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BQ25171-Q1: Minimum Charge Qualification Voltage and General Voltage Threshold Explanation Request

Nicholas Zambetti
Prodigy 160 points
Part Number: BQ25171-Q1

I cannot find any mention of a minimum qualification voltage in the datasheet; does this IC have a minimum voltage for charge qualification?

For example, some battery charger ICs refuse to charge batteries that are lower than 350mV.

I suppose this is a protection so the IC does not attempt to charge old/damaged batteries or a battery that is inserted backwards

Does this chip have such a minimum voltage for charge qualification?

Also a general question about voltage thresholds...

 • The OVP of 1.7V seems very high, too high to hit repeatedly in daily/regular use.

On NiMH battery datasheets, 1.7V while charging is an extreme maximum ... https://eneloop101.com/wp-content/uploads/2017/02/HR-3UTGB.pdf

• The Vrecharge is stated in the datasheet as 1.330V (1.305-1.355V), which also seems high, too high to begin a charge cycle

Measured open-circuit, I would expect the OVP to be ~1.4V and the Vrecharge to be ~1.2V

Can you explain why these voltages are so high?

Perhaps these voltages are the elevated voltages while the IC is active or in a charge cycle?

Thank you

  • 0
    TI__Mastermind 18885 points

    Hi Nicholas, 

    Nicholas Zambetti said:
    Does this chip have such a minimum voltage for charge qualification?

    No the BQ25171-Q1 does not have a minimum voltage qualification for charging. The device monitors the NiMH cell voltage and if it is below Vrechg the charge cycle will be initiated. 

    Nicholas Zambetti said:
    Can you explain why these voltages are so high?

    As you stated, the 1.7 V overvoltage threshold is an absolute maximum. The NiHM charge cycle is time based where the battery is charged with a constant current for a time set via the CHM_TMR pin. If the charge timer is set correctly according to the battery specifications the cell voltage should never get that high. If the overvoltage threshold is reached, charge is stopped until the voltage falls below 1.45 V.  It is also important to note that in general NiMH batteries are more tolerant to overvoltage than other chemistries.

    The V_recharge threshold is at this seemingly high level because it's main function is to initiate the optional intermittent charge feature. Intermittent charging is designed to replenish the natural self-discharge of NiMH batteries. When this feature is enabled anytime the IC is on and the cell voltage falls below 1.33 V a short charge cycle (25% of the full charge time) occurs. The Vrechg threshold is high so that if intermittent charging is enabled the cell will not discharge further and be closer to full capacity. 

    Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 160 points

    Hi Garrett, thank you for this very helpful explanation. The V_recharge threshold in particular makes much more sense to me now.

    I have two follow-up questions...

    Garrett Kreger said:

    No the BQ25171-Q1 does not have a minimum voltage qualification for charging. The device monitors the NiMH cell voltage and if it is below Vrechg the charge cycle will be initiated. 

    Might you know how it handles cases of batteries inserted backwards (so the polarity is inverted)?

    Must additional protection be in place to handle this, or can the BQ25171-Q1 handle this gracefully?

    Garrett Kreger said:
    If the charge timer is set correctly according to the battery specifications the cell voltage should never get that high.

    How does this chip handle situations where batteries are installed that are lower than which CHM_TMR is configured? (eg. 1300mAh installed when CHM_TMR is configured for 2700mAh)

    Would it then rely on the thermistor or OVP to prevent over-charging/venting? Or are there other ways you suggest to handle this scenario?

    For context, in our application, customers are free to install a battery of their own choosing. We can print a suggestion for a capacity on the product ("use only 2500mAh or higher"), but many people do not read or understand these sorts of messages.

    Thank you,

    Nicholas

  • 0 in reply to Nicholas Zambetti
    TI__Mastermind 18885 points

    Hi Nicholas, 

    Nicholas Zambetti said:
    Might you know how it handles cases of batteries inserted backwards (so the polarity is inverted)?

    The bq25171-Q1 does not have protection against a backwards inserted battery. An additional circuit is needed to protect the device against this. Here is a link to an application report on protection circuits. Reverse Battery Protection Circuits 

    Nicholas Zambetti said:
    How does this chip handle situations where batteries are installed that are lower than which CHM_TMR is configured? 

    You are correct, the device will rely on the battery thermistor and OVP to prevent over-charging of a lower capacity battery. For NiMH charging with this device the timer is the only termination method other than a fault. If your customers are free to install a battery of their choosing then ideally you would want them to also be able to select the appropriate resistors for CHM_TMR and ISET to set the charge time and current according to the specifications of the battery being charged. 

    Regards,

    Garrett