Hi guys. I'm completely new to electronics. I need a little help actually. I will keep it short.

Let's assume I'm having a battery of 9000mAh(11.42 V, 74.9 Whr) and a device rated as 5V, 55mA. My question is if I'm powering the 5V, 55mA device with that battery. How long will the battery last in hours assuming it had full charge? I would like to know the calculation steps for this. Thanks.

 

If you just put a resistor or a linear regulator in to get the voltage down to 5 volts, the drain is 55ma.
9000/55 = 163.6 hours
If you use a switching regulator at 90% efficiency, the calculation is 74.9 WHr x 0.9/ (5V x 55 ma)

ps, 9AH x 11.42V = 102.78 WHr

 

If you just put a resistor or a linear regulator in to get the voltage down to 5 volts, the drain is 55ma.
9000/55 = 163.6 hours
If you use a switching regulator at 90% efficiency, the calculation is 74.9 WHr x 0.9/ (5V x 55 ma)

ps, 9AH x 11.42V = 102.78 WHr

Actually I was talking about my laptop. I'm plugging in a USB device to it which is rated 5V, 55mA and the laptop has a battery of 6000mAh(11.42 V, 74.9 Whr). So the hours will be calculated by 6000/55=109 hrs, right? or Should I take the Voltage and Whr into account?

 

Your laptop almost certainly uses a switcher to get the 5 V USB power rail. But it is not going to be 100% efficient. As a guestimate, use something in the 80% to 90% range. But you also have the load of the rest of the laptop while it is sitting there supplying power to the USB -- how much that is is anyone's guess.

If you already have the device, plug it in an time how long it takes for the battery to drop to somewhere around 50% charge.

 

Your laptop almost certainly uses a switcher to get the 5 V USB power rail. But it is not going to be 100% efficient. As a guestimate, use something in the 80% to 90% range. But you also have the load of the rest of the laptop while it is sitting there supplying power to the USB -- how much that is is anyone's guess.

If you already have the device, plug it in an time how long it takes for the battery to drop to somewhere around 50% charge.

So 6000mAh/55mA=109 hrs will be the answer while ignoring any other factors, right?

 

So 6000mAh/55mA=109 hrs will be the answer while ignoring any other factors, right?

Why do you think that your computer is using a linear supply for the USB supply rail?

 

The laptop will typically shutdown by design. The 5V calculation will likely be irrelevant.

 

The laptop will typically shutdown by design. The 5V calculation will likely be irrelevant.

I don't follow.

Yes, at some point the laptop will shut down, but when that will happen will depend on the state of charge of the battery. By default, it might also shut down after so many minutes of "inactivity" and having a device pulling power from a USB port may or may not be considered activity. But the user can almost always adjust the settings to prevent this type of shutdown, so now they are going to be shutdown based on battery state.

 

What you said. Then there are those USB charge ports too.