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multiple currents in series
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multiple currents in series

by dragonuv on Sat Mar 07, 2009 9:34 am

I know what happens to batteries in series in terms of voltage and batteries in parallel in terms of current, they sum up.
but what happens to multiple batteries in series in terms of current(when the current of each battery varies) and batteries in parallel in terms of voltage(when voltage of each battery varies)?

is it like a resistor in parallel? (1/Rt=1/R1+1/R2)
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Re: multiple currents in series

by zener on Sat Mar 07, 2009 3:34 pm

I am not sure I quite understand your question but here is my answer:

Regarding current, when you have a circuit "loop", regardless of what makes up the loop, the current is the same at any point in the loop. Batteries do not have an inherent current the way they have an inherent voltage. They have a maximum current they can deliver, but they will only give what is needed. For example, when the battery is disconnected, it "gives" no current.

When you put two batteries of different voltages in parallel, for example, the battery with the higher voltage will see the lower voltage battery as a low impedance load. So, the higher voltage battery will "charge" the lower voltage until they are the same. This is tolerable if they are chargeable type, but not so good if they are primary (non-chargeable) type.

Does that help?

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Re: multiple currents in series

by dragonuv on Sat Mar 07, 2009 8:27 pm

thanks zener, it pretty much helped. although i knew the first explanation when the battery deliveres the needed current.
now, what happens when the 2 batteries with different voltages in parallel are solar panels in stead of batteries? does the panel with the higher voltage damages the one with the lower voltage?

and what happens to 2 solar panels in series that provide 300ma and 500ma?
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Re: multiple currents in series

by magician13134 on Sun Mar 08, 2009 1:47 am

There was once a man who wondered exactly the same things. His name was Kirchhoff (wayyy too many letters or something) and he came up with a couple of laws that explain everything that you need to know. Basically Zener nailed it, but if you wanted to look a little further into it, here's some good reading: http://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws

As far as I know, solar panels will simply act like batteries in this case. Also, when charging a battery with a solar panel, if the solar panel output voltage drops below that of the battery, the battery will drain back into the panel. To prevent this, people use diodes, while you lose a little voltage, it'll be better than losing the panels. "Solar" diodes are usually Schottky (sorry, Zener :) ) because they lose the least amount of voltage. (Although I've read that diodes are really only necessary in panels producing 36V or higher, but you may just want to play it safe)
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Re: multiple currents in series

by dragonuv on Sun Mar 08, 2009 6:59 am

too bad this year in physics they dont teach kirchhoff's laws :(

i got it, batteries in series with different voltages is not good because the battery with the lower voltage can be charged by the one with the higher voltage.

http://img15.imageshack.us/my.php?image=parallel.png

what about the picture i attached? in this case the battery with the lower voltage cannot be charged because it has a diode, what can be the output voltage in this case? and is it the same with solar panels?

and what about batteries\solar panels in series with different currents? what is the output current with a circuit like that?
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