Self discharge to zero volts of NiCd cell?

[treez]

We are using this NiCd cell in our emergency lighting unit (five in series).

NiCd cell (part number 20500)
https://www.tenergy.com/Store?search=20500&ext=F

If the unit is in rest mode, then no current is drawn from the battery.

Under such conditions , how long will it take for this cell to self-discharge to zero Volts?

It it true that a cell cannot possibly be stopped from eventually self discharging to zero volts if left uncharged?

 

[GSM Man]

All battery chemistries exhibit self-discharge. Typical NiCd cells will self-discharge approximately 10% per month at 20°C. The higher the temperature, the higher the discharge rate. A good source of infomation on batterys is: https://batteryuniversity.com

 

[treez]

Thanks,

that site is good but doesnt say if self discharge would take a NiCd cell down to 0V.

 

[tpetar]

Re: Self discharg1,99V per celle to zero volts of NiCd cell?

http://www.camlight.com/techinfo/techtips.html


Emergency lighting unit should get power periodically or always, with a purpose to keep batteries in full charge. Additional to that periodically should make some self test for some time.

Even car lead acid should be removed from shops on every 6 month and back to factory or some official service to recharge them.

If you keep NiCd in rest state for longer time, dont expect full capacity from them when you need them.

NiCd cell is discharged completely on 1V, similar to car lead acid (starter type six cells) 11,99V (1,99V per cell). Below this level this is permanently destruction of battery, and in that situation anything else about thise batteries is irrelevant in the context of normal usage.

 

[treez]

sorry but i never heard that a NiCd cell is dead-forever when its cell voltage goes down to 0V.

 

[tpetar]

Let’s look at what a weak cell does in a pack that is strung together with strong ones. The weak cell holds less capacity and is discharged more quickly than the strong brothers. Going empty first, the strong brothers overrun this feeble sibling and the resulting current on a continued discharge pushes the weak cell into reverse polarity. Nickel-cadmium can tolerate a reverse voltage of minus 0.2V and a reverse current of a few milliamps, but exceeding this level will cause a permanent electrical short. On charge, the weak cell reaches full charge first and it goes into heat-generating over-charge while the strong brothers still accept charge and stay cool. The low cell experiences a disadvantage on both charge and discharge. It continues to weaken until finally giving up the struggle.




Some people claim to recover dead NiCd batteries – i.e., batteries showing 0 V on a voltmeter and that doesn’t revive by hooking them to a charger – by performing a high-current quick charge, process known as “zapping”, and them putting them back to the charger for a regular charge.

In fact, this will work if the battery has an internal short circuit caused by a small dendrite, which is a small piece of material connecting the two battery poles internally. What the zapping does is to burn this dendrite, like if it were a fuse, solving the short-circuit problem.

But the problem can come back, because not only other dendrites can be formed but also the material that was vaporized is now inside the battery, which can act as a resistor, making the battery to hold less charge than when it was good.

However, zapping the battery if the problem isn’t an internal short circuit can damage the battery even more. As we mentioned, if you let your NiCd battery completely discharged, it can be damaged – i.e., completely discharged forever – and the problem here won’t be any dendrite inside creating an internal short-circuit.

Keep in mind that this technique has nothing to do with “memory effect”. Some people can claim that they solved the “memory effect” of a battery by doing this but in fact the battery problem was another one (internal short circuit).


However this brings another problem: NiCd cannot be fully discharged or they will be damaged. Fully discharged usually means having a voltage below 1 V per cell (NiCd batteries are usually formed by grouping several 1.2 V cells).

**broken link removed**



Batteries are devices with internal chemical process, which are dependent from current flows through battery (charge/discharge). If you go out from chemical margines, internal processes will become unpredictable. Because of that we have voltage ranges for normal cycling of each type of batteries, and on first place manufacturer datasheet is relevant for battery usage.