When you connect the USB output to USB input, you are just connecting the negative (ground) terminal to the negative, and positive (+5V) to positive. Like when you jump-start a car.
When you connect two ends of a battery, you are connecting the positive to the negative.
Because that is not what is happening. The positive of the USB input connector is connected (eventually) to the positive of the battery which is connected (eventually) to the positive of the USB output connector. Ditto the negative.
In addition to the above advice, also watch out for shoddy products. Once a month, I like going to youtube, and watching some of the content creators there post 'tube’s about crappy electronics.
Inside a fake "10400mAh" eBay USB power bank. - YouTube <- this fella is from the Isle of Man, IIRC, so he is dealing with some stuff that might not be available in the U.S. The point is the same, though. Buyer beware!
Unlikely. If you supply voltage to to the input jack, the input SMPS delivers a voltage to the battery that is slightly higher than the battery’s own voltage, causing the battery to charge. this means the output SMPS receives a slightly higher supply voltage than it’s expecting from the battery. You might think this results in an elevated output voltage from the output SMPS, but nope. SMPSs are generally designed to tolerate a range of input voltages, from which they deliver a constant output voltage. Example, have you seen flashlights advertising how they deliver constant brightness for most of the battery’s life? They’re using an SMPS to deliver constant voltage to the lamp, even as the battery’s voltage decreases over time. This is why we use them.
The only issue is that if the input and output SMPSs are both delivering power, then they are both generating waste heat (instead of just one of them). This means you’re dumping more heat into the case, but not much. Part of why SMPSs are so popular is they’re pretty efficient. If you’re moving 7.5 watts through one SMPS with an efficiency of 95%, then you’re dumping 0.375 watts of waste heat into the case. If you’re running both SMPSs because you plugged your device’s face into its ass, then you’re dumping 0.75 watts of waste heat into the case - still not much at all. I wouldn’t expect an overheat condition from this.
Note that laptop computer power supplies generally allow pass-thru charging: you can plug them into the wall, and the output will continue to supply your computer while the battery charges.
:smack:
A more accurate description of what is happening is this is equivalent to connecting the + and - terminals of a battery with a wire and also a large resistor that limits the flow of current. This is what the circuitry in these battery packs effectively does. So you expect the same thing to happen - the resistor would get warm and the battery would deplete over some time.