Two Identical Cells in Parallel: What happens?

[justadream]

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I know that placing two identical cells in parallel does not affect voltage. But what if the two cells are different? Do you just average their voltages?



Also, I know that when you have 2 identical cells in parallel, the current is the sum of those of the individual cells. Does this still apply if the cells are not identical?

 

[Cawolf]

I don't think you can connect cells in parallel that are not identical.

I think it would destroy the cells and I doubt we could see a question like that on the MCAT.

Unless there is a problem that asks about it?

Also the formulas would then show that x=y and this doesn't seem possible. I am relatively certain it wouldn't work.

 

[justadream]

@Cawolf

Wait, really?

Why would it destroy the cells?

Also, if that is true, doesn't that mean you could never connect cells in general? Because isn't it impractical to have identical cells (for example, 1 cell could contain some impurity and be .00001% different)?

 

[Cawolf]

Yes - the cell with the greater emf would drive a voltage through the lower emf cell that is greater than it's own.

If there was a 1 V cell and a 2 V cell the 1 V cell would have some minor resistance by the nature of all things. A very small amount but it would be there. The 2V would drive a massive current through it resulting in prompt overheating and destruction.

 

[justadream]

@Cawolf

So are you saying that any batteries that you connect (whether in series or parallel) need to be identical?

 

[Cawolf]

No just in parallel. If they are in series then the potential sums.

 

[JMMTB]

If the batteries are different and in parallel then current will flow. Current flows because of a difference in voltage. If one battery is 9V and the other 6V then there is a 3V difference between the cathodes and the weaker battery would be treated as a resistor, based upon its internal resistance (usually very small).

Because the resistance is small the current will be fairly high and the batteries can heat up. Also not all batteries can be recharged.

If it was a rechargeable battery, the stronger battery would discharge into the weaker battery until they were equal.

 

[Cawolf]

To recharge they also need to be wired in opposite directions though, correct?

 

[justadream]

@Cawolf
@JMMTB

Wait so JMMTB, you are saying connecting different batteries in parallel is possible?

Are you saying the resulting voltage would be the difference of the two cells? (what if you had 5 cells?)?

And the resulting current of this setup would be:

Voltage of the most powerful battery - [sum of internal resistance of the other batteries]

?

 

[JMMTB]

To recharge they also need to be wired in opposite directions though, correct?

Not necessarily. A simplified way of recharging a battery is thinking of the current flowing internally from the cathode(+) to the anode(-). In the case of two unequal batteries the stronger battery will force current to go internally from the cathode to the anode in the weaker battery and recharge it (or cause it to overheat).

Wait so JMMTB, you are saying connecting different batteries in parallel is possible?

Are you saying the resulting voltage would be the difference of the two cells? (what if you had 5 cells?)?

And the resulting current of this setup would be:

Voltage of the most powerful battery - [sum of internal resistance of the other batteries?

It is possible, but often very dangerous. (the MCAT would then add a question regarding chemical burns afterwards and whether is would be best to wash the area with water, a strong acid, a strong base, or a weak base)

To find the voltage we need to implement Kirchoff's loop rules. So sticking with a 9V and a 6V in parallel, cathode to cathode. Say we call the anode of the 9 V battery 0V or our "GND." As we travel through the 9V battery we are at 9V at the cathode and along the wire. Once we reach the smaller battery we drop 6V traveling from the cathode to the anode and then we find the problem. We have 3V on a wire that should be 0V (anodes connected). So there must be a 9V drop across the entire 6V battery, and there is. 6 V drop due to it's value and 3 V drop because of its internal resistance.

Say R=.1 and thus V/R=I 3/.1=30 amps! P=IV=90W!

Most batteries can't handle that and thus would fail internally[/user]

 

[Cawolf]

So to sum it up - it doesn't work in a standard circuit with standard cells.

 

[justadream]

@JMMTB

"(the MCAT would then add a question regarding chemical burns afterwards and whether is would be best to wash the area with water, a strong acid, a strong base, or a weak base)"

What would be the answer to this?

 

[2Pints]

@JMMTB

"(the MCAT would then add a question regarding chemical burns afterwards and whether is would be best to wash the area with water, a strong acid, a strong base, or a weak base)"

What would be the answer to this?

I think @JMMTB was kidding around. To answer your question, you generally wash chemical burns with water for a long time.

 

[JMMTB]

Ya I was just joking but hypothetical...

We can immediately eliminate the strong acids and bases because that would be really not smart to put on yourself. That leaves us with weak base and water.
For practicality rinsing with water is the best, but because this is the MCAT the passage would probably have a sentence that says something like the internals are super reactive with water and then you would answer weak base (baking soda or soap).

 

[justadream]

Okay so going back to the identical cells in parallel scenario:

The current of multiple identical cells in parallel is defined by TPR as the sum of the individual currents. How does that work?



Like how do you know what the individual currents are?

 

[JMMTB]

The current into a junction would be the sum of the individual currents leaving a junction but as far as solving for the current being pulled out of a two batteries in parallel you would need to know the full circuit. Just use Kirchoff's loop rules and junction equations.