# Internal Resistance of a Battery

I am currently doing a lab trying to calculate internal resitance of a battery. Do I use the formula V=IR?

## Answers and Replies

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What do you denote by R? The circuit connected to the battery also has resistance?
$$Emf = I(R_{batt} + R_{circ})$$

Using a variable resistor, take different values of R and find the values of the current (I) and voltage (V). Draw a graph of V against I, find the slope as it'll give you -r (which is the internal resistance of the battery) .

you mean to say, that an ammeter or voltmeter would come to effect my results?

and that Internal Resistance is relative to the electric current.

JoakimvE said:
you mean to say, that an ammeter or voltmeter would come to effect my results?
What do you mean? . Surely you know that if you increase resistance, current will decrease and voltage will increase?

yes... ofcourse i know that... but i mean, is Internal Resistance of a Battery a constant or not?

Yes, it is constant.

okay...
damn, my values are confusing the hell out of me...

JoakimvE,
Hey it's simple. What are the steps of the experiment you're conducting to find internal R? Are you using a variable resistor or only adding resistors to the circuit?

Integral
Staff Emeritus
Gold Member
JoakimvE said:
you mean to say, that an ammeter or voltmeter would come to effect my results?
This depends on the meters you are using. If you are using a modern digital meter, then then I doubt that you will be able to detect their presence. If you are using an old fashioned analog meter with a D'Arsonal/needle movement, then yes, they will have a measurable effect.

An ammeter is an addional resistance in series and a voltmeter is a resistance in parallel to the measured resistance.

variable resistor

okay, thanks a lot. im understanding this way more now (even though im not much of an electrician :zzz:).

anyhow, i guess the main problem is how much i varied the resistance by. i just varied the electric field (E)... im guessing i cant calculate the resistance i added.

Emf = V(terminals) + V(lost)

JoakimvE said:
anyhow, i guess the main problem is how much i varied the resistance by. i just varied the electric field (E)... im guessing i cant calculate the resistance i added.
You actually don't need to.. First, short circuit the battery to find its emf. Set up your circuit with the variable resistor, voltmeter and ammeter connected properly. Now increase the resistance by moving the slide and record 5 reading for voltage and current (from the voltmeter and ammeter)

Plot a graph of V against I and calculate the slope as it would represent -r The straight line you've drawn is represented by this equation:

V = (-r)I + E
E: battery emf
-rI: voltage lost due to internal resistance
V: voltage across terminals, measured by voltmeter

Hope that cleared things out for you, if you need me to explain it more let me know, I'd be glad to help.

no i got it. thanks a lot.