NiCd battery capacity degradation over years?

[treez]

Hello,

We have a NiCd Battery consisting of five 4000mAh NiCd cells.

It is used in an emergency light.

It is charged constantly with a charging current of C/15. (If the battery voltage starts going above 8V then we stop charging it until it goes back below 8V)
Every month it is discharged for 3 hours with a constant current of 710mA.

What will be the capacity of this battery after 4 years?

NiCd cell datasheet:

 

[BradtheRad]

From the datasheet it appears you should 'tickle' (trickle) charge under 200 mA. This is C/20. It may be okay for Nicads to get this much continually.

The very last graph shows a decline in capacity at the four year mark. Seeing the country where the batteries are manufactured, and since it is a market prone to overly optimistic claims, I believe a service life of three years is more realistic.

 

[keith1200rs]

I don't think you can assume 'it comes from China so they are lying'. I have done some independent testing of batteries from China for a UK company and found the products to be excellent. You could either do some testing yourself or ask the supplier to provide data/test results on which the data was based.

Keith

 

[treez]

The battery requires trickle charging at C/20......however, our charging current source is fixed at C/10, and it has to be this, so that it can charge the battery up within 19 hours.......however, C/10 is too high for constant trickle charging of the battery.

We can however, switch this current source on and off, so will it be ok if we PWM this current source with a duty cycle of 0.5....eg turn the current source on and off at 100Hz with 50% duty cycle?......is that going to have the same effect as a C/20 constant current, and give the same lifetime of the battery?

 

[keith1200rs]

Try asking the battery manufacturer. They may have some data on the effect of the higher charge current.

Keith

 

[BradtheRad]

Yes, C/10 pulsed at 50% will yield the same net effect as C/20.

I have several Vinnic Nicad cells (C and D size) which I purchased mail order 12 years ago, at a bargain price. They are labeled 'China' and when I have used them they still work okay. I have not verified their Amp-hr capacity.

When I put them on a charger at 100mA, they get slightly warm after several hours. This is normal and is not harmful to them.

Questions:

Do you monitor total volt level, or individual cell levels?

Do you intend to detect whether any of the cells has lost capacity?

Do you have an alarm circuit to warn that one of the cells is below nominal volt level, as compared to the other cells?

 

[treez]

Do you monitor total volt level, or individual cell levels?

..total volt level

Do you intend to detect whether any of the cells has lost capacity?

I am not sure how to do this, -maybe monitor the rate of volt drop after known discharge?

Do you have an alarm circuit to warn that one of the cells is below nominal volt level, as compared to the other cells?

...no, this seems extravagant?

 

[dick_freebird]

In my experience, which has yielded about half a cubic
foot of dead and weak NiCd batteries, there is no
consistency or predictability. Other than that the pack
will find its own weakest link. When I take apart drill
packs I can find about 3/4 of the cells seem to still be
good, and a couple won't take or hold a decent charge.

So I would place zero faith in any prediction.

 

[sky_123]

NASA technical reports is often a good resource for technology like this. I searched for Ni-Cd and Nickel Cadmium,
and found lots of hits.
The nice thing about Ni-Cd is that it has been around for so long, so that surely lots of information is available on this.
Of course, your cells may be very different from the ones NASA uses, but there may be some guidelines (I didn't
check).

 

[BradtheRad]

I forgot to mention that although I can still use most of my Chinese Nicads...

Some of them have gone bad over the years. It may not always have been a gradual deterioration. Sometimes it was a sudden inability to take a charge.

I do not know if it is caused by:
* dried-out electrolyte
* a terminal losing contact internally
* dendrites (metallic whiskers growing in the electrolyte)

I don't suppose there is any source who can guarantee that every one of their batteries will last for four years.

I wrote:
Do you monitor total volt level, or individual cell levels?

You wrote:
..total volt level

I suppose it is sufficient if you monitor the total volt level. That way you can recognize if a cell will not take a charge, because the total volt level will not rise to full charge level.

You can also notice if a cell does not retain a charge, because the total volt level will drop quickly from full charge.