# Why do cell phone chargers drop speed when multiple devices are connected?

What are the 'limitations' that prevent a wall charger from charging multiple devices at their maximum power?

Let's say I have a 4-port cell phone charger that is rated at an unknown wattage. When I connect a single phone, it charges fine. But when I connect a second device, there is a visible drop in the charging currents.

Are these ports connected in 'serial' and share the power? If yes, what prevents from building a device that can have separate power from the source to the port?

• "Share" is ambiguous here because it could be interpreted as duplicating the same value across everything or dividing something across everything. It's more clear to say that series connections will maintain the same current through everything while parallel connections will maintain the same voltage across everything. A 4-port charger charges everything with the same voltage, so it can't be series. Dec 12, 2022 at 3:16
• @DKNguyen, OP correctly says share the power across phones/devices, which is how multiple charging outputs would work, with a separate charging circuit for each phone/USB/etc output:s voltage and current from a common internal power supply. You describe multiple outputs sharing either the same voltage or same current, which is not how chargers work. This goes even further for Quick Charge and Fast Charge outputs with switchable voltage ranges. Dec 12, 2022 at 7:03
• @TonyM A 4-port USB charger is not actually a charger though. Dec 12, 2022 at 14:24
• "what prevents from building a device that can have separate power from the source to the port?" - such device is called "several chargers".
– IMil
Dec 13, 2022 at 2:34

What are the "limitations" that prevent a wall charger from charging multiple devices at their max power?

Power supplies are only rated for so much current. Evidently the one you bought uses too small of a power supply to run all the chargers at max rate.

Are these ports connected in "serial" and share the power?

Parallel, not serial.

If yes, what prevents from building a device that can have separate power from the source to the port?

Nothing. You just didn't spend enough money to get something like that.

• I have not found any charger that can do this. I have been talking to some manufacturers and they insist that the whole circuit is limited to xAmps and cannot be changed. Do you mind linking me to a charger that can do this? Is there a specific term I need to search for? Dec 12, 2022 at 3:24
• @deppfx I have an industrial USB hub that provides 2A to 16 ports. It comes with an external 200W power supply, and internally has a row of buck converters to ensure that all ports get the rated power concurrently. Not hard to find similar things online. Dec 12, 2022 at 3:28
• @deppfx I've got a 2-port Anker USB-A that will do 2x2.4A (though none of my devices will charge above 2A and I'm not sure I've tested 2x2A). Their specs are honest IME. Dec 12, 2022 at 15:20
• @deppfx yes, the whole circuit has a limit that can't be changed. So if you have two 100W outputs and want to be able to run them both at max, the converter that feeds them had better be capable of 200W. But most people don't want to pay for that, so instead they build it for, say, 130W, which will let you charge one device at 100W or two at <=65W, but not two at 100W. "All you need to do" to make the thing you want is to build the conversion electronics that much bigger. But most people won't buy that bigger, more expensive device. Dec 12, 2022 at 18:42
• The point is that 100 Watt is cheaper than 130 Watt which is cheaper than 200 Watt. And 100 Watt charges a 30 Watt device at full speed, plus a 100 Watt device at 70% speed, until the 30 Watt device is fully charged and the 100 Watt goes up to 100% speed. In this case 130 Watt would be more expensive and a bit faster, while 200 Watt would be a lot more expensive and wouldn't gain you anything. Dec 13, 2022 at 15:12

In order of cost:

1. One 5V 2A current-limited AC-DC supply feeds 4 ports in parallel, limiting total current to 2A, so if several loads are connected they will get a lower current.

2. One 5V 8A current-limited AC-DC supply feeds 4 ports in parallel. However 8A exceeds the capabilities of a USB socket and cable, so each port requires its own 2A current limiter to avoid melting stuff and fire. Each port gets 5V 2A no matter what is plugged in the other ports.

Same for USB-C:

1. One variable voltage current-limited AC-DC power supply feeds all ports in parallel. If several USB-C devices are connected but they request different voltages, it will have to output the lowest voltage, throttling the device capable of higher voltage. How current is negotiated will depend on how much they spent on parts (separate per port, or global).

2. One 20V high current AC-DC power supply, one voltage negotiation chip per port (or a micro that does it on all ports), and one buck converter per port: every port gets the requested voltage and power.