leidig32
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batteries in a flashlight

Postby leidig32 » Thu Feb 05, 2015 11:20 am

my son is in 2nd grade and loves playing with flashlights. I found a simple idea to find out which brand of battery is the best. I bought three AAA eveready LED mini flash lights. Used eveready, Kroger and Duracell. from everything I have read these should last 12-24 hours. We have now had these flashlights turned on for 4 days and they are still going strong. From your experience how long could these end up lasting? If more than one more week we are in trouble. Figuring we have to do this same experiment over again to determine the average. Is this just crazy that after 4 days all three flashlights are still working?
Thanks for your expertise!
LeAnne

HowardE
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Re: batteries in a flashlight

Postby HowardE » Thu Feb 05, 2015 12:01 pm

Except for the cheapest junk batteries, most AA or AAA batteries are pretty similar across brands. The question is how much light does the flashlight put out and consequently, how much power/current is the LED drawing?

Many of these cheap LED flashlights pull perhaps 20mA through the LED. A AAA battery typically can provide 1000 mAh which at 20mA is (more or less) 1000/20 or 50 hours. It gets a little more complicated when you go from one red/yellow/green LED to a white one and to two batteries, but a white LED requires a higher voltage. So the rough mAh capacity calculation is still somewhat reasonable. I'm actually surprised it's still lit after 4 days but I'll bet it's a lot dimmer than it was. The battery manufacturers consider the battery to be pooped out when it drops from 1.5V to 1.1V with a load on it. I'll wager that after 4 days of bring on, your batteries are technically 'dead'. You have to determine at what point you'll call the test completed. It may linger with a very dim glow for a while longer if it's lasted this long. But yes, I'm a little surprised you got more than 2-3 days.

Duracell publishes all sorts of test results for their batteries. Take a look at http://ww2.duracell.com/en-US/Global-Te ... heets.jspx (your son may want to include some charts if he's making a report or entering this in a fair). From their data, their AAA batteries should have had trouble lasting a full day - so something is odd. It would be wonderful if you had access to a multimeter and could measure the current draw to make some calculations.

It's a cool experiment at that age. All kids love flashlights - even us grownup kids!

Howard

leidig32
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Re: batteries in a flashlight

Postby leidig32 » Thu Feb 05, 2015 12:34 pm

One flashlight is MUCH more dim than the other 2. It went dim the first day but is still holding on. So, maybe make the experiment how much battery life is left after 24 hours and buy a multimeter to check this? Or do you think they will finally turn off soon? UGH!!! What to do?????

HowardE
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Joined: Thu Nov 20, 2014 1:35 pm
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Re: batteries in a flashlight

Postby HowardE » Thu Feb 05, 2015 12:45 pm

Flashlight vendors refer to life as "usable light". That is, sure the light is still visible if you stare at it in a dark room, but does it actually light anything up? You have to decide some scientific and deterministic way if declaring a flashlight "dead". If you have a light meter you could measure the output of the beam at some distance. You could watch the voltage of the batteries and when they drop from 1.5V per cell to 1.1V, call that dead.

If you do measure the batteries with a multimeter, remember that the voltage will be higher when the flashlight is off. So you need to measure it with the flashlight on. Some thin wires taped to the batteries before you stick them in the flashlight ought to be able to poke out so you can make the measurement.

You're going to have to invent your own method here, but I'd suggest one of those two. Whatever you do, make it as repeatable as possible if you want it to pass scientific muster and win that Nobel prize. :)

Howard

leidig32
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Joined: Thu Feb 05, 2015 11:01 am
Occupation: parent

Re: batteries in a flashlight

Postby leidig32 » Thu Feb 05, 2015 12:48 pm

Thank you Howard! I do not beleive we are near the Nobel Prize just trying to keep up with second grade!!!!

bfinio
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Re: batteries in a flashlight

Postby bfinio » Fri Feb 06, 2015 1:20 pm

Hi leidig32 -

Howard has provided some great advice so far. I just wanted to throw in a couple more things:

1) You can get "lux meters", which are used to measure the intensity of light, pretty cheap on Amazon: http://www.amazon.com/s/ref=nb_sb_noss? ... =lux+meter. So, if you wanted to quantify "how bright" the flashlight is with a number (instead of just looking at it with your eyes and assigning values like "really bright, bright, medium, and dim"), you could use one of those. Note that you would need to take the readings in a dark room where the lux meter's reading will not be contaminated by other light sources, like ceiling lights or windows.

2) If you decide to go with a multimeter and voltage readings instead, we have an entire tutorial on how to use one (just in case you haven't before): http://www.sciencebuddies.org/science-f ... rial.shtml. You can also find plenty of multimeters on Amazon, or get one at a Radio Shack if you have one nearby.

3) You might not have time to do this now, but for future reference, you might be able to see the results "faster" if you use something like a toy car that has motors or other moving parts, instead of a tiny flashlight. The toy car will draw much more power, draining the batteries quicker. We have actually noticed that with some of our robotics projects, where Energizer or Duracell last much longer than off-brand ones.

Good luck!

-Ben


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