Li-polymer battery wire voltage drop concern

[hcglitte]

Hi all!

I am about to order 50 pcs of a li-polymer battery from China. It is a 2500 mAh 3.7 V nominal. During the design of the charger, I chose a switching type from TI. In order to be safe about noise, I added a ferrite bead on the + side to suppress switching noise. The least DC resistance 0805 version I could find was 9 mohms (3 Amps max). If charging with 1C (2500 mA), the voltage drop across the FB is 22,5 mV. Then I started to think about the cable voltage drop. The cable is 5 cm, so 10 cm total length is 21 mV. This adds up to 43,5 mV which is 1 % of the constant voltage phase (4.2V). I know this voltage needs to be very accurate in order to fully charge the battery. Since the charger IC also has 1% accuracy, we could be, worst case, 2 % off spec. Then I took a closer look at the charging curve for a Li-polymer battery. The current will decline sort of fast when entering the CV phase. So, the battery voltage may be about 4.1 V in the beginning during the CV phase, but then quickly going to 4.2 V. So perhaps the voltage drop is not that important after all...

Anyone who have any experience or thoughts about this dilemma? Please let me know!

Best Regards

[Harvs]

By the time you get to the voltage cut out level the current will be much lower than 2.5A. So it will be much less of a concern.

However, 2.5A is pretty close to the beads max rating.  So why not parallel a couple of the ferrite beads, split the current through them.

[ConKbot]

Lithium battery charging generally terminates at C/20, so your voltage sag should drop from 43.5 to 2.2 mV when charging is terminated, making the error insignificant compared to the charger accuracy. 

Depending on how fast youre discharging though (if its though the same lead) it may be worth it to parallel a 2nd ferrite.