# USB power bank: How can both voltage and current be given

• greypilgrim
In summary, the conversation discusses confusion about basic electricity and the use of a USB power bank with multiple outputs. The resistance value is not the same as the supplied value, and connecting a device with a lower resistance to a higher rated output can cause problems. The specific problem depends on the supply, but generally it is a lowering of the output voltage. The difference in charging capabilities between an iPad and a smartphone may be due to different charge electronics rather than battery capacity. It is suggested to turn off the iPad while charging to potentially increase charging speed.

#### greypilgrim

Hi.

I'm still confused about basic electricity. I have a USB power bank with two 1 A and one 2.1 A outputs. They all are specified with 5 V (all numbers from the manual, not measured myself).

I just can't bring this into agreement with Ohm's law U=RI. Shouldn't the resistance R only depend on the load that is connected to the outputs? So how can both U and I be given beforehand? Or is one of them or both a maximum value?

greypilgrim said:
Hi.

I'm still confused about basic electricity. I have a USB power bank with two 1 A and one 2.1 A outputs. They all are specified with 5 V (all numbers from the manual, not measured myself).

I just can't bring this into agreement with Ohm's law U=RI. Shouldn't the resistance R only depend on the load that is connected to the outputs? So how can both U and I be given beforehand? Or is one of them or both a maximum value?
You have a common misunderstanding. The RATED value is not the SUPPLIED value. The rated value is a maximum, above which the supplied value will start to cause problems w/ the source. The supplied value is whatever the load requires (up to the rated value, when, again, problems start).

davenn
Thanks, that's what I figured.

I can only charge my iPad on the 2.1 A output. If I connect it to one of the 1 A outputs, it shows the charging symbol but the battery level doesn't increase. So I guess the resistance of the iPad is lower than 5 ohms, but this causes "problems", as you put it. What exactly happens then? Does the power bank increase some internal resistance until the current is 1 A? Or will the voltage drop from 5 V to a value such that the current becomes 1 A? Or does this depend on the power bank?

Another question: Since I can charge my smartphone on the 1 A output, but my iPad only on the 2.1 A output, this should mean that the iPad's resistance R=U/I is smaller than the smartphone's. Can this somehow be explained by the different battery capacities or are there different charge electronics or something like that?

greypilgrim said:
Thanks, that's what I figured.

I can only charge my iPad on the 2.1 A output. If I connect it to one of the 1 A outputs, it shows the charging symbol but the battery level doesn't increase. So I guess the resistance of the iPad is lower than 5 ohms, but this causes "problems", as you put it. What exactly happens then? Does the power bank increase some internal resistance until the current is 1 A? Or will the voltage drop from 5 V to a value such that the current becomes 1 A? Or does this depend on the power bank?
The specific problem depends on the supply, but generally it is a lowering of the output voltage. It can also be overheating and damage to the supply circuitry.

Another question: Since I can charge my smartphone on the 1 A output, but my iPad only on the 2.1 A output, this should mean that the iPad's resistance R=U/I is smaller than the smartphone's. Can this somehow be explained by the different battery capacities or are there different charge electronics or something like that?
I think it has to be a difference in the charge electronics, not the battery capacity, but I'm not positive about that.

greypilgrim said:
I can only charge my iPad on the 2.1 A output. If I connect it to one of the 1 A outputs, it shows the charging symbol but the battery level doesn't increase.

If the iPad is ON then you are asking the Power Bank to provide current for two things... Charging the battery and running the iPad. If running the iPad takes more than 1A there won't be any left over to put into the battery.

I don't have an IPad so I can't check but... If you really get stuck it might be possible to charge the iPad from a 1A output if you switch the iPad OFF first rather than just allowing it to sleep. Many tablets will charge faster if you do this because virtually all of the available current can be used to charge the battery. I think on an iPad holding the power button down for 4 seconds will switch it off properly (requiring a reboot when you use it again).

davenn

## 1. How does a USB power bank provide both voltage and current?

A USB power bank has a built-in circuit that regulates the flow of electricity from the power bank to the device being charged. This circuit converts the bank's stored energy into a constant voltage and adjusts the current accordingly to provide a safe and efficient charge.

## 2. What is the difference between voltage and current in a USB power bank?

Voltage is the measure of electrical potential difference, while current is the measure of the flow of electric charge. In a USB power bank, voltage is kept constant while current can vary depending on the device's charging needs.

## 3. How do I know the voltage and current output of a USB power bank?

Most USB power banks have their voltage and current output listed on the packaging or in the product specifications. You can also check the labels on the USB ports or use a multimeter to measure the output.

## 4. Can a USB power bank damage my device by providing too much voltage or current?

No, a USB power bank is designed to provide a safe and regulated charge to your device. It will not provide more voltage or current than what your device can handle. However, using a power bank with a higher voltage or current output than your device's requirements can damage your device's battery over time.

## 5. Why do some USB power banks have multiple USB ports with different voltage and current outputs?

Different devices have different charging needs, and a USB power bank with multiple ports allows for flexibility in charging multiple devices simultaneously. The varying voltage and current outputs cater to different devices' requirements, providing optimized charging for each device.