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

Discussion in 'Electronics Homework Help' started by shyronnie, Feb 20, 2012.

1. ### shyronnie

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Nov 1, 2011
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??

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
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.

3. ### shyronnie

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Nov 1, 2011
Why did my thread get deleted?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
I must have had your post selected when I deleted some other posts. Sorry. I've put it back.

5. ### shyronnie

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Nov 1, 2011
Steve: Which formula du i have to use for the simultaneous equations. I can't figure this one out.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
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.

7. ### shyronnie

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Nov 1, 2011
I found out. thanks.

8. ### john monks

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Mar 9, 2012
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: Mar 10, 2012  