BQ25713: Charging 4x NiMH batteries in series (4.8V nominal, 5.6V fully charged)

Hi Daniel,

1.Is the BQ25713 suitable for charging NiMH batteries? The TI's web page for this part clearly states that it is suitable, but the datasheet isn't so clear. The whole datasheet assumes approximately 3V per cell as in lithium polymer batteries, instead of 1.4V per cell as with NiMH batteries.
>> The BQ25713 can be used for charging NiMH batteries; however, the default settings that are loaded on POR as a result of CELL_BATPRESZ are configured for Li-ion. In order to use with a different chemistry, please update the registers at POR with an external microcontroller. More details on this in response to your questions below.

2.What voltage value should be connected to the CELL_BATPRESZ pin for my purposes? The datasheet states: "CELL_BATPRESZ pin also sets SYSOVP thresholds to 5 V for 1-cell, 12 V for 2-cell, and 19.5 V for 3-cell/4-cell." These settings clearly isn't suitable for NiMH batteries and only appears to allow for 4 voltages. Can I set a voltage that is different to these 4 specified? Can I set this pin to say 6V using a resistor divider instead of the 4 voltage levels specified in the datasheet?
>> The input to CELL_BATPRESZ is not continuous, it is sorted into buckets for 1/2/3/4 cell. As you have pointed out the defaults for these are set for Li-ion, so the values will need to be overwritten for NiMH.
>> In order to charge up to 5.6V, you will need to apply CELL_BATPRESZ of 2 cells (40% divider of REGN) because the 1 cell setting has a SYSOVP of 5V, and with NVDC architecture, VSYS must be higher than the battery voltage in order to charge. Note that the SYSOVP setting is determined from CELL_BATPRESZ only and cannot be overwritten via I2C registers. Please refer to datasheet section 9.3.3.3 for more details.

3. In the event of a full system reboot, how can I ensure that the output voltage from the charger does not exceed 6V with a 4 cell NiMH batteries in series? The BQ25713 does not appear to have non-volatile memory for the registers.
>> From POR with the CELL_BATPRESZ set for 2 cells, VSYS will be 6.2V. This can be overwritten by the uC once the uC boots, but there is a chicken-and-egg situation here where the VSYS will be 6.2V for some amount of time before uC can write to registers 0x0c/0x0D to change the minimum system voltage.


4.I need a 10uH inductor. What resistor should be connected to IADPT pin? The datasheet states: "The resistances recommended for 1uH, 2.2uH and 3.3uH inductance are 93kΩ, 137kΩ and 169kΩ, respectively." That is, it does not have any other resistor value recommendations and no equation has been provided for other inductances.
>> The BQ25713 requires a resistor on IADPT pin that is used to reconstruct inductor current for slope compensation for average current mode control. As with CELL_BATPRESZ, the resistance applied here does not provide a continuous range but is latched into predefined bins. 3.3 uH inductor via 169 kOhm is the largest inductance supported. The device has not been validated with larger inductor value, and there is no way to correctly set the IADPT pin for a larger value.


5.I need a 10uH inductor. What is the maximum value of the output capacitance? The datasheet states: "To get good loop stability, the resonant frequency of the output inductor and output capacitor should be designed between 10 kHz and 20 kHz." However, the datasheet also states: "Minimum 6 pcs of 10-μF 0805 size capacitor is suggested to be placed by the inductor." Using the suggested minimum of 60uF caps, the resonance frequency is 6.5kHz [ f = 1 / (2*pi*sqrt(LC)) ] which is outside of the 10kHz to 20kHz limits specified. This is even before adding in other capacitors in the system circuit. This appears to assume a maximum of 3.3uH inductance, which is odd, and is not mentioned in the inductor selection section of the datasheet.
>> You are correct that 3.3 uH is the largest inductor value supported by BQ25713. More detail in #4 above.

Regards,
Steve

Thank you Steve for the spot on answers. I wonder why TI didn't include these information in the datasheet.

Could you answer a couple more questions please.

• Does this charger have the ability to sense an NTC that is in the battery pack to adjust charge rates and voltages accordingly, and also offer battery thermal protection? I couldn't find any reference to battery NTC connection. Can CMPIN be utilised for this function?

• Is the ILIM_HIZ pin also expecting only 4 distinct voltage levels? Namely, 1.2V, 1.6V, 2.2V, 2.6V for input currents of 0.5A, 1.5A, 3A, 4A respectively. The datasheet does not state such restrictions, but in light of your response for other settings, I'm not so sure.

• Now that am aware that 3.3uH is the largest inductor that I can use, will the charger function adequately with low currents, say 

• I would like to use this charger to charge in Constant Current mode until the NiMH batteries are approximately 80% to 90% charged (not critical) but then, drop down to low trickle charge until 100% charged or more, at which time, it can either stop charging or continue trickle charging, which does not damage NiMH batteries. Is this possible? or will this charger continue to operate in CC mode even after reaching 100% charge?

• I'm using "SLUSD83A –JUNE 2018–REVISED JULY 2018", but it is difficult to work with as it is full of gaps and ambiguities. Is there a more recent version of the datasheet?

Best regards.

Daniel Park