Voltage to Light Problem
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JerrySchoolinYou
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Voltage to Light Problem
I’ve been working on a science project dealing with different brands of AA batteries. In my experiment, though, sometimes a lower voltage battery would output more light than a higher voltage. They’re all in the same flashlight model, and all ran at the same time. Does anyone have an idea why this is?
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bfinio
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Re: Voltage to Light Problem
Hi - this is a bit of a crash-course in electronics, so this may make more sense if you've take a physics class and are familiar with concepts like voltage, current, resistance, and Ohm's Law.
What you are seeing is probably related to the internal resistance of the different batteries. Batteries are not "ideal" - there is some resistance inside the battery itself. When you measure the voltage of a battery when it's not connected to anything else, you are measuring the open circuit voltage, because there is no current flowing from the battery. As soon as you connect the battery to a load (like a flashlight or motor etc), some current will flow. There will be a small voltage drop over the battery's internal resistance, meaning the actual output voltage of the battery "under load" will be lower than the open circuit voltage.
You can try to measure the voltage under load, but that can be difficult depending on the device you're using and whether you have easy physical access to the batteries when they're installed in the device, i.e. can you remove the battery cover and use the multimeter probes to measure the battery voltage while the device is operating. If that's hard to do, another option would be to purchase an external battery holder https://www.amazon.com/s?k=AA+battery+holder and use alligator clips to connect it to the battery terminals inside the flashlight. That will make it easier to access the batteries and measure the voltage while the device is on.
What I'm guessing you'll see is that the batteries producing less light have a lower voltage under load (and higher internal resistance) even if you measured a higher open-circuit voltage.
Depending on the brand, sometimes you can also look up the battery's datasheet online and find the internal resistance (it may be listed as "impedance" but explaining exactly what that means is a topic for another day - for now, consider it the same as resistance). For example, here's a datasheet for a Duracell AA battery, which lists the impedance (internal resistance) as 81 milliohms: https://docs.rs-online.com/2a27/0900766b814ef4c0.pdf. That is for a fresh battery. The internal resistance will increase over time, so if you want to compare battery brands, it's important to use new batteries for your tests.
If you want to learn more about battery internal resistance, Energizer has a good document explaining it here: https://data.energizer.com/pdfs/batteryir.pdf
Sorry if that was a lot of information, but I hope it helps!
Ben
What you are seeing is probably related to the internal resistance of the different batteries. Batteries are not "ideal" - there is some resistance inside the battery itself. When you measure the voltage of a battery when it's not connected to anything else, you are measuring the open circuit voltage, because there is no current flowing from the battery. As soon as you connect the battery to a load (like a flashlight or motor etc), some current will flow. There will be a small voltage drop over the battery's internal resistance, meaning the actual output voltage of the battery "under load" will be lower than the open circuit voltage.
You can try to measure the voltage under load, but that can be difficult depending on the device you're using and whether you have easy physical access to the batteries when they're installed in the device, i.e. can you remove the battery cover and use the multimeter probes to measure the battery voltage while the device is operating. If that's hard to do, another option would be to purchase an external battery holder https://www.amazon.com/s?k=AA+battery+holder and use alligator clips to connect it to the battery terminals inside the flashlight. That will make it easier to access the batteries and measure the voltage while the device is on.
What I'm guessing you'll see is that the batteries producing less light have a lower voltage under load (and higher internal resistance) even if you measured a higher open-circuit voltage.
Depending on the brand, sometimes you can also look up the battery's datasheet online and find the internal resistance (it may be listed as "impedance" but explaining exactly what that means is a topic for another day - for now, consider it the same as resistance). For example, here's a datasheet for a Duracell AA battery, which lists the impedance (internal resistance) as 81 milliohms: https://docs.rs-online.com/2a27/0900766b814ef4c0.pdf. That is for a fresh battery. The internal resistance will increase over time, so if you want to compare battery brands, it's important to use new batteries for your tests.
If you want to learn more about battery internal resistance, Energizer has a good document explaining it here: https://data.energizer.com/pdfs/batteryir.pdf
Sorry if that was a lot of information, but I hope it helps!
Ben