Power bank output ports: why two devices charge slower than one

The first bank, a 20,000 mAh model, has three outputs: a USB-C socket, a built-in USB-C cable and a USB-A socket (Xiaomi). Its spec page lists the same lines for each one: 5V⎓3A, 9V⎓2.23A, 10V⎓2.25A, 12V⎓1.67A (Xiaomi). Volts × amps gives 15 W, 20.07 W, 22.5 W, 20.04 W. The best is 10 V × 2.25 A = 22.5 W, the “22.5W (Max.)” on the page.

On the same page, before the per-port lines, sits “multiple ports: 5V⎓3A” (Xiaomi). That is 5 × 3 = 15 W, against 22.5 W from one port alone. The voltage falls too: alone, a port reaches 12 V; together, the only line is 5 V.

The second bank, a 30,000 mAh model rated 18 W, prints its multiple-port line first: 5V⎓3.6A. Then the USB-A port on its own (5V⎓2.4A 9V⎓2A 12V⎓1.5A) and the USB-C port (5V⎓3A 9V⎓2A 12V⎓1.5A) (Xiaomi). The USB-C lines carry no “single port” tag of their own, but they are printed apart from the multiple-port line, so we read them as that port alone. Here the watts stay level, 18 W alone and 18 W together, but the 9 V and 12 V lines are gone.

Neither page says how the multiple-port current is divided between devices. The labels still show that it is shared. On the 20,000 mAh bank, each output alone gives 3 A at 5 V; three of them would add up to 9 A, and even two to 6 A. The multiple-port line is 3 A. On the 30,000 mAh bank, the two ports alone list 3 A + 2.4 A = 5.4 A at 5 V; the multiple-port line is 3.6 A.

Some wall chargers say more about the split. One maker's two-port wall charger says it allocates power by what each device needs and the order you plug them in (Xiaomi). For chargers in the USB-IF certification programme, a shared port does not have to supply its stated power all the time (USB-IF), and the packaging must say that the other ports drop (USB-IF). The two power bank pages here give the multiple-port line and nothing about the split.

USB Power Delivery runs at fixed voltages of 5, 9, 15 and 20 V (USB-IF). Without USB Power Delivery, a USB-C port advertises 1.5 A or 3 A at 5 V (USB-IF), which is 5 × 3 = 15 W. A phone that charged from the 9 V or 12 V line on its own is back at 5 V once a second device is plugged in, and the bank's 5 V current is now shared.

The second device does not have to be a phone. Earbuds or a watch on the second port also put the bank on its multiple-port line, and the phone on the first port loses its higher-voltage lines.

• Charge the phone alone first, then the earbuds or watch. Plugged in together, both are on the 5 V line.
• The built-in cable is an output. A device left on it while you use another port puts the bank on its multiple-port line.
• Before buying, find the "multiple ports" or "simultaneous" line on the spec page. No such line means the maker has not told you what happens.
• Neither bank here lists a line above 12 V, so neither offers the 15 V or 20 V USB PD levels. Check what your laptop asks for before counting on a bank like these.
• Watts can stay level while the voltage drops (18 W alone, 18 W together on the second bank). Compare the volts, not only the watts.