battery charge and discharge characteristics

Hello Harini,

Your charge and discharge curves look healthy, which part are you referencing that you need help with? Is the capacity reducing after each cycle? Can you share the log files of the charge and discharge?

Sincerely,

Wyatt Keller

NCR18650B_Test_Data.xlsxHello Wyatt,

How are you doing? Thank you for the response. 

In the trailing post, I have posted curves for Discharge and Charge of the battery pack 3s2p(li-ion) At full it is 12.55V.

When the ~100% filled up battery pack is discharged at 1C (obviously it will draw 6.4 AMPS from 2P pack). BMS kicked in at 8.7V and turned off the discharge FET. And the total discharge capacity is ~5800mAH.

Later we continued it with charging at 0.5C, and by the end of the charge completion i.e when it reached 12.54V the battery was pumped with only 3550mAH.

So my question is How in our physical world is breaking the laws? or Do I have an over-unity battery pack? Because when it discharged with for 5800mAH then obviously it should be taking in ~5800mAH to fill up to the level it was before discharging the battery pack?

Am I missing anything?

Appreciate your help.

Hello Harnini,

There is no TI part name or number attached to this post, I'm not sure how to help resolve your without more information regarding which TI parts were used.

Are you getting the current and mAh information from a gauge, charger, monitor? Are all your connections secure? Do you have any interconnection losses between the cells causing high loss during charge/discharge?

Sincerely,

Wyatt Keller

Hi Wyatt,

We are using TI BQ24715 Charger IC and BQ7791501 BMS controller. 

The current is monitored from the BQ24715 Charger IC IOUT pin. All our connections are secure we do not have the interconnection losses.

Hello Harnini,

I'm going to redirect your inquiry to the charger team as I am not familiar with this charger.

Sincerely,

Wyatt Keller

Hi Harnini,

The BQ24715 IOUT pin only provides a measure of 40 times adapter (input) or 16 times discharge current, computed using the differential voltage across the sense resistor.  Are you computing capacity by multiplying charge current by time?  How are factoring in the charge current as it tapers down from constant current due to battery impedance?  You might consider using a gas gauge to provide a more accurate measure of battery capacity.

Regards,

Jeff 

Hello Jeff,

Thanks for the response.

When you say BAT pin are you referring to IOUT PIN (where Charge or Discharge current can be monitored).

We are using the IOUT pin in Charge current monitor mode to compute the capacity by multiplying it with time. Probably from your post I understand that we cannot integrate IOUT charger current to find out how much Q has been delivered to the battery? If I am wrong correct me.

We have not used any factoring in the charge current as it tapers down from CC since we are just measuring the IOUT thinking that it is the absolute current going into the battery.

We have BMS BQ77915 in our application and It just happened today while charging the battery Iout was reading 2.6A (0x14 set 0x0B80 sense resistor is 7.5mOhm on BQ24715) and I happened to measure the voltage drop across the BMS sense resistor(4.5mOhm and drop were 18mV) and calculate the current value to be around 4Amps. Then I understood that I have missed something basic here. Please correct if I am misinterpreted here.

 Thank you for the suggestion to use a gas gauge? Could you suggest us cost-effective method, I am thinking like to have a Current sense circuit hooked at BMS sense resistor? Any suggestions, please

Thank you

Harini 

Hi Harini,

Yes, I meant IOUT pin.  The IOUT pin monitors battery discharge current or input current to the charger from the adapter, not charge current.  To compute charge current, you would need to do an efficiency power balance: eff = VBAT*ICHG /(VBUS*IBUS).  Hopefully the datasheet efficiency curves provide enough info for you to estimate efficiency for your given input voltage.

I will send to the gauge team to help with choosing a gauge.

Regards,

Jeff

Hi Harini,

For 3S2P li-ion, please consider BQ4050 or BQ40z50-R2. BQ40z50-R2 is based on our Impedance Track technology, which provides for a more accurate state of charge and state of health reporting.