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Suppose you are at a coffee shop and your phone battery is low. You decide to plug it in to your laptop via USB to charge it. At this point the laptop's battery serves as our energy source for charging the phone.

But then you realize your laptop's battery is also low, so you plug your laptop in using the AC adapter it came with. So the coffee shop's electricity is charging the laptop battery.

Now, do most charging circuits in laptops switch to source power (specifically on USB ports) from the AC adapter or not? Would the current draw from the AC adapter increase as a result of having the phone being plugged in, or would it just take longer to charge the laptop battery to full capacity?

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    \$\begingroup\$ What's the aim of this question ? What is the problem you need to solve ? \$\endgroup\$
    – doom
    Nov 30, 2019 at 20:49
  • \$\begingroup\$ You can't charge the laptop's battery and at the same time draw power from it so any USB output power must come from the charger supply. The situation will be further complicated by maximum voltage and current negotiation between the laptop and its power supply and between the phone and the laptop. \$\endgroup\$
    – Transistor
    Nov 30, 2019 at 20:55

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If the voltage at the bus connected to the battery is higher than the battery voltage (given by the State of Charge of the battery), current will flow into the battery, charging it.

In this case, current can obviously not flow from the battery at the same time. Thus, the energy to supply the laptop has to come from the power supply brick attached to it.

This may change fast, as switch mode power supplies can regulate voltages fast. It may go from charging to discharging the battery depending on cpu load, if the PSU is not able to supply the maximal consumption by the laptop, for instance. When battery is discharging, the voltage of the battery is higher than the bus, even if there's other supplies connected the same bus.

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  • \$\begingroup\$ The decision to charge or not charge the battery is made by the PMIC, which could decide to charge or discharge the battery regardless of the voltages involved based on its programming. I've seen portable devices that will actually discharge the battery while charging if the user puts enough load on them. \$\endgroup\$
    – user1850479
    Dec 2, 2019 at 17:00
  • \$\begingroup\$ @user1850479 to charge a battery, you have to apply a higher voltage to them. The battery charger controls this voltage. The voltage dictates current flow. If the PMIC wants to discharge the battery, it has to decrease the voltage on the battery bus. \$\endgroup\$
    – vidarlo
    Dec 2, 2019 at 17:05
  • \$\begingroup\$ Obviously the PMIC will need to apply voltage to move current, but this is independent of any bus voltages since the battery is NOT connected to anything but the PMIC. There are no "other supplies connected to the same bus". The phrasing there makes it sound like the battery is hooked up to some shared power bus, this is not so. \$\endgroup\$
    – user1850479
    Dec 2, 2019 at 21:11
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do most charging circuits in laptops switch to source power

Yes, they all do the switch

(specifically on USB ports)

Regarding USB ports, if a device (laptop) receives the power externally (via AC-DC adapter or else), it is called "high-powered device". In this case a USB-compliant port must supply "at least 5 units of load", which would be 500 mA for USB 2.0 port, and 900 mA for USB 3 ports.

Would the current draw from the AC adapter increase as a result of having the phone being plugged in, or would it just take longer to charge the laptop battery to full capacity?

For most laptops there is a way to communicate the power capability of AC-DC adapter to the laptop, so the laptop knows its limit, and current won't increase beyond the adapter capability. And, since you said your battery is "low", the adapter is likely already maxed out. However, smart laptop will re-distribute the internal power paths depending on design quality and selected power policy. In some cases the system will limit CPU performance, so the battery might continue to charge at optimal rate. But if your power policy is high-performance/presentation, the CPU will work at full power, and the battery might charge at slower rate. It all depends...

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Would the current draw from the AC adapter increase as a result of having the phone being plugged in, or would it just take longer to charge the laptop battery to full capacity?

As I understand it laptops will generally have an intelligent charge control system with a bypass path*. When the laptop is plugged in, power is routed from the AC adapter to the system via the bypass path and also routed from the mains input to the battery via the charge controller. The charge controller will also monitor system power consumption so it can prevent overloading of the adapter.

So if the charge controller thinks the adapter has spare capacity and you increase system power consumption by plugging in your phone then the charge controller will just allow the extra current to be drawn from the power adapter. On the other hand if the charge controller thinks the adapter is already maxed out then it will lower the charge current to the battery to keep overall input power levels within acceptable limits.

* The existence of this bypass path is why laptops can typically work without a battery present, unlike mobile phones.

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