[billybob_jcv]

Hi everyone! My 10 year-old daughter wants to do a science fair experiment on the classic "Which battery lasts the longest?" I need a little help from the gurus...

My plan is to use a 2-battery AA holder to power a small clock and some sort of load device (motor, fan, light, etc). We'll set the clock to 12:00, and start the test. When the battery can no longer power the circuit, the clock will stop, and she can record the time the battery lasted. Simple enough!

However, I'm having trouble finding a device that will have the right load to drain the batteries in a reasonable amount of time. Ideally, the batteries would last LESS than 12 hours, so there's no confusion on whether the clock wrapped past 12:00, and she can just read the time directly off the clock every morning for however many batteries she wants to test. I need some ideas for a device that will do *something* (so it is interesting - a resistor load that just sits there and gets warm is too boring for a 10 year old), will work off 3v, and will draw enough current to drain AAs in less than 12 hours.

I tried a small hobby motor, but it was still running after almost 24 hours. I'm currently testing a small 12v computer fan, but I'm not hopeful. Darn it - too many things have been made too efficient!

Does anyone have any ideas for a cool (CHEAP) device that would fit the requirements?

 

[Dept_of_Alchemy]

You could wind your own homemade motor with some wire and two magnets, this way you can control how draining the device is. Better yet, you could teach your daughter how to wind a motor and let her try it herself. If DIY (cardboard and wire) is too downmarket, you could try buying parts for something like this and wind the motor inside it (these things stock will EAT AA batteries in minutes). When I was ten I was way into that, no kidding.

Larger gauge wire gives more torque, lower gauge more RPM. It's easy to fit some gears on it and make it do pretty mechanical things.

Quote:

Originally Posted by billybob_jcv /img/forum/go_quote.gif My plan is to use a 2-battery AA holder to power a small clock and some sort of load device (motor, fan, light, etc).

You meant to say 'her' plan, right?

 

[billybob_jcv]

Well of course it's her plan! She's the Project Manager, I'm just the poor grunt working under her relentless whip...

Thanks for the link - looking at the datasheet on that motor showed me something that I had completely neglected - that the current draw of the motor will increase dramatically when the motor is under load. Doh! For the tests we had the motor free running. We'll try again with something large enough attached to the spindle to just keep the motor from stalling.

If I shake the cobwebs off my brain, I think it works out to:

Typical AA Alkaline = ~2700 ma-h, so for 12 hours, 2700/12 = 225 ma. Therefore, I need to draw greater than 225 ma to have the battery completely drained in less than 12 hours. Is that correct?

 

[whiz]

Quote:

Originally Posted by Dept_of_Alchemy /img/forum/go_quote.gif Larger gauge wire gives more torque, lower gauge more RPM. It's easy to fit some gears on it and make it do pretty mechanical things.

and the number of windings what does it do?

 

[picklgreen]

Another motor you may want to try is the ones found at hobby shops used in remote controlled cars. They are fairly high torque motors so they drain alot of juice from a battery. Also...you mentioned you are going to just be using AA batteries. Be sure to find all 4 types of batteries, Carbon-Zinc(normal cheapy no name batteries), Zinc-Chloride (usually called heavy duty), alkaline (actually these are made of manganese-zinc - duracell and energizers), and Nickel-Cadmium (rechargeable). You are going to find that all batteries are the same that are made of the same type of materials. Alkalines last longer than Nickel-Cadium batteries and everything lasts longer than a rechargable. You might want to change your experiment to AAA size since these are lower current than AA so wont last as long thus giving better results (and less waiting time!).

What other types of batteries there are:

Mercury Cell (Watch batteries)
Silver Oxide Cell (Hearing aide batteries)
Lithium Cell (Cell phones, laptops)
Lead-Acid (Car battery)
Nickel-Metal-Hydride (Cell phones)
Nickel-Iron Cell (Electric trains)
Nickel-Zinc (Electric trains)
Fuel cells (cars)

 

[crazy_bob_uk]

Nickel-Metal-Hydride (Ni-MH) can also be found in AA bateries, these tend to be higher capicity than Nickel-Cadmium (Ni-Cad), but they cant provide as high a current.

 

[picklgreen]

I also relised that you can get lithium batteries in AA size now too!

 

[stewtheking]

If you are attaching gears to the motor, you could have the gears hooked up to some string and a weight, then when the motor pulls it would slowly lift the weight... The height the weight reaches would then give you an accurate measure of which battery pulled for the longest....

 

[OtterMarc]

billybob_jcv,

Just wondering. The motor may no longer turn, but the clock might keep right on going. Just a thought.

Marc

 

[Gontran]

You could also put a power resistor in paralel with the fan or motor.

You can find blinking led's at mouser, digikey, maybe even rat-shack, or one of those pretty multi-color flashing LED novelty things people hang from earrings and other body piercings...(I'm sure there's a name for those, I don't get out much...) or those spinning ever changing LED thingies they sell at the amusement parks (around central FL anyway...)

 

[eVITAERC]

Quote:

Originally Posted by OtterMarc /img/forum/go_quote.gif billybob_jcv, Just wondering. The motor may no longer turn, but the clock might keep right on going. Just a thought. Marc

Just want to stress that this is a HUGE issue. You must consider how to solve this first.

 

[OtterMarc]

Here's a possible solution for your daughter: Power the clock through a relay. Once the relay opens due inadequate current to hold it closed, the clock will stop. It needs to be started at the same time, of course.

An additional load may be required, a resister, or something that moves for the fair would be more appealing.

Relays are mechanical. This will not be as precise as our usual circuits we build here, though that may not matter unless the variance between one test set and another is small. That could become the issue with the relay actually, and using different loads may become part of the experiment to spread out the results in a meaningful way. Sounds like fun.

Find a way to help her figure out this solution on her own, if you can. A knowledge of relays might help. 'Here's an interesting device - a relay. Ever heard of it?..."

You'll need a DPDT relay wired to be self-keeping.

 

[error401]

Quote:

Originally Posted by OtterMarc /img/forum/go_quote.gif Here's a possible solution for your daughter: Power the clock through a relay. Once the relay opens due inadequate current to hold it closed, the clock will stop. It needs to be started at the same time, of course. An additional load may be required, a resister, or something that moves for the fair would be more appealing. Relays are mechanical. This will not be as precise as our usual circuits we build here, though that may not matter unless the variance between one test set and another is small. That could become the issue with the relay actually, and using different loads may become part of the experiment to spread out the results in a meaningful way. Sounds like fun. Find a way to help her figure out this solution on her own, if you can. A knowledge of relays might help. 'Here's an interesting device - a relay. Ever heard of it?..." You'll need a DPDT relay wired to be self-keeping.

A MOSFET like the 2N7000 might also work well here (better?), using the same technique. The threshold voltage is typically 2V so it shouldn't run for too long. They're both pretty crude though. To get any kind of scientific measurement what you want is a current source - with the transistor you're solely voltage switched, which will cut off for e.g. Alkaline batteries before they're dead - with the relay you're dependent both on current supply and voltage which means it'll react differently to different battery types. But it's just a science fair project, right

.

If I were doing it for my own experimentation, I'd probably use an LM317 configured as a CCS and a big power resistor (though a motor would work fine as well, provided you configure the CCS not to overpower it - or even the LM317 itself, but I don't like that idea...). I'd choose a voltage to call 'flat' across the resistor and choose a resistor value to generate maybe 0.5-1V more when in the current loop. Conveniently, the ~2V threshold of the 2N7000 works well with a 2-cell ~3V battery pack to be the switch here. When the voltage drops, the batteries can no longer supply the selected current at the cutoff voltage, and you'd cut power to the clock.

Maybe a bit much for a 10 year old, but maybe gives you some ideas...

I don't know if she (or you) care, but if you want to get more in-depth, it might be worth doing some investigation into how load current affects delivered energy, even with your simple configuration. The rate of discharge often has a great deal of effect on the amount of usable energy you'll get out of a battery - and this varies greatly between different types. Say, compare the runtime with 1 motor to the runtime of 2 motors in parallel. You should (might) find that running with 2 motors gets you nowhere close to 1/2 the runtime. Could be fun to discover that...

 

[OtterMarc]

Quote:

Originally Posted by error401 /img/forum/go_quote.gif ...with the transistor you're solely voltage switched, which will cut off for e.g. Alkaline batteries before they're dead - with the relay you're dependent both on current supply and voltage which means it'll react differently to different battery types. But it's just a science fair project, right .

Quote:

When the voltage drops, the batteries can no longer supply the selected current at the cutoff voltage, and you'd cut power to the clock. Maybe a bit much for a 10 year old, but maybe gives you some ideas...

Quote:

I don't know if she (or you) care, but if you want to get more in-depth, it might be worth doing some investigation into how load current affects delivered energy, even with your simple configuration.

Excellent ideas, Error401. Maximising output energy can be a science fair project for years to come as new battery chemistries emerge.

I'd be careful to not under-estimate a modern kid, especially if they've been looking over Dad's shoulder for years as he tinkered.

Makes me wonder if BillyBob has seen any of this.

 

[karimoh]

You could let your daughter do her own project?