A brain teaser...
 

You have a battery which measures 12.8volt off-load. When you place a 1amp. load on it (a 12watt lamp) the terminal voltage drops to 12.7volt.
What is the internal resistance of the battery? Two sig. figs. will do.
Where would you find such a battery?

Leedsman.

If you want an accurate answer then measure the resistance of the bulb, the figure of 12W is a nominal figure and any resistance you calculate from it could be off by quite a bit.

As for finding a battery by a specified internal resistance, I know Yuasa give it on their data sheets but never noticed it on car batteries

Regards

richard

Upon trying this with a real 12volt lamp, the measured current flowing was exactly 1amp. (The hot resistance will consume 12watt. The filament resistance is much lower when cold of course).
Try again. The data provided will indicate the internal resistance of the battery, be assured.

Leedsman.

0.098 ohms

Regards
Richard

Correct answer Tfb, and the place you're likely to find one like this is on....
A motorbike.

Leedsman.