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Draining a AAA battery

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npig

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Draining a AA battery

Hi,

My daughter has a science fair project that I need to help her with. She's testing whether the storage condition of AA batteries has an effect on their life.

I'm looking for some suggestions on how to build something that can drain a AA battery in around 10 minutes or so. Any suggestions on something cheap, easy and safe to build would be much appreciated.

Thanks!

EDIT: Mistakenly typed AAA instead of AA. AA is correct.
 
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You can completely fully discharge a cheap Chinese carbon-zinc AAA battery cell just by looking at it.
But if you try to discharge a name-brand AAA alkaline cell in only 10 minutes then it might be dangerous because the battery and the load will get very hot.
 
They are Rayovac alakaline AA.

10 minutes was just my way of saying "fast". Whatever fast and safe equates to is fine.
 
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The capacity of a standard alkaline AAA is about 1000 mAh but the capacity also depends on discharge rate. The faster you discharge the battery the less capacity you'll get. Just for the sake of this example we'll disregard that and assume that the capacity is set at 1000 mAh. To use up 1000 mAh in 10 minutes you would have to draw about 6 A of current out of the battery.

Capacity = Current draw*battery life

So

1000 = I*.1667hrs

I ≈ 6A.

You will have to connect a resistor between the positive and negative terminal of the battery to drain it but you have to make sure to choose a resistor suitable for this.

Since V = IR, to draw 6A we divide the battery voltage of 1.5 by the current 6

R = V/I = 1.5/6 = .25 Ω

Once agian to bound the problem we'll assume that battery voltage is constant even though it will decrease as the battery gets discharged.

The other consideration you need to address is the amount of power this resistor is going to have to dissipate. All the resistor is doing in essence is converting the stored energy from inside the battery into heat. If you don't size the resistor accordingly, it can burn up. To determine how much power will be dissipated use the equation:

P = IV

I = 6A
V = 1.5

So P = 9W

You need to make sure that the resistor is rated for at least 9W. Somewhere along the lines of 15W to 20W would probably be best.

Since I don't know how easy it's going to be for you to find a .25Ω resistor what you can do instead is place 4 1Ω resistors in parallel. That will have an equivalent resistance of .25Ω. it will also have the added benifit of spliting up the current draw and the power disipatted between the 4 resistors.

Each of the 4 resistors will only carry 1/4 of the total current and will only have to dissipate 1/4 of the total power so you can get away with 4 1 Ω 5W power resistors. like this:

Battery Discharge.PNG

Now all tha being said, I wouldn't suggest you wait until the science fair to try this. The battery will get hot and so will the resistors. Maybe to unsafe levels. Try this first in your garage or something with the kids far away.
 
Thank you for your reply. Could you break down the schematic for me? I'm not familiar with the correct way of reading them.

I actually made a mistake, She is using AA batteries.

I read, on Wikipedia, that "a general rule of thumb is that an AA alkaline battery can deliver 700mA without any significant heating". Would you say that's accurate?
 
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Thank you for your reply. Could you break down the schematic for me? I'm not familiar with the correct way of reading them.

I actually made a mistake, She is using AA batteries.

I read, on Wikipedia, that "a general rule of thumb is that an AA alkaline battery can deliver 700mA without any significant heating". Would you say that's accurate?

AA batteries are around 2600 mAh so a discharge rate of 700 mA would take:

T = 2600mAh/700mA = 3.7 hrs to drain the battery.

Probably less though because of what was stated in my previous post. Use the equations from my last post to determine the correct resistance and power rating to discharge your now AA battery in 10 minutes or whatever time you decide is practical.

Break down of the schematic:

1. Buy parts.
2. Connect each end of all 4 resistors together.
3. Connect opposite end of all 4 resistors together.
4. Connect one of the ends from all 4 resistors that were previously connected to the - side of the battery
5. Connect the other end from all 4 resistors that were previously connect to the + side of the battery.
6. Release from hand.
7. Step back.

The battery will get hot and I'm not sure what else. Be careful. If you acidentally open up a worm hole, you don't know me.

Edit:

I read, on Wikipedia, that "a general rule of thumb is that an AA alkaline battery can deliver 700mA without any significant heating". Would you say that's accurate?

I don't know.
 
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I would recommend not discharging the battery at more than a few amps (which would probably take a half hour to an hour to discharge). You could use one or more bulbs in parallel such as these for a load. That will give an visual indication of the discharge, and the brightness would give you a voltage indication.
 
AA batteries are around 2600 mAh so a discharge rate of 700 mA would take:

T = 2600mAh/700mA = 3.7 hrs to drain the battery.

Probably less though because of what was stated in my previous post. Use the equations from my last post to determine the correct resistance and power rating to discharge your now AA battery in 10 minutes or whatever time you decide is practical.

Break down of the schematic:

1. Buy parts.
2. Connect each end of all 4 resistors together.
3. Connect opposite end of all 4 resistors together.
4. Connect one of the ends from all 4 resistors that were previously connected to the - side of the battery
5. Connect the other end from all 4 resistors that were previously connect to the + side of the battery.
6. Release from hand.
7. Step back.

The battery will get hot and I'm not sure what else. Be careful. If you acidentally open up a worm hole, you don't know me.

Edit:



I don't know.


It sounds like 10 minutes is a bad idea. Since she doesn't have to demonstrate this at the science fair, I think we'll play it safe and try to build something that takes a an hour or so instead. I definitely don't want to open up any wormholes. :D

Thanks again!
 
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I definitely don't want to open up any wormholes. :D

I say go for wormholes!
Think big!
If you can make it repeatable, your daughter can pass on the science fair and go for a Nobel Prize.

JimB
 
Or they have to explain to the insurance adjuster why a school project burnt down their house....
 
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