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Topic: 2 different battry 's parralel (Read 544 times) previous topic - next topic

nickn4

hi there,

what would happen with the current and voltage, if i put two different batterys parralel?

for example, when i link 1 battary wich has 6V & 200mA with a battary wich has 0.5V & 800mA parralel..
will the result be something like 6V & 1A?

i want to know this so i can but my power sources together for my projects.

thanks for helping me out,
nick
humankind cannot obtain anything without
sacrificing something of equal value in return.

majenko

The result will be one battery trying to recharge the other and, at a minimum, power wasted.

The batteries should be isolated from each other with diodes or p-channel mosfets.

nickn4

i made a schematic..
so the blue's are the readings.. are these like i though or have i messed up the diodes?
humankind cannot obtain anything without
sacrificing something of equal value in return.

majenko

#3
May 30, 2012, 01:12 am Last Edit: May 30, 2012, 01:23 am by majenko Reason: 1
Overkill on the diodes there.  You only need one per battery.

You'll get 5.3V out.  The diodes (unless you select schottky diodes) have a 0.7v voltage drop across them.

You will be able to draw the 200mA from the 6v battery.  The 800mA from the 0.5v battery will be doing nothing.

The potential at the anode of the diodes will be 5.3V.  This is larger than the potential at the cathode of the low-voltage battery diode, so the diode will be not conducting.  The low voltage battery will be switched out of the circuit and will therefore be completely pointless.

oric_dan

#4
May 31, 2012, 07:23 pm Last Edit: May 31, 2012, 08:55 pm by oric_dan(333) Reason: 1
Quote

what would happen with the current and voltage, if i put two different batterys parralel?


IF you're gonna play with electronic circuits, you need to invest some time in learning
basic circuit theory. The first thing to learn is Ohm's Law.

http://en.wikipedia.org/wiki/Ohm%27s_law

As indicated, if you tie 2 nodes together, the current between the nodes is related
to the voltage difference between the nodes and the resistance from one to the other.

In your case, V = 6V - 0.5V and R = 0 ohms. Now calculate the current using
I = V / R.



majenko


jackrae

#6
May 31, 2012, 07:38 pm Last Edit: May 31, 2012, 10:54 pm by jackrae Reason: 1
And what type of battery gives 0.5 volts when charged ???

No questions should ever be considered as silly since the asker is obviously seeking an answer to a problem but he/she really needs to consider what is being asked.

In answer to the specific question :  the battery with the highest voltage will endeavour to force the voltage of the lower battery up to a point where both batteries have equal voltage.  This is accomplished by a current (usually quite high) flowing from the higher into the lower.  The nett result is that the higher voltage battery will lose AH capacity and the lower battery will gain AH capacity.   However, if the voltages are vastly different, as in this case, and the battery impedances are relatively low (not specified in this query) the high current may catastrophically destroy either or both batteries, ie they may explode.  They certainly will heat up unless some device (eg resistor) is used to limit the current.

To be safe, only batteries of similar design and capacity should be paralleled to increase capacity and ideally should have an isolation diode for each battery to prevent one discharging into the other or others.

You also have problems with how to evenly charge batteries in parallel - a whole different topic governed by battery chemistry.

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