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## Re: 9V battery testing; Thevenin equivalent; car headlamps.

 Subject: Re: 9V battery testing; Thevenin equivalent; car headlamps. Stanislaw Flatto Tue, 01 Aug 2006 12:23:13 GMT sci.electronics.basics, sci.electronics.repair
 ```Adam Funk wrote: ``````I recently tested a 9V alkaline battery by measuring its open-circuit voltage (9.0 V) and then measuring it with a car headlight lamp (R = 1 Ohm) across the terminals (4.0 V). The lamp lit up brightly and got warm, but from the significant voltage drop I conclude that the battery is basically dead. Correct? From those measurements I get a Thevenin model of the circuit as follows, where Rb is the battery's internal resistance and Rl is the load (lamp). - Vb + Rb -----|||||-----/\/\/\----- | | | | o o | | | Rl | -----------/\/\/\--------- With the load removed, and assuming the voltmeter is an open circuit, Vb = 9.0 V. With Rl = 1 Ohm in place and the voltage across o-o measured as 4.0 V, the loop current is 4 A. So Rb is 1.25 Ohm. Correct? Is there a rule of thumb for judging a battery as "still OK" or "dead" based on the calculated Thevenin resistance? I measured the headlamp as 1.0 Ohm, which in a 12 V car circuit ```(assuming a negligeable series resistance) should have a power of 144 W. Does that sound reasonable? ``` ```Your calculations are correct but they are on paper and your testing arrangement is in real life with real components so: a) R1 can and does change depending on brightness produced by ~1-10 or 20. (Read the Watts rating at 12- 13.7V --> car battery on charge). b) Battery has I dependant on rate where the production of current has some upper limit and then the voltage drops independant of Rb ``` ```So to test your measurment introduce in this circuit an ampermeter and do your calculations again. ``` Have fun Stanislaw Slack user from Ulladulla. ```
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