End Cell Failures

[woolyhead]

when cells in an electric  battery are charged, sometimes the end cell in the string of cells fails. Why is it the end cell and not one from somewhere else in the cell chain? I have experienced this several times in my life and I've just had it happen again, this time with CdS cells. Why the end cell?

Edited July 9, 2020 by woolyhead

[Ghozer]

It is due to the way they are some times wired - when you are not using the device (or its on standby some times)  the rest of the cells in a battery may discharge into the first (or last) cell in the chain - (again, depends how cheaply the battery is made, and how it's wired)

This will obviously cause problems, - it can also cause a polarity reversal in that cell, which can cause the whole battery to fail..

[woolyhead]

Thanks Ghozer. I think I understand what you're telling me. But I would have thought the discharge could take place into any of the cells in the loop, not necessarily the first or last one, couldn't it? 

[Ghozer]

like I said, It really depends on the design of the battery as a whole, and how 'cheaply' it's made..

more expensive ones would have an internal charge and discharge controller, that all the cells would connect directly to, and this will balance the charge/discharge, and prevent cell discharge into other cells, and reverse polarity issues etc..

a cheaply made battery, may bypass this, connect them all in series or parallel (or a combination of both) and just have an 'overcharge' detector IC and nothing more...

I'm trying to simplify a little here though, there's a whole minefield of if's and buts and maybe's when it comes to this sort of thing

[woolyhead]

Yes, I see. My C cells are from a cheap electric drill so I suppose they would be the cheapest variety of cell.  If they are, then with them simply connected in series, would the end cell sometimes fail, other than with chance failures of course? When my end cell failed I wondered whether the battery voltage overall had maybe risen to the off load voltage of the charger because several cells had each produced more that 1.35 volts  so that there was insufficient voltage push left to charge the rest of the  cells any more and that because of this the end cell failed to get a full charge. But that doesn't explain why it's always the end cell that suffers. Unless you know different, maybe? I just calculated that since there are 12 cells in the battery, if 11 of them happened to have an on-charge voltage of 1.636  volts then in series they would give18 volts. This is the off load charger voltage. Is it possible that when on charge these cells could generate 1.636 volts each? I know the nominal fully charged value is 1.35 volts for a new cell but that isn't the on-charge voltage. Or is it? The cells are quite old, ie about fifteen years. So does my theory hold water or not?

Edited July 10, 2020 by woolyhead

[Ghozer]

You have to think about the flow of electricity (It actually flows negative to positive) and how the batteries are connected

it's hard to explain in text on here,

But let's try... 

Imagine it like a row of buckets, with a pump at the end of the row. (the pump is like your electrical load)
Each bucket is connected to each one next to it with a pipe.
Each bucket, has an equal amount of water (let's say, they are all half full).
The pump runs from the last bucket, back into the first (to complete the circuit).
You turn the pump on, What happens to the water in those buckets?
 

Edited July 11, 2020 by Ghozer

[woolyhead]

Nice analogy Ghozer. Thanks for your answers, they are helpful in understanding.

[Ghozer]

2 hours ago, woolyhead said:

Nice analogy Ghozer. Thanks for your answers, they are helpful in understanding.

Thanks, and no problem..

I was obviously way over simplifying it, there's much more involved when we're talking batteries and such, but I hope you got the idea at least