NiCd charging needs steady DC?

[eem2am]

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?

 

[Harald Kapp]

I think the current waveform as described is no problem in itself. What may be difficult is the detection of end of charge using the standard dV/dt method. The varying current will produce a varying voltage across the battery's electrodes. If you want to use dV/dt charge temination you'd have to ensure that you measure the voltage always at the same current level, which at 100kHz is possible, but less than easy.

You could smooth the ripple by sending the current through a simple LC lowpass filter before feeding it to the battey. The less ripple, the easier the voltage measurement.

You can read up here on charge methods.

 

[eem2am]

Thanks Harald,

Great article from mpower.....they say pulse charging is good as long as the "gaps" are at least 30ms.....

the main question though, was , "which is less damaging to batteries, high frequency ripple charging current, or low frequency ripple charging current, given that the rms values of the low and high frequency current waveforms are the same.

 

[Harald Kapp]

high frequency ripple charging current, or low frequency ripple charging current

That depends on what you call high and low. With "high" frequency, e.g. >10kHz (note: this is a totally arbitrary limit set by me within the last second) you will increase the proportion of capacitively phase shifted current vs. voltage. This might reduce the efficiency of charging. It will certainly lead to warming of the battery without adding effective charge (due to the phase shift). Therefore I'd keep the ripple frequency as low as possible.

I'm not an expert on charging batteries. This is my gut feeling.