BQ25756EVM: BQ25756

Hello Ethan,

i have set VAC_DPM to 40320 mV but on multimeter it is showing 30.065V on the battery side and CHARGE_STAT as Fast Charging (CC Mode)  The power supply connected is showing 24V and 0.002A

* Created: Thu Feb 20 15:48:57 IST 2025
*
* Format: Register Name tab Character,\t Register Address tab Character,\t Hexadecimal register value.
* BQZ Container: Charger_1_00-bq25756.bqz
*
Charge Voltage Limit 00 10
Charge Current Limit 02 640
Output Voltage Limit 04 00
Input Current DPM Limit 06 640
Input Voltage DPM Limit 08 1F80
Reverse Mode Input Current Limit 0A 640
Reverse Mode System Voltage Limit 0C 3E8
Precharge Current Limit 10 140
Termination Current Limit 12 A0
Precharge and Termination Control 14 0F
Timer Control 15 1D
Three-Stage Charge Control 16 00
Charger Control 17 C9
Pin Control 18 C0
Power Path and Reverse Mode Control 19 20
MPPT Control 1A 20
TS Charging Threshold Control 1B 96
TS Charging Region Behavior Control 1C 57
TS Reverse Mode Threshold Control 1D 40
Reverse Undervoltage Control 1E 00
VAC Max Power Point Detected 1F 00
Charger Status 1 21 23
Charger Status 2 22 80
Charger Status 3 23 00
Fault Status 24 00
Charger Flag 1 25 29
Charger Flag 2 26 80
Fault Flag 27 82
Charger Mask 1 28 00
Charger Mask 2 29 00
Fault Mask 2A 00
ADC Control 2B 60
ADC Channel Control 2C 0A
IAC ADC - Use Field View 2D 00
IBAT ADC - Use Field View 2F 00
VAC ADC 31 00
VBAT ADC 33 00
TS ADC 37 00
VFB ADC 39 00
Gate Driver Strength Control 3B 00
Gate Driver Dead Time Control 3C 00
Part Information 3D 12
Reverse Mode Battery Discharge Current 62 02

Hello Aseem,

Thanks for the new information. Just to clarify, you are reading 30V on the battery and 24V on VAC? Do you have a battery attached to VBAT?

I'm reading through the registers and this looks like normal behavior for a BQ25756 in VAC_DPM so far. The BQ25756 is in fast charge mode, but the BQ25756 should not be providing current to the battery while in VAC_DPM.

Best Regards,
Ethan Galloway

Yes reading 30V on the Battery and 24V VIN. I have connected 8 Lithium Cells in Series. Total Charging Volt Should be 33.6V and 3 A

What should be the register values for 24V charger with 33.6V battery. Charging current to be 3 A

Hello Aseem,

Thanks for being patient with this. I'll get back to you next week on this question.

Best Regards,
Ethan Galloway

Hello Ethan,

Thanks for the reply. For 8 Lithium cells in series I think the charging voltage should be 33.6V. Please suggest! also in Battery Management Studio It Says the registers are not in the correct form to load. 

Best Regards,

Aseem Gupta

Hello Ethan ,

I have set the values as told by you. I have also put discharged cells now. But still not showing the expected charging voltage.  Can we have an online session to resolve the issue.

Best Regards

Aseem Gupta

Hello Aseem,

The register values for the BQ25756 charging a 24V compared to a 33.6V should be the same if the charger is in fast charge mode. Also, I'm using the BQStudio BQZ file from here. You'll need to load the registers while in the field view. https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1379347/faq-bq25756-bqz-file-for-bqstudio?tisearch=e2e-sitesearch&keymatch=BQ25756%20bqz

Aseem Gupta1 said:

But still not showing the expected charging voltage

Can you go into more detail for the BQ25756EVM showing the expected charging voltage? Have you adjusted the feedback resistors to charge an 8S battery? Are there any other adjustments to the EVM?

Does the BQ25756 start charging if VAC_DPM is lowered from 40V to 20V?

Best Regards,
Ethan Galloway

Hello Ethan ,

Thanks for the reply. I have not done any changes to the EVM. It is as is. I have to develop a system to charge 8 Lithium cells connected in series with 3A 24 V power supply. And the load is 24V 1.2A connected to the Battery. I can increase the 24V 3A power supply to 24V 5A also if needed.

The charging time should not be more than 45Min. and the backup should be 4 hours. I am using 6800mAh 3.7V 18650 Lithium cells in series.

Best Regards

Aseem Gupta

Hello Aseem,

You'll need to change the battery voltage feedback resistors on the EVM to be able to charge an 8s Li-Ion battery. You can check this FAQ for the resistor values for different battery types.

I also recommend using BQStudio or the TI Charger GUI to turn down the charge current from to 1A or so while you are doing the initial testing.

Also, can you try test the charger with a constant voltage E-Load?

Best Regards,
Ethan Galloway

Hello Ethen,

I have done the prototyping and has the final issues in the register settings. Total Battery Voltage is 26.139V for 8S. Resistors installed are 249K and 12K with setting as 1546mV.  Charging voltage adaptor connected is 24V 3A. Still not charging. The registers I have set are as follows:

* Created: Tue Mar 04 12:10:25 IST 2025
*
* Format: Register Name tab Character,\t Register Address tab Character,\t Hexadecimal register value.
* Device: BQ25756
* BQZ Container: Charger_3_00-bq25756.bqz
*
Charge Voltage Limit 0010
Charge Current Limit 00F0
Output Voltage Limit 0000
Input Current DPM Limit 0640
Input Voltage DPM Limit 0348
Reverse Mode Input Current Limit 0640
Reverse Mode Input Voltage Limit 03E8
Precharge Current Limit 0140
Termination Current Limit 00A0
Precharge and Termination Control 0F
Timer Control 1D
Charger Control C9
Pin Control C0
Power Path and Reverse Mode Control 20
MPPT Control 20
TS Charging Threshold Control 96
TS Charging Region Behavior Control 57
TS Reverse Mode Threshold Control 40
Reverse Undervoltage Control 00
VAC Max Power Point Detected 0000
Charger Status 1 0C
Charger Status 2 80
Charger Status 3 00
Fault Status 00
Charger Flag 1 01
Charger Flag 2 00
Fault Flag 10
Charger Mask 1 00
Charger Mask 2 00
Fault Mask 00
ADC Control 60
ADC Channel Control 0A
IAC ADC - Use Field View 0000
IBAT ADC - Use Field View 0000
VAC ADC 0000
VBAT ADC 0000
TS ADC 0000
VFB ADC 0000
Gate Driver Strength Control 00
Gate Driver Dead Time Control 00
Part Information 12
Reverse Mode Battery Discharge Current 02

This is the last issue to be resolved. 

Best Regards

Aseem Gupta

Hello Aseem,

I'm looking through your registers and it looks like the charger is outputting a BAT_OVP flag. I've got a few test questions:

• Can you measure the VBAT voltage near the BQ25756 with an oscilloscope?
• Can the BQ25756 charge the battery when the battery is a lower voltage?
• If possible, can you measure the FB pin with an oscilloscope? Make sure to power down the BQ25756 before you put the oscilloscope probes on the FB pin.

Best Regards,
Ethan Galloway

BMS042A_SCH_page1IGS.pdf This is the schema I am using. Please let me know the issue. 

Best Regards

Aseem Gupta

Hello Aseem,

Thanks for the schematic. I have a few suggestions:

• Can you remove R59 and C25 and see if that improves the behavior?
• Can make sure the RC filters on SW1 and SW2 are removed as well?

Have you gotten any progress on the tests?

Best Regards,
Ethan Galloway

Hello Ethen

i have removed as told. Still 2.2 V at Regn pin. And No charging.

Best Regards

Aseem

What are the connection changes for stand alone use?

Hello Aseem,

You are seeing 2.2V at the REGN pin?

With the battery removed and the BQ25756 charging, what voltage do you see on VBAT?

Do you see a difference in the VBAT voltage if you set EN_PRECHRG=0?

Best Regards,
Ethan Galloway