Degradation of NiCd cell capacity over time?

[Flyback]

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:

 

[MrAl]

Hi,

According to that data sheet, the cell is supposed to last for more than 4 years with a constant charge of 0.05 C and if it stays under 55 degrees C and under 65 degrees C only once in a while. It may also require what they call "Maintenance" which means discharging completely every so often so that the cells do not develop what they usually refer to as the "Memory Effect", which causes an apparent loss of capacity.

Since 0.05 C isnt that high (100ma for a 2000mAhr cell, 200ma for a 4000mAhr cell) this could be acceptable data. The truth of these things comes out when you actually use them. If you are prepared to buy new cells you wont be disappointed

Which makes me wonder just how much these cells cost you that you are so worried about this. What if they only last 2 years, is that going to be a big problem?

I used 2Ahr rated NiCd cells for years. I found that the charger that came with the unit i bought did not function the way it should so i designed one. Since the cells were not used very much (once per month perhaps) the main idea was to just keep them charged so that when they were needed they would be ready to go. I found that the only problem was to maintain a current that would overcome the self discharge of the cells over time. But the other problem was that the required constant current was too low to be deemed effective, so i had to build a charger that would pulse the NiCd pack twice per day with enough energy to keep them primed and ready for when i needed them. The charger pumped a regular level charge (like 150ma) for a short time period, twice per day. That was enough to keep the cells ready. The cells lasted for five years.

After that, i gave up on NiCd's because i felt that i would have liked to get more life out of them over the years even though i got 5 years out of them. So i switched to Lead Acid. Lead Acid have their own problems, but they are cheaper for the capacity you get and i need a lot of capacity. The NiCd's were not really that great anyway at only 2 ampere hours. Lead Acid batteries are readily available at huge Ahr ratings and they also supply more instantaneous power, so i went to those. My main application at the time was power tools, but now it's expanded to a lot of other things too so i go with Lead Acid now.

I also use NiMH for some small applications like flashlights and small emergency lighting, but i only use the low self discharge type because the others loose their charge even over short periods like a week or two. Just to note, the NiCd's you use loose about 35 percent of their charge over a one month period.

 

[MikeMl]

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

It'll be shriveled and dried out...

I would put a timer on it, and trickle it for an hour once every 24 hours. I used to do this on GE and Motorola handy-talkie batteries, and got much longer utilization than folks who just left the trickle charge applied 24/7.

 

[MrAl]

Hello there Mike,

Note in my previous post (just before yours) i talked about a charger that was designed to pulse the battery twice per day.

 

[MikeMl]

Hi, glad that there is something we agree on

 

[MrAl]

Hi, glad that there is something we agree on

Hi Mike,

Im sure there will be other things too.

The data sheet is specifying a very low trickle charge so it might not be too bad, but i prefer the pulse too either once or twice a day. I also dont like very low trickle charge because i dont think that's enough to keep the cell up to charge due to the very low current required.

I did an experiment with NiMH cells keeping them on charge for 24/7 and found they only lasted 6 months. They were slightly older versions though so maybe the newer designs will hold up better.

 

[jpanhalt]

Here is one of the more up to date and enlightened comments I have seen on maintenance of nickle-based batteries.

**broken link removed**

John

 

[Flyback]

Actually, we also have a requirement for a C/10 charging current. In other words, charge at C/10 for 19 hours. However, the trickle charge rate is C/20.
Will it be ok if we PWM our C/10 charging source at 50% duty in order to make it C/20 on average, or will this kind of pulse charging make the battery life degrade quicker?

Our charging current source is a resonant inductive coupler, and these bad boys only chuck out one current level.....however, we can turn it "off" by shorting it out.

I fear that if we constantly trickle charge at C/10 , then our battery capacity wil degrade too quickly?
Also, i feel that C/10 constant trickle charging could result in the batteries overheating, do you agree?

 

[MrAl]

Hello,


Pulse charging is supposed to actually be better so you're ok there. C/10 sounds too high for long life though.

 

[Flyback]

incidentally , this is what out battery charge current looks like, its 379mA average, peaks up to 1 amp...i know you say pulsey is good, but is this current waveform ok for battery charging?

 

[jpanhalt]

This comment applies to NiCd batteries. NiMH may be similar. It is certainly not true of all battery chemistries, e.g., lithium.

**broken link removed**

Interspersing discharge pulses between charge pulses is known to improve charge acceptance of nickel-based batteries. Commonly referred to as a “burp” or “reverseload” charge, this method assists in the recombination of gases generated during charge.

The implication I took away from that source is that adding a "burp" to the charge cycle may be good for the battery. What you show looks more like relatively high frequency PWM. Since we know that PWM discharge is worse than average current discharge, I would be very suspicious of any claim that PWM charging was actually good for the battery.

Is Mr. AI talking about burp charging or PWM? And also, I think any data related to the effect must be for NiCd batteries, not lead acid or others.

John

 

[MrAl]

Hello again,


PWM charging is ok but that look like 1MHz pulsing. The normal PWM would be maybe around 10kHz at most. So i cant say with certainty that the high frequency (1MHz) will charge the battery well enough or not. I can guess that it would, but without a test i would still have to wonder a little.

But a test isnt that hard. Charge it for the required amount of time knowing the average current, and then test the discharge of the battery to see if you get the full capacity or close too it.

I assume that you've used that waveform to calculate the average current. If not, post the amplitude of the highest peak and the lowest valley.

Things like this come up quite a bit because not everything has been tried and proven, and even if it has been done before it may not be documented or if it was documented it may not be available to the public. So we have to be ready to do our own testing sometimes and try to get a feel for what works and what doesnt. The only way sometimes is to do a test or two, no way around this.