What would happen if you connected two chargers to each other?

[miles_muso]

Hi All,

 

Newbie to this forum. I did a searxh on 'scientific forums' and this one looked quite likely to answer my query. I hope you can help........

 

A friend and I were discussing a theoretical situatuation where two USB power packs (the type that act as emergency chargers for phones, iPads, etc) were connected to eachother in such a way as they could both draw power from the other and could both charge the other. (It must be possible to connect power packs in this way, as a single power pack can draw electricity from the mains whilst providing power to a device via a USB charger, for example.)

 

We assumed that the power packs would start off at 100% full, but can this be the case? Power packs drain (atrophy?) if left for some time, so this must mean that a fully charged power pack actually starts to lose power immediately after being charged to 100%, even if the rate at which it loses power is a fraction of a percent. We also need to factor in the amount of energy lost as heat from the cables connecting the power packs.

 

That being the case, as soon as you plug one power pack into another they would start to draw top-up power (dependent on the accuracy of the governing firmware). And a further assumption would be that, unless the packs were connected to each other at EXACTLY the same time, one would take presidence over the other and would be the 'most greedy'.

 

Is there any scientific principal behind our theoretical situation, or is the answer always going to be 'it depends'?

 

Please note: I am not, in any way, a scientist. The terms I am using above are probably incorrect, and there are quite possibly some basic priciples and accepted theory that I have neglected. We're just two friends who wants to know a little more about a topic we were discussing.

 

Thanks

 

 

[studiot]

Hello, Miles and welcome.

 

It is not generally a good idea to run batteries (for that is what usb power packs are) or other sources directly in parallel.

 

Running in parallel means that you connect in the way you are suggesting.

 

Trying to charge two packs together from one charger may overload the charger if it is not self protected.

Or it may simply be unable to fully charge either pack.

 

Yes if you connect two (partly) charged packs together then the charges will be distributed in both.

So you can recharge one pack from another, but obviously a pack of small capacity will not be able to fully charge one of a larger capacity.

This will never be as good as a proper charger since the packs are designed to dupply the usb votlage of 5 volts, whereas a charger will offer a higher voltage.

[Mordred]

to add details to studiot's post chargers are not just voltage and current, they use a type of circuit called pulse width modulation, the amount of load the circuit can support is specifically designed for the load application. A PWM circuit can provide for a higher load however this requires a change in its duty cycle, Ideally the duty cycle is 50% higher current loads than the designed specs require a higher duty cycle which leads to faster component failure. A good example is a PC power supply.

 

Yes a 300 watt power supply can supply higher wattage but its lifetime decreases.

 

http://en.wikipedia.org/wiki/Pulse-width_modulation

 

the circuit for usb voltage and current levels is also a PWM signal

the circuit here is a type of pwm controller

 

http://www.electroschematics.com/4983/usb-mobile-charger/

Edited July 28, 2014 by Mordred