# What's the capacitance of a typical car's battery?

1. Feb 5, 2012

### fbs7

Question for fun.

A car's battery is not a capacitor, but it seems to "accumulates" charge the same way. So, if you take a typical fully charged car's battery, what would be the equivalent capacitance of a 12V capacitor that would have the same charge?

For an extra kick - try to guesstimate its order of magnitude before comparing to the actual calculations! My guesstimate: 500F.

2. Feb 5, 2012

### Jony130

The typical car battery has a 55Ah = 198000 coulombs
C = Q/V = 198000C/12V
But such comparisons don't have any sense.
Capacitor is not a Battery.
The capacitor has a different discharge characteristics than the battery.
But probably you know that already.
So what is the purpose of this thread ?

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3. Feb 5, 2012

### fbs7

Thread is for fun - a battery and a capacitor have the same function, as they store and release an electric charge - so I hoped to have a bit of interesting discussion here. The operating principle is quite different - one does that through an electric field and a dieletric, while the battery does that through chemical energy (which is another form of an electric field), therefore the discharge profiles are different - but then an actual capacitor is similar, but not exactly the same as an ideal capacitor, specially if one chooses an odd dieletric.

Answer of about 15,000 F is right, at maximum charge, what is pretty amazing. At smaller charges the equivalent capacitance decreases. I bet if anyone would imagine that just by guessing.

To this I was planning to link a question about what's the equivalent capacitance of a cloud, but it doesn't look like many people finds this kind of discussion interesting. Perhaps in the physics forum...

Last edited: Feb 5, 2012
4. Feb 5, 2012

### Jony130

From what I remember our mother earth has capacity in the area of 70μF.

5. Feb 5, 2012

### Antiphon

Closer to 700uF.

6. Feb 5, 2012

### fbs7

Wow, both 70u and 700u are pretty amazing!

7. Feb 6, 2012

### psparky

Hmmmm....not sure a battery can be compared to capacitance.

What brings me to this amazing statement? Well....this.

A capacitor has a resistance....or more accurate....a reactance of 1/JWC.

Or in simpler terms....1/WC.

In other words the reactance of a capacitor is dependent on the frequency.

A battery is DC....therefore it has a frequency of ZERO.

If you plug zero into the reactance of a capacitor...1/WC....you get infinite resistance.

A battery clearly has a finite resistance.

Just food for thought to spur further discussion. Interesting question though.

Then again....a battery does stop charging when full and it becomes an open circuit just like a capacitor when fully charged....and when the battery is low current flows much faster like a capacitor when being charged.

So we are actually talking about the transient state of a capacitor in a DC enviroment......produced by the battery itself...or produced by an alternator...lol

Wow...now I'm really confused...lol.

Last edited: Feb 6, 2012
8. Feb 6, 2012

### Averagesupernova

I don't think there are any arguments here about the differences between capacitors and batteries. However, just because there are differrences does not mean that there are no similarities.

9. Feb 6, 2012

### psparky

Clearly....because you can charge a capacitor with battery......and you can charge a battery with a capacitor!

10. Feb 6, 2012

### fbs7

I think that Jony130 got a good approach on this thing of capacitor x battery, which is that in an ideal capacitor Q = CV, while a battery has a non-linear relationship between charge and voltage. Therefore, a single equivalent capacitance would be one that depends on voltage, not a constant.

An interesting thing, though: this is NREL (National Renewable Energy Laboratory)'s model "2RC" of a battery:

http://www.batterydesignstudio.com/system/files/images/NREL-2RCo.preview.jpg

So they simulate a battery as being two capacitors and 3 resistors. This is the simplest model I could find for a battery - the other ones seem pretty more complicated. Model diagram courtesy of "http://www.batterydesignstudio.com". [Broken]

Last edited by a moderator: May 5, 2017
11. Feb 6, 2012

### psparky

This makes sense. When I was spewing out my dissertation....I was thinking there must be some sort of capacitor either in series or in parallel with eachother....or that monstrosity you just showed us.

So to answer the 2nd question of this thread...what is the purpose?

The purpose is simple....to learn.