Using AD5941 to design a battery impedance measurement for 8V to 100V battery

Hi,

In CN0510 schematic, the power transistor (TIP31BG) used in Darlington pair supports upto 80V. 

For battery voltages upto 100V, this transistor need to be changed to a higher rated one.

Thanks for your advise. I wonder why such high current rating power transistor should use? TIP31BG is 3A rated. But according to CN0510 circuit note, typical excitation signal is 50mA only.

Hi,

      Higher rating is required for voltage, not current. 

So I can choose a high voltage , small current SMT transistor with smaller size?

yes

thanks

I have modified C36 to C39 of CN0510 to high voltage rating capacitors. (100Vdc) And measure a battery around 15V with SensorPal. But the result was not as expect. The real part shows negative value. Below was my test data. Is there any further modification need for my application?

Hi,

The code does complex division of voltage across battery and RCAL as below:

Zbat = Vbat/Vrcal * Rcal

where 

Zbat = battery impedance.

Vbat = current through battery X RTIA

Vrcal = current through Rcal X RTIA

As per above expression, the negative real values are due to the reactive component of battery and RCAL at the particular frequency

 Zbat.real = (Vbat.real* V.rcal.real   + Vbat.imaginary* V.rcal.imaginary)*Rcal