Simple electronics question - need easy help!

[bbb74]

Ok I want to make a device that will let me flatten 4 x 2000mAh AA NiMH batteries in series in about 1 hour, for as little $$ as possible, and it doesn't have to be fancy.

I was going to get a 4xAA battery holder (series) and then attach that to something. I know little about electronics and I think I need a power resistor? How many ohms resistance would I need? If you can show me how to do the maths I will learn something... :) I have tried already and came up with 2.4 Ohms, and capable of handling 10 watts, but that is probably wrong...

 

[Mr Happy]

Ok I want to make a device that will let me flatten 4 x 2000mAh AA NiMH batteries in series in about 1 hour, for as little $$ as possible, and it doesn't have to be fancy.

It's really not good to do this. If you try there is a danger you will damage one of the batteries by reverse polarization at the end of the discharge.

But let us suppose you are intent on harming your batteries. Then the calculations would go like this:

Approximate voltage = 4.8 V
Current required = 2 A
Resistance required = 4.8 / 2 = 2.4 ohms
Power required = 4.8 x 2 = 9.6 W

Therefore a resistance of about 2.5 ohms and 10 W power handling would indeed do the job. The resistor will get quite hot: don't let it touch anything that might be damaged by heat.

The safer way to do this would be to discharge each cell individually using a resistance of 0.5 to 1.0 ohm. That way the danger of reverse polarization is removed and the amount of heat generated in the resistor will be less.

 

[HKJ]

The resistor will get quite hot: don't let it touch anything that might be damaged by heat.

Depending on resistor, this might be a understatement, some power resistors has working temperatures up to 300 degrees Celcius.

 

[bbb74]

It's really not good to do this. If you try there is a danger you will damage one of the batteries by reverse polarization at the end of the discharge.

But let us suppose you are intent on harming your batteries.

You are correct! :) The reason why I want to do it is to see how much reverse charging the cells hurts them :) Its for my battery shootout thread. Got 4 cells each of different brands, and will give them a few full discharges to simulate being run down flat in a 4xAA torch. Cells with higher variation will suffer more than cells with low variation.

Thanks for checking my maths.

 

[Wrend]

Why not just test the discharge capacities of the cells to find how consistent they are? Less guess work that way to arrive at more conclusive findings, unless the point is to see how well a cell can be recovered and perform after it has been reverse charged.

This is what I'd do instead:

Slowly discharge the cell, fully charge the cell at X% of its rated capacity/hour, then let it rest for 1 hour.

Discharge the cell at Y% of its rated capacity/hour (preferably at 0.2C/h rate) to determine its usable capacity, then dead short the cell for 1 hour.

Reverse charge the cell at Z% (you'll want this to be relatively low) of its rated capacity/hour for 1 hour, then dead short the cell for 1 hour.

Fully charge the cell at X% of its rated capacity/hour, then let it rest for 1 hour.

Discharge the cell at Y% of its rated capacity/hour to determine its new usable capacity. Compare this with the former usable capacity test to see how well the cell holds up.

X, Y, and Z are somewhat arbitrary, but should be kept consistent between the various cells tested to see how they compare.

 

[Lynx_Arc]

How about finding a 2-3 Amp 4.8v light bulb?
Or maybe several 4.8v light bulbs in parallel.

 

[bbb74]

How about finding a 2-3 Amp 4.8v light bulb?
Or maybe several 4.8v light bulbs in parallel.

That's not a bad idea at least I can see when its finished...

Why not just test the discharge capacities of the cells to find how consistent they are? Less guess work that way to arrive at more conclusive findings, unless the point is to see how well a cell can be recovered and perform after it has been reverse charged

I am testing the discharge capacites etc on a C9000. But I also wanted to do "real world" tests by running the cells flat to show how worse off they get after being reverse charged. I didn't really want to leave a 4xAA torch lying on turbo for this test because a) I don't have one and b) i would probably prefer not to wear out a new LD40 for a battery test :)

 

[bbb74]

As a follow up to this thread, I was wondering what happens, electrically, when I connect 2 x AA's and a 1.5 ohm resistor all in series, but accidentally put one of the batteries around the wrong way? "Nothing much" was my observation but I'm sure something would have been happening...

 

[Mr Happy]

As a follow up to this thread, I was wondering what happens, electrically, when I connect 2 x AA's and a 1.5 ohm resistor all in series, but accidentally put one of the batteries around the wrong way? "Nothing much" was my observation but I'm sure something would have been happening...

Nothing much is pretty much it. If you reverse one battery in a pair then when you connect them in series the voltages cancel out; you get no voltage across the resistor and no current flows.

 

[xul]

As a follow up to this thread, I was wondering what happens, electrically, when I connect 2 x AA's and a 1.5 ohm resistor all in series, but accidentally put one of the batteries around the wrong way? "Nothing much" was my observation but I'm sure something would have been happening...

George came to class with a shirt that was stained and full of holes. He was making a go-cart out of 3 car batteries and a starter motor and he put one of the batteries in backwards.

 

[HKJ]

George came to class with a shirt that was stained and full of holes. He was making a go-cart out of 3 car batteries and a starter motor and he put one of the batteries in backwards.

It is a bad idea to reverse a battery in a parallel coupling.

 

[xul]

It is a bad idea to reverse a battery in a parallel coupling.

They were in series. There might have been 4 batteries total, with 3 charging the reversed one through the starter motor.Those were the days. . .

 

[xul]

How come my 'edit post' screen is blank and I can't put in carriage returns?

 

[uk_caver]

If you must flatten the pack http://www.candlepowerforums.com/vb/newreply.php?p=3766883&noquote=1(to a total pack voltage of zero) then a bulb probably is a good idea - for a start, it radiates away most of the power put into it, is designed to cope with being hot, and will generally at least be glowing while it is hot.

 

[xul]

If you must flatten the pack (to a total pack voltage of zero) then a bulb probably is a good idea - for a start, it radiates away most of the power put into it, is designed to cope with being hot, and will generally at least be glowing while it is hot.

Plus it acts like a constant current sink - the cold resistance is 10x to 15x less than the hot resistance.