BQ24172: Charging and Re-charging Current Profile

Dear Jackson,

The bq24172 uses an external resistor divider to connect to the FB pin. This pin is regulated at 2.1 V during constant voltage regulation. The top of this voltage divider will be regulated to the set regulation during constant voltage mode when FB is regulated to 2.1 V.

1. The connection to the battery will determine the voltage difference between the actual battery voltage and the FB pin voltage divider regulation (the top of the divider being connected to the same node as the battery). As said earlier, the charger regulates to an internal 2.1 V reference on the FB pin during the voltage regulation phase. The charger will increase the current until the FB pin voltage divider reaches regulation. If the FB pin connection reaches regulation before full charge current is reached, the part has entered the constant voltage mode. This means that the impedance between the top of the FB divider and the actual battery connection is such that the lower value of charge current makes the FB divider regulate to full regulation voltage. If the FB pin connection does not reach regulation by the time the full charge current is reached, there will be full charge current until the regulation voltage is reached. At that point, the charger will transition from the constant current phase to the constant voltage phase.

2. This question is related to the previous. When the charger enters the regulation voltage phase, the FB pin is regulated to a fixed voltage. However, as the current is slowly tapered down, the voltage of the actual battery will slowly rise to the regulation voltage until it terminates. At this point, the actual voltage of the battery will be very close to the top of FB divider. During the constant regulation phase, the top of the FB pin divider will be regulated. The charge current is slowly tapered down in this constant voltage phase to ensure full regulation of the battery voltage.

Thanks,

Mike Emanuel

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Mike:

Thank you for the additional detail about the charging and re-charging profiles.

I have a related question about charge termination:

We want to use the TTC capacitor as a safety backup to terminate charging after (for example) 4 hours.  However, I tested this by choosing a deliberately small TTC cap (4.7nF = 26 minutes).  However, I do not see charge terminating, even after 1.5 hours.  Is there some other required configuration to enable this function?

Thanks,

Jackson

Jackson,

NiMH battery termination is very different from Lithium Ion battery termination. I would encourage you to read the following as they explain the inherent differences.

https://www.ti.com/lit/an/slyt468/slyt468.pdf

https://www.ti.com/lit/an/slua015/slua015.pdf

TI has an application note for charging NiMH batteries using the bq24105-Q1. It requires a microcontroller to correctly charge the NiMH batteries.

https://www.ti.com/lit/an/slua843/slua843.pdf

Thanks,

Mike Emanuel

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Mike:

Thanks for the additional information.  We already have looked at the first AN, and that is what led us to believe that we could use the BQ24172 for charging NiMH packs.  The TI product selector also indicates this.  A NiMH cell provider familiar with the BQ24172 also supported this decision.  We need a compromise solution since our customer may go with Li or NiMH chemistries.  I know that we are not terminating charge in the standard ways for NiMH (temperature rise or voltage dip).

However my current question is about the TTC timer not seeming to work:

We want to use the TTC capacitor as a safety backup to terminate charging after (for example) 4 hours.  However, I tested this by choosing a deliberately small TTC cap (4.7nF = 26 minutes).  However, I do not see charge terminating, even after 1.5 hours.  Is there some other required configuration to enable this function?  I see the TTC pin oscillating (triangle @ 11KHz, between 1.55V and 0.95V).

Thanks,

Jackson

Jackson,

In the datasheet, the range for the fast charge timer is 1 hour to 10 hours.

Can you please experiment with other capacitor values that put the fast charge timer in the correct range?

Thanks,

Mike Emanuel

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Mike:

Yesterday I tried 22nF, which should have terminated at approximately 2 hours.  But the device charged for about 3.5 hours, until 10% Ichg was reached.

The charge cycle with 4.7nF also just completed, at 10% Ichg, after about 3.5 hours.

Please let me know if you need more information to help resolve this.

--Jackson

Jackson,

Are you charging with Lithium Ion batteries or Nickel Metal Hydride Batteries when doing this? If you are using the NiMH battery, the default termination scheme of the bq24172 will not follow the process outlined by the previous documents. In fact, the voltage of the cell will drop when it reaches peak voltage. The bq24172 uses constant current followed by constant voltage, while the NiMH requires peak-voltage-detection. External circuitry is required to correctly charge the NiMH battery. Please refer to the previous documents shared.

Thanks,

Mike Emanuel

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Mike:

Again, I understand these points about terminating charge with the NiMH.  That is not the issue we are dealing with right now. 

My question is still about how the TTC cap is supposed to function.  I confirmed in my last post that a 22nF cap (about 2 hours) does not terminate charging.

Why would the presence of NiMH vs. Lithium batteries have an effect on this function of the BQ24172?

Thanks,

Jackson

Jackson,

As shown earlier, the batteries respond differently when reaching full charge voltage. If the cell voltage drops, as in NiMH, then the charger will charge longer to reach the desired regulation. The key here is that the cell voltage drops. This inherently alters the termination. In addition, if the cell voltage regulation is set too low, the part may cross the recharge threshold and restart charge. If you want to accurately charge a NiMH battery, you need a microcontroller for proper termination.

Thanks,

Mike Emanuel

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Mike:

What if I just want to stop charging after 2 hours, regardless of chemistry or how fully charged the battery is?

Why does TTC not terminate charging i this case?  That is the observed behavior, and it has nothing to do with regulation at the FB pin.

Datasheet section 9.3.9 "Enable and Disable Charging" says:

"One of the following conditions stops ongoing charging...TTC timer expires or precharge timer expires."

--Jackson