High frequency ripple current charging NiCd cell is bad?

[Flyback]

Hello,

Supposing a i have a triangle wave of average value 400mA, and peak = 500mA and trough = 300mA, and duty cycle = 50% ('up' slope time = 'down' slope time), frequency = 100KHz.

If i charge a NiCd cell with this triangle current, then would the charging efficiency be the same as if a 400mA flat DC current had been used?

Over time, whereby the cell was repeatedly discharged and re-charged, Would the cell capacity degradation with time be worse with the triangle charge current or the flat DC charge current?

 

[schmitt trigger]

Charging a battery with DC produces heat, which is detrimental to the batteries health.

The ripple current will produce additional heating.
This is similar to what happens to a capacitor filtering ripple current.

This additional heat should have some impact on the battery's life.

 

[MikeMl]

I agree with Trigger. 60 or 120Hz ripple is one thing; 100kHz ripple is another.

 

[Flyback]

The ripple current will produce additional heating.
This is similar to what happens to a capacitor filtering ripple current.

This additional heat should have some impact on the battery's life.

Thanks but high frequency ripple current tends to increase a capacitors life...is this the same with cells?

 

[MikeMl]

Thanks but high frequency ripple current tends to increase a capacitors life...

Huh??

 

[schmitt trigger]

Thanks but high frequency ripple current tends to increase a capacitors life...is this the same with cells?

No, it doesn't.
Ripple cause I2R losses
Losses create heat
Heat reduces a capacitor's lifetime, specifically electrolytics.

Most major capacitor manufacturers have notes explaining that.

 

[Flyback]

thanks but high frequency ripple current tends to increase a capacitors life...is this the same with cells?

..what i mean is, for capacitors, ripple at high frequency "sees" a lower ESR than ripple at lower frequencies.
-Does this same situation apply for cells?

As we all know, "no ripple at all is better than high frequency ripple" , forgive me if i was not divulgent enough.

 

[schmitt trigger]

The original question was how would a triangle wave current would compare to a flat DC current,, with respect to battery degradation over time.

The answer still applies, irrespective of the frequency........that the battery will most likely degrade worse, since ripple will increase self heating.

 

[Flyback]

The original question was how would a triangle wave current would compare to a flat DC current,, with respect to battery degradation over time.

The answer still applies, irrespective of the frequency........that the battery will most likely degrade worse, since ripple will increase self heating.

please forgive me, ........the answer kindly profligated here does not make the comparison between high frequency ripple and low frequency ripple, which was the original question.

(i concur with you most emphatically that both high frequency and low frequency ripple cause heating...)

 

[ronv]

Is there any load on your battery during charging or is it just charging?

 

[Flyback]

...just charging.....apart from a 20mA current to the control circuitry.

 

[ronv]

Then it would seem to me that all the current is going into charging and there is really no current being draw out of the battery - so no real ripple current only a variation in charging current. The only risk might be if the increased current caused the the voltage or current to go to high for the charge rate.

 

[Mr RB]

A quick google found a ST datasheet on designing a high freq NiCd fast charger using a forward converter, which charges the NiCd in current pulses at 100kHz;
https://www.electro-tech-online.com/custompdfs/2013/02/5-10522.pdf

So I doub't charging "ripple" or even pulse charging NiCds has any significant bad effects. When the current is there, charging happens, when it is not there, nothing much happens.

 

[schmitt trigger]

please forgive me, ........the answer kindly profligated here does not make the comparison between high frequency ripple and low frequency ripple, which was the original question.

(i concur with you most emphatically that both high frequency and low frequency ripple cause heating...)

Ok, I suggest the following, to prove one way or another if this has an effect.

1. Completely discharge a battery.
2. Properly attach a thermocouple to the battery's body. You may require thermal adhesive.
3. Charge with high frequency ripple. Log the temperature at 1/2 hour intervals until fully charged.
4. Fully discharge battery again.
5. Repeat but this time use pure DC.
6. Compare the temperature rise between the two methods.

Please note, for the temperature readings, subtract the ambient temp, to obtain the batteries actual self heating.