I have come across multiple YT videos (about the new higher USB power delivery and battery degradation) mentioning that chargers communicate with the phone or laptop when charging to avoid battery degradation, or avoid supplying too high a power. Is that really true and is there some kind of standard for that? Do nearly all chargers support it?

  • \$\begingroup\$ Transistor and user1850479 both gave you great answers. One thing to keep in mind we can only take a SWAG as you did not specify the phone. There are many phone chargers that do not communicate, especially the older ones. \$\endgroup\$
    – Gil
    May 30, 2021 at 22:18
  • \$\begingroup\$ Some do. \$\endgroup\$ Jun 1, 2021 at 12:53

3 Answers 3


Yes - modern devices absolutely negotiate what charge current they will accept, and what the charger can provide. In fact without negotiation, the device will only get 500 mA, which may not be enough to increase the battery charge while the unit is working hard.

Example: My work iphone can use power faster than my car charger can provide, noticeably when hotspotting.

Consider that an electric car charger is a big version of the phone charger, and that some of these ideas may trickle down into smaller hand-held items in the future.

The fancier chargers can negotiate with the attached device to calculate a mutually-agreeable charging current, and can even consider the time of day the car will be needed and can work out a slower charge that gets the battery to capacity before needed.

Some fancy multi-slot chargers can work out which devices are needed soonest, and prioritise charging them first, which is why some chargers ask when you will need the vehicle back.
Imagine if you had a phone/tablet charging alongside your electric hair trimmer that is only used once a week.

Finally, some cars can charge, and then tell the charger to keep providing power which is used to pre-heat the cabin rather than taking the power out of the batteries.

I've noticed my phone charges up in the early part of the night, and then bounces from 95% To 100% until morning. It would be fantastic if the phone could take power from the charger to run itself directly, and leave the battery at 100%.

  • 2
    \$\begingroup\$ Bouncing between 95% and 100% is rather unusual and may even be an indication of a problem. Most modern phones/tablets/laptops just charge to 100% and stay at 100% as long as an external power source is available. \$\endgroup\$
    – fraxinus
    May 31, 2021 at 8:12
  • \$\begingroup\$ @fraxinus true - the android power graph shows that. I know the battery is tired too so it probably drops to 95% as soon as the charger shuts off - I was trying to make the examples of car/charger communication relevant to USB and cellphones, which is what OP was asking about. Yeah its slightly tenuous example. \$\endgroup\$
    – Criggie
    May 31, 2021 at 11:17
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    \$\begingroup\$ I recommend clarifying that they are power supplies, not chargers. The charger is in the phone and the power supplies can be used for non-charging applications. \$\endgroup\$
    – Transistor
    May 31, 2021 at 12:19
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    \$\begingroup\$ It has always been my understanding that all lithium-type batteries charge the way Criggie describes - charge to 100%, allow the battery to discharge naturally to some lower value such as 95, and then charge again to full, in order to prevent the wear and tear of keeping a battery at peak charge for hours and hours every night. The unusual part to me is that Criggie's battery status indicator doesn't hide this fact. \$\endgroup\$
    – Steve V.
    May 31, 2021 at 20:38

USB-PD (power delivery) lets the phone communicate with a power supply to pick a voltage/current that both support. For example, a power supply might support 5, 9, 12 and 20V, but a phone might only support 5, 9 and 12v. Supplying the phone with 20V when it can only handle 12 could destroy it, so in reality USB-PD is there to protect your devices from destroying one another while still being able to use higher voltages when both devices are compatible.

USB-PD is just for negotiation of voltage/current. What the load does with that is its own business, so battery charging is not something the power supply knows about. The battery charge controller in the phone handles battery charging.

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    \$\begingroup\$ there are not only USB-PD. You can negotiate voltage even before USB-PD, and there are many standards for that \$\endgroup\$
    – phuclv
    May 31, 2021 at 3:16
  • \$\begingroup\$ @phuclv Yes there are, but the question was prompted by a video about USB power delivery. \$\endgroup\$ May 31, 2021 at 3:25
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    \$\begingroup\$ USB-PD is indeed the (relatively recent) standard. Before that there was basic negotiation up to 5V x 500 mA, and there have been lots of other ways for chargers and devices to negotiate power. The most common is probably Qualcomm's Quick Charge protocol,(note the many versions) but there are quite a few others. \$\endgroup\$
    – jcaron
    May 31, 2021 at 10:04

You will find the answer if you research "USB charge negotiation".

What you are calling "chargers" are power supplies. The charge controller is in the phone. The phone tries to communicate with the power supply to learn how much current it can safely draw. If it can't communicate it assumes 500 mA max.

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    \$\begingroup\$ I think it'd be a good idea to add, since they asked if there's a standard, some mention of USB-PD and probably also Qualcomm QC. \$\endgroup\$
    – Hearth
    May 30, 2021 at 16:55
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    \$\begingroup\$ @Hearth, I don't know enough about either. The stage is yours! \$\endgroup\$
    – Transistor
    May 30, 2021 at 16:57
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    \$\begingroup\$ I thought – at least for USB 2.0 – it’s 5V, 100mA? You can request 500mA in the device descriptor and are only allowed to draw it after enumeration has completed. \$\endgroup\$
    – Michael
    May 31, 2021 at 11:43

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