BQ25172: Question about charging profile and charging termination

Hi Chase

Q1 -- The BQ25172 is timer-based NiMH charger and termination of charge when timer expires is preferred.  The time will be set by TMR pin.

Shortest charge time (TMR) is 4 hr, for this condition charge current (ISET) should be 25% of C.  As charge time (TMR) is increased the charge current (ISET) will decrease.

The OVP trip point is 1.7V per cell it is not preferred to terminate with OVP.  

Chase Jeong said:

Datasheet says about nonrecoverable fault mode. (BAT OCP, ISET pin short, Charge timer expires)

Q2 -- Charge Timer expires will be charge complete condition, no fault.  For Nonrecoverable fault the only option is to cycle input power or pull TS low.

Dear Bill,

Thank you for your support.

Q1. You mean that BQ25172 charges the battery at least 4 hours after the charging start?

How about the battery remains 90% or higher? I think that the battery would be fully charged within very short time if the battery is 90% or higher.

Does BQ25172 charges the battery at least 4 hours even the battery is 90% higher? I think it is risky because the charger supplies constant current even the battery is fully charged. What do you think about it?

Q2. Then it is the only way to cycle input power or pull TS low using external MCU or switch, etc.

Then I should use MCU if the charger is recovered from nonrecoverable fault mode. Am I right?

Please check these issues. Thanks.

Best regards,

Chase

Hi Chase

Chase Jeong said:

You mean that BQ25172 charges the battery at least 4 hours after the charging start?

Yes, but the 4hr depends on setting of the TMR pin

Chase Jeong said:

Does BQ25172 charges the battery at least 4 hours even the battery is 90% higher? I think it is risky because the charger supplies constant current even the battery is fully charged. What do you think about it?

NiMH are very tolerant to overcharge and this will not be a problem.  The current needs to be kept proportional to the charge time (4hr 25% to 33% C).  No fast charge 1C option.  

This was review with NiMH cell manufacture.

Chase Jeong said:

Then I should use MCU if the charger is recovered from nonrecoverable fault mode. Am I right?

If the power cannot be removed from the charger than TS is the best option.  Controlling TS from MCU is possible.

Dear Bill,

Thank you for your support, Hope you had a good weekend.

1. I have to wait at least 4 hours even the battery is fully charged before 4 hours passed.

-> Is there any way to recognize if the battery is fully charged before the timer is terminated?

2. I cannot confirm that the battery is fully charged or not. just wait the timer is over.

Please confirm these 2 situations.

Best regards,

Chase

Hi Chase

The BQ25172 is designed with backup battery applications in mind vs a portable application.  It will not support a 1C fast charge conditions, charge currents will be lower.

Is this a battery backup application.

Chase Jeong said:

I have to wait at least 4 hours even the battery is fully charged before 4 hours passed.

-> Is there any way to recognize if the battery is fully charged before the timer is terminated?

Battery voltage may give some indication.  Also if a gas gauge is used it would give some indication.

With lower charge currents the typical fast charge indicators of delta T or neg delta V are not reliable.

.  

Chase Jeong said:

I cannot confirm that the battery is fully charged or not. just wait the timer is over.

Is the customer trying to avoid a battery overcharge condition or just end charge quicker? 

Dear Bill,

Thank you for your support.

Yes, My customer is worried about 2 things.

- Battery overcharge : Fully charged before timer is terminated. How about if the battery start charging with 90% battery remaining?

- Cannot recognize fully charged : Same situation as above. End customer do not know even battery is fully charged before timer is terminated. They should wait.

Do all NiMH battery charger have same specifications in charging termination?

Best regards,

Chase

Hi Chase

The BQ25172 will check battery voltage before starting charge -- if the battery voltage is above 1.33V per cell battery is considered full and charge will not start.

Chase Jeong said:

Battery overcharge : Fully charged before timer is terminated. How about if the battery start charging with 90% battery remaining?

If the battery voltage is above 1.33V per cell a charge will not start.  But once charge starts it will continue until timer expires.  NiMN will tolerate overcharge.

Chase Jeong said:

Do all NiMH battery charger have same specifications in charging termination?

Yes, they will all behave in a similar way.

Dear Bill,

Thank you for your support, and hope you had a good weekend.

I will share your opinion with my customer.

- Do you have any tested data which NiMH batteries tolerate overcharge?

Best regards,

Chase

Hi Chase

I can discuss the NiMH battery maker and test data in more detail using the private messenger feature of forum.

Look for a message / request from me.