AA Battery Question

[bobcat2000]

helloWorld.cpp,

May I ask you a question regarding AA battery discharge question?

I have a flashlight that uses 3 AAA batteries in series.  I put 3 rechargeable AAA batteries to the flashlight. (assuming all 3 are equally charged to the same voltage)  The batteries are the same brand, rating, and bought at the same time.  The batteries begin to run out. So, the light gets dimmed.

I take the 3 AAA batteries out and measure the voltage.  Two of them are pretty much full but 1 of them is low.

Should all 3 batteries be low (or about the same voltage) instead of just 1 of them is low?

Voltage is the potential difference.  So, it is about pressure being pushed to different direction.

Is the electricity from this 1 battery being used to charge the 2 other batteries somehow?
What can be done to make all these 3 batteries to discharge the same rate but not charging each others?

Thanks!

[Zero999]

The batteries are not manufactured to a precise capacity. One battery has a much lower voltage, than the other two, because it has the lowest capacity.

[tunk]

What do you mean by full and low?
A fully charged NiMh is 1.4-1.5V, and depending on usage, fully
discharged down to 0.9-1V. Some typical discharge curves:
https://lygte-info.dk/review/batteries2012/Ikea%20Ladda%20AAA%20900mAh%20%28White%29%20UK.html
Also, one of them could be defective.

[bobcat2000]

Sure, let me clarify my vocabulary.
I use a Amprobe BAT-250 to test the batteries.
If the needle swings very fast all the way to the green area, I consider this battery full.
If the needle swings slowly to close to the green area, I consider this battery low voltage.

I turn my flashlight on until it turns itself off.  Actually, I have other devices like alarm clocks that use multiple batteries in series. they all have similar behavior.

I check the 3 AA batteries using the Amprobe.  Majority of the time is that 2 AA batteries will be full and 1 AA will be low voltage.

Majority of the time, only 1 battery will run out of power but the other 2 will still be full.

I am wondering why 1 battery will run out but not all 3 of them.

Danken!

[BradC]

I am wondering why 1 battery will run out but not all 3 of them.

Re-read the answer from Zero999 above.

The discharge curve on nickel based chemistries is pretty flat. It doesn't take much of a capacity difference at the knee to cause considerable voltage disparity and you are measuring them right at the knee where the smallest capacity cell is empty.

[bdunham7]

Replace just the low battery with another--or recharge it only--then put the other two and the replacement/recharged battery back in the flashlight and see how long it runs.  I would guess it won't run very long and then you'll find that at least one of the two original 'good' batteries will also be similarly depleted.

[IanB]

helloWorld.cpp,

May I ask you a question regarding AA battery discharge question?

I have a flashlight that uses 3 AAA batteries in series.  I put 3 rechargeable AAA batteries to the flashlight. (assuming all 3 are equally charged to the same voltage)  The batteries are the same brand, rating, and bought at the same time.  The batteries begin to run out. So, the light gets dimmed.

I take the 3 AAA batteries out and measure the voltage.  Two of them are pretty much full but 1 of them is low.

Should all 3 batteries be low (or about the same voltage) instead of just 1 of them is low?

Voltage is the potential difference.  So, it is about pressure being pushed to different direction.

Is the electricity from this 1 battery being used to charge the 2 other batteries somehow?
What can be done to make all these 3 batteries to discharge the same rate but not charging each others?

Thanks!

First, a word of advice: never do this to rechargeable batteries. You will damage them.

If you use two or three NiMH batteries in a lamp like this, always remove them and recharge them before the lamp shows signs of dimming.

Why, you may ask?

Basically, NiMH cells maintain a reasonable voltage right to the end of their discharge curve, and then the voltage falls off a cliff. If you have three cells in series, one of these cells is going to be the first one to fall, since no two cells are identical. Whichever one runs out first, will end up having current pushed through it by the other cells that are still going. This is called "reverse charging", and it is very bad news for an NiMH cell. It will damage it. So you really don't want to let this happen.

Also, this now explains why you see one battery with a low voltage while the others are still high.  The battery with the low voltage was the first one to run out. If the low voltage is zero or even negative, then definitely don't do this to your cells.

[bobcat2000]

He assumes 1 of the batteries is bad.  I am sure all the batteries are good.

I use 2 identical flashlights to test a total of 6 AA identical batteries.  One battery will always run out in each of these 2 flashlights.

My word "identical" means I bought the same brand of flashlights and the same brand of batteries new at the same time.

I will then take 1 battery out from 1 flashlight and put it into the other flashlight.  I let this flashlight run out and test these 3 batteries again.  I will have the same behavior.  One battery will always run out but the other 2 will still be full.

I can try non-rechargeable batteries.  Same behavior.

[BradC]

My word "identical" means I bought the same brand of flashlights and the same brand of batteries new at the same time.

Re-read the answer from Zero999 above. Keep reading it until you either understand it, or have further questions.

[bobcat2000]

Ok.  Let me try to understand what you are saying.

You are saying that 1 of the 3 batteries will always run out of power first.  As soon as this battery reaches that "cliff", it will just "short out" in a fraction of a second.  So, this will break the circuit.  The remaining 2 batteries will have "no draw" so to speak.  When I test the batteries, one will be low but the other 2 will be full because there is no more "draw" to the remaining 2 batteries.

So, if I use non-rechargeable batteries, I will see the lights keep dimming but not "short out".  The "cliff" is more like a "slope" for non-rechargeable batteries.

[BradC]

It doesn't "short out", but its voltage tanks quickly which is why you see the light dim considerably. The others will be close to empty at this point, but it's always prudent to stop discharge at the first sign of this happening.

In the case of one cell being considerably weaker than the others, then it's not uncommon for it to start charging in reverse, quickly causing damage to that cell.

The point is, you buy 6 cells brand new but none of them have "identical" characteristics (capacity or internal resistance). They will always mismatch to a degree and one will always die first.

[bdunham7]

Ok.  Let me try to understand what you are saying.

You are saying that 1 of the 3 batteries will always run out of power first.  As soon as this battery reaches that "cliff", it will just "short out" in a fraction of a second.  So, this will break the circuit.  The remaining 2 batteries will have "no draw" so to speak.  When I test the batteries, one will be low but the other 2 will be full because there is no more "draw" to the remaining 2 batteries.

So, if I use non-rechargeable batteries, I will see the lights keep dimming but not "short out".  The "cliff" is more like a "slope" for non-rechargeable batteries.

The 'cliff' or 'slope' depends on the type of battery, the load and the cutoff voltage of the device.  If you have an incandescent light, there should be no complete cutoff, but if you let it get dim enough, the weaker cell will be subject to reverse polarity.  For an LED light with 3 AA cells, you need 3V+ to have any light at all, so all three cells have to contribute.  Once one gets weak, the light will go out.  Keep in mind that the voltage under load in your device, the open circuit voltage measured by a voltmeter and whatever your battery tester is measuring are three different things.

[bobcat2000]

Thanks guys.

I think I understand what is going on now.

[BrokenYugo]

Sounds like your tester isn't calibrated very well, that or your flashlight is a poor design that doesn't use up the batteries fully. My no name one will find one deep in the red and two in the yellow or red, in the situation you outline, the three will all bounce like a dead one too.

[SilverSolder]

Sounds like your tester isn't calibrated very well, that or your flashlight is a poor design that doesn't use up the batteries fully. My no name one will find one deep in the red and two in the yellow or red, in the situation you outline, the three will all bounce like a dead one too.

Are there any flashlights that extract the energy from each battery separately??

[IanB]

Are there any flashlights that extract the energy from each battery separately??

Yes, the flashlights that run on a single cell 

[tunk]

The tester may have been designed for 1.5V alkalines and not 1.2V NiMh,
so the readings may not be entirely correct. The end voltages could also
show some information.