supply is simply a power supply, except perhaps for very old phones with NiCd batteries (Li-ion batteries require sophisticated chargers to ensure that the battery never overcharges, the circuitry is pretty much always inside the device and not the “charger”).

This. Keeping the screen on takes a huge amount of power. If you have a crappy charger and a power-hungry phone, sometimes having the screen, 4G, and GPS on (such as when using google maps), my charger can’t even keep up. It drains the battery slightly slower than without the charger, but not enough to actually make the juice go up.

That said, as has been pointed out the screen will usually turn itself off after a few seconds. It bothers me too but I just click the screen button to turn it back off as soon as I plug it in.

yep. the iPad for example will helpfully say “Not Charging” in the status bar if you have it connected to a PC USB port. n.b. it actually will charge- albeit slowly- if it’s left in sleep mode, but the second the display turns on it will stop doing so.

Is this even true for screen LEDs, which run brighter? I know CCFLs are one of the commonly failing parts of LCDs. And OLEDs definitely have a short life which may be affected by this.

8 seconds!?! Well, why couldn’t they have made it only 4?

Well it took some time but I tracked down the person responsible for the screen illuminating for 8 seconds at Apple.
A very nice young lady named Locks. I asked why 8 seconds? Here is her response

If you have any further question refer them to Goldie.

IME when a CCFL backlight fails it’s usually the inverter and not the tubes themselves. As for backlight LEDs, I’m sure Michael63129 will correct me if I’m wrong but I don’t think they qualify as “high power” in this context. At least, they’re not run as hard as the diodes used in things like LED light bulbs (where much of the “bulb’s” mass is heatsink.)

as far as OLED, AIUI the limiting facter is still the blue element. Supposedly the red and green have reasonably long lifetimes, but blue “wears out” (grows dimmer) at a substantially faster rate.