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# Calculating electromotive for, and internal resistance of a battery.

#### shyronnie

Nov 1, 2011
25
From a battery a current of 2.0 A, with a voltage of 114.2 V. When Amperage is increased to 8.0 A, the voltage drops to 112.6 V.

Find electromotive power, and the internal resistance of the battery.

How do i do this? Simultaneous equations with the formula r_i=(E-U)/I or by differentiation, or??

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Think of a battery as being a perfect voltage source in series with a resistor.

In this case you don't know the actual battery voltage, OR the value of the resistor.

But you do know how much extra voltage the resistor drops when the current changes.

You can use that to determine the value of the resistor.

You could also solve it as a pair of simultaneous equations, but this way is easier if you are familiar with Ohm's Law.

#### shyronnie

Nov 1, 2011
25
Why did my thread get deleted?

My thread got deleted. Why?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
I must have had your post selected when I deleted some other posts. Sorry. I've put it back.

#### shyronnie

Nov 1, 2011
25
I must have had your post selected when I deleted some other posts. Sorry. I've put it back.

Steve: Which formula du i have to use for the simultaneous equations. I can't figure this one out.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
You need an equation that has as unknowns the battery voltage and ESR as unknowns, and where the current and measured output voltage are the knowns.

You will have two equations with two unknowns and you can then solve for them.

#### shyronnie

Nov 1, 2011
25
You need an equation that has as unknowns the battery voltage and ESR as unknowns, and where the current and measured output voltage are the knowns.

You will have two equations with two unknowns and you can then solve for them.

I found out. thanks.

#### john monks

Mar 9, 2012
693
The change in battery voltage is small. Therefore you can come up with a close approximation of the internal battery resistance by ohm's law. You take the change in battery voltage and divide it by the change in current. You get [the answer -- which we don't provide]

Last edited by a moderator:

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