9V battery testing; Thevenin equivalent; car headlamps.

Nice experiment and good math practice but that battery was not designed to supply that much current. You really can not tell much about the battery from that test. Get the test specs on the battery and load it properly and repeat your experiment. BTW: The resistance of the lamp filament will not stay the same as it heats up. Tom

No, 1,25 ohm ESR (Equivalent Series Resistance) for a 9V battery is quite good. For a 1,5V "D" cell i would consider it too high.

Yep. But remember that the 1 ohm of the lamp is measured in cold state. At 9V it is probably a lot more.

It depends of the size, technology and voltage. A large "D" cell will have in its new state an Rb of about 0,1 ohm or even less. A new 9V battery might give you somathing like 1 ohm.

No, the resistance will increase by a factor of 2 or 3 as the lamp gets hotter. Connect it to a decent power supply (or car battery) and measure the current. That will give you the corrent operating wattage and resistance.

--
Joop van der Velden - pe1dna@amsat.org
Het hele jaar door radiovlooienmarkt op:

If this is a typical 9V battery as in:

then the usual load is in the neighborhood of a few tens of mA up to maybe a couple of hundred mA at greatly shortened service life. If your load current was really as high as 4A, then if the battery wasn't finished before it probably is now.

Also, the car headlight will increase in resistance as its filament gets hotter, so your load might have started at 1 ohm, but would have risen considerably very quickly.

"Joop van der Velden"

** More like 10 or 12 times, actually.

Depends just how white hot the filament becomes.

Halogen projector lamps with 50 hour rated live increase by a factor of about 16.

....... Phil

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

1. (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.

Adam Funk ha escrito:

Problem is that a headlamp isn=B4t an adequate load for a 9V battery. A

5W tail lamp would be a better load, so the test you made is esentially meaningless, even if the math model is perfect.

I haven't got an ammeter but "fun" would be a good excuse!

Interesting. I though a 44% voltage drop sounded like a lot, but as you and others have pointed out, the load resistance I've used is very low. What sort of resistance do I really need for this sort of test?

Right. I measured the lamp's resistance with an ohmmeter, which of course puts very little current through it.

But I took the measurements by clipping the voltmeter (actually it's the same meter) leads onto the battery terminals, reading the open-circuit voltage, then pressing the lamp's terminals against the battery terminals (the spacing was convenient --- that's where I got the idea from) and immediately reading the loaded voltage (before the lamp heated up).

Thanks!

So a load in the area of 270 to 620 Ohm (the examples in that data sheet) would be much more suitable.

Oops!

It might be dead now that you've run it at 4 amps, but 9.0 V open ckt is perfectly fine. When the o.c. voltage drops down to 7 V or so it might be considered dead, but that would really depend on what you're using it for (i.e. how much is the load).

Next time, try testing it with 470 to 1k ohms. Even better would be if you know the load resistance of your intended application.

Regards,

Mark

By the way, some 9 volt batteries are made up using cells that are not welded together, and only touching each other, which can lead to higher ESR and unreliability. I don't know if there is another battery which has been made like this. Beware.

GS

How about the ASCII art? ;-)

No. A 1-ohm load is way too low a resistance to use with a 9V battery; the excessive current that flowed (over 4 amps) will damage the battery. The battery may be dead *now* because you killed it. Or, if the abuse was only momentary, it may be OK.

I would have tried maybe a 100-ohm load, drawing 90 mA, which is a typical (rather heavy) load on a 9V battery.

If you're sure the load was 1.0 ohm, then the total resistance in the circuit is 9/4 ohm, so indeed, Rth is 1.25 ohms.

That is a rather low Rth for a 9-volt battery. As much as 5 or 10 ohms might be OK.

Indirectly, yes. At the current that the battery is designed to supply, the voltage should be at least 90% of full voltage.

Actually, in this era of digital voltage meters, I measure Vth, not Rth. Vth drops from well over 9.0 V for a new battery to 8.8 V for one that is showing its age.

Yes. Headlamps normally draw something like 10 amps at 12 V, which would imply about 1.2 ohms. And that is approximate.

For the handful of batteries I've measured, I get 9.4 V when they are brand new.

Mark

I was under the impression that the open-circuit voltage wasn't a reliable measure of how much "juice" is left in a battery.

Adam Funk: You are absolutely correct about open circuit testing of batteries is useless.... many otherwise almost dead batteries will have a surprisingly high open circuit terminal voltage...... ....BUT the test LOAD (car headlamp) that you are using is not much more than a dead short compared to the design capabilities of the 9V battery. These batteries were designed with practical average load currents of 20ma and occassionally will be found in devices using 100ma or possibly a little more. A good and practical load that I have used for actual evaluation and comparison with other 9volt batteries of questionable freshness is a 270 ohm , 1 watt resistor, which gives a load current of about 33ma.... this will "test" the battery terminal voltage with a suitable load and will not ruin or drain the battery while doing it. Best Regards, Daniel Sofie Electronics Supply & Repair

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It wasn't back in the days of analog meters with 10% accuracy. It is now that we have digital voltmeters with very high input impedance and 1% accuracy.

Shorting this kind of battery for some seconds won't do permanent harm.

At high loads, the ESR of the battery will change with time, too. Connect a 9v alkaline battery to an ammeter. It will start at, say, 2 amps and drop off as internal polarization kicks in; the decay and recovery time constants are very roughly in the area of a minute. At lower currents, ESR is pretty steady.

John