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BQ25172: Ni-MH charging circuit

user6526132
Prodigy 100 points
Part Number: BQ25172

(1) I'm trying to configure a Ni-MH charging circuit using BQ25172.

ISET=500mA@600ohm
VSET=3cell+Inter,18kohm
TS=Therm 10kohm
VIN=5V-1A

In this case, VOUT_OVP=1.7V×3cell=5.1V, which exceeds VIN=5V.
Is it possible to detect full charge based on voltage?
In that case, at what voltage will it be detected?

(2) I am using an electronic load to check the charging stop voltage.
As the VIN increases, the charging stop voltage appears to decrease, which is unexpected.
On the contrary, it is not constant.
What kind of problem is there with the settings?

  • 0
    TI__Guru 68296 points

    Hello

    Question 1:

    "Is it possible to detect full charge based on voltage?" -- For a NiMH this would not indicate full charge, could give rough indication of state of charge.  The BQ25172 is timer based, charge at a lower current long enough to fully charge. 

    Question 2:

    "What kind of problem is there with the settings?" -- If this is an electronic load it may not be increasing the output voltage independent of the current to simulate a battery charging.  An example of a battery emulator can be seen in figure 2 of the BQ24040 user guide.

  • 0 in reply to Bill Johns
    Prodigy 100 points

    Hi, Bill Johns-san. Thank you for your response.

    In that case, the BQ25172 is just a timer charger, so
    Should we consider that it is necessary to set the current according to the time?

    Since voltage and temperature monitoring are stopped only in the event of an 
    abnormality, does it mean that it will not stop until the time comes, unless
    an abnormality occurs even if too much current is applied?

    Also, if the power is turned off after the initial charge is completed, will 
    intermittent charging not occur? Or will the charging time be reduced by looking 
    at the voltage when the power is turned back on?

  • 0 in reply to user6526132
    TI__Guru 68296 points

    Hello

    user6526132 said:
    Should we consider that it is necessary to set the current according to the time?

    Yes, the charge time can be set from 4hr to 22hr.  The current is typically set to about 25% of battery capacity (C).  A 2000mAh battery would be 500mA current for 4 or 6hr.  

    user6526132 said:
    unless
    an abnormality occurs even if too much current is applied?

    A NiMH will tolerate overcharge if the current is limited to prevent overeating, 25% of C.

    user6526132 said:
    Also, if the power is turned off after the initial charge is completed, will 
    intermittent charging not occur?

    Power is turned off, when power is turned back on the charger will check battery voltage.  If it is above 1.33V / Cell  no charge will start, below 1.33V / Cell full charge will start.  If configured for intermitter charge it will start when voltage drops below 1.33V / Cell