weird thing happening with batteries...

[pammy]

....I'm pretty thick when it comes to electrics - but My Dad has come up with something rather weird with two duracell batteries.

Imagine if you will - you have a normal 1.5v battery, you put the probes on each end from a volt reader and it reads 1.5v as you would expect. You then do exactly the same with battery number 2 - except the reading goes the opposite way suggesting -1.5v. Then you put the two batteries in sequence - positive to negative and the reading instead of getting 3v - you get 0v. reverse the coupling so you have positive to positive and it reads 3v. It seems that one of the batteries has reversed its polarity or is going through a major identity crisis. We've never come across this before and he's wondering what it might be. Might it be a dodgy battery or could something have happened to make it behave this way.

 

[grasmere]

Hi Pammy - a quick answer if it helps

Ni-cad rechargeable batteries can sometimes go through cell reversal when cells are connected in series, ie the one with the lowest capacity may be driven into reversing its polarity if it fully discharges first. The more cells in the battery, the greater the chance of cell reversal.

CELL REVERSAL
What is cell reversal? Batteries made up of more than one cell have the potential for cell reversal problems when the discharge is deep enough to bring one or more of the cells in a battery to zero voltage. If discharge continues beyond this point, the voltage on the depleted cell will reverse polarity. The positive electrode is usually the first to run out of capacity. Continuation of the discharge will cause the reversal of the negative electrode and the voltage will be further reduced to about -1.4 volts. The problem that occurs is the generation of hydrogen gas. As the electrodes change polarity they will generate hydrogen. Since the hydrogen will not recombine, the internal cell pressure will build up to a level that causes the cell to vent if the reverse charge current is maintained for a significant period of time.

The solution to cell reversal is to avoid applications where the cells will be disharged repetitively or very deeply. At high discharge rates, such as we find in power tools, the electrical reversal occurs before the electrochemical reversal with a significant fall off in performance of the product.

HTH

 

[pammy]

quick response back - they were normal duracells - not rechargables

 

[grasmere]

they were normal duracells - not rechargables

Hmm - in that case 'something weird is happening with your batteries'

. . but dont throw away, try it the way round that it works . . .

 

[pammy]

Hmm - in that case 'something weird is happening with your batteries'

. . but dont throw away, try it the way round that it works . . .

That's what I told him to do

 

[Brian WH]

That's what I told him to do

Ooooh you are clever.

 

[garystu1965]

Hi Pammy - a quick answer if it helps

Ni-cad rechargeable batteries can sometimes go through cell reversal when cells are connected in series, ie the one with the lowest capacity may be driven into reversing its polarity if it fully discharges first. The more cells in the battery, the greater the chance of cell reversal.

CELL REVERSAL
What is cell reversal? Batteries made up of more than one cell have the potential for cell reversal problems when the discharge is deep enough to bring one or more of the cells in a battery to zero voltage. If discharge continues beyond this point, the voltage on the depleted cell will reverse polarity. The positive electrode is usually the first to run out of capacity. Continuation of the discharge will cause the reversal of the negative electrode and the voltage will be further reduced to about -1.4 volts. The problem that occurs is the generation of hydrogen gas. As the electrodes change polarity they will generate hydrogen. Since the hydrogen will not recombine, the internal cell pressure will build up to a level that causes the cell to vent if the reverse charge current is maintained for a significant period of time.

The solution to cell reversal is to avoid applications where the cells will be disharged repetitively or very deeply. At high discharge rates, such as we find in power tools, the electrical reversal occurs before the electrochemical reversal with a significant fall off in performance of the product.

HTH

Give the man an anorak