BQ24172: Power management forum

Jackson Wilson

Part Number: BQ24172
Other Parts Discussed in Thread: BQ2000, BQ24105-Q1

In other posts (https://e2e.ti.com/support/power-management/f/196/t/915362 and https://e2e.ti.com/support/power-management/f/196/t/918802I have received conflicting information regarding the suitability of the BQ24172 for charging NiMH packs.

The TI landing page for the BQ24172 (https://www.ti.com/product/BQ24172?qgpn=bq24172#product-details) indicates that it can be used with NiMH chemistry.

Can someone from TI please confirm whether we can safely and effectively use the BQ24172 with NiMH batteries?

I understand that we will need to manage charge termination in a different way than is usually used for NiMH cells, and that we may not charge to full capacity with this solution.

We have reviewed the TI app note (https://www.ti.com/lit/an/slyt468/slyt468.pdf) which describes how to use Li-ion chargers with NiMH batteries.

Thanks,

Jackson

over 5 years ago

0 Jeff F over 5 years ago

TI__Guru**** 166626 points

Hi Jackson,

The BQ24172 can only provide the constant current portion of charging. If the charger goes into constant voltage mode, as it would for a lithium ion battery, the charger will likely not terminate properly. I do not know which LiIon charger Charles Mauney used for his appnote. He is no longer with TI. We have other customers who have used the BQ24172 for NiMH charger but they are either using the safety timer to terminate charge with <0.5C charge current and the CV set point set to an OVP point, similar to that explained in the app note below.

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

Or, they are using another IC (for example, bq2000) or host processor with ADC to perform termination.

Regards,

Jeff

0 Jackson Wilson over 5 years ago in reply to Jeff F

Intellectual 330 points

Jeff:

Thanks for your reply. The app note about NiMH in automotive applications will be useful.

We have MCU connections to TS and FB pins (ADC channels), and STAT and ISET pins (GPIO). Our intention is to monitor STAT, TS, and FB, and disable charging by holding ISET low, when voltage on FB reaches a certain level, and/or temperature on TS rises above a certain threshold.

Another issue is that I have not been able to get the safety timer to work with our NiMH packs. I placed a 15nF cap (should be 84 minutes), but charging would not stop when using NiMH cells (safety timer did function correctly with a LiPo pack).

I was told in another post on this forum (https://e2e.ti.com/support/power-management/f/196/t/918802) that the safety timer would not work for NiMH cells. Can you confirm whether this is correct?

Also, the charge profile we see when using a NiMH pack looks like has a sort constant current phase (30 minutes - 1 hour depending on the NiMH pack we are using, and depth of discharge). Then there is a longer phase of slowly ramping voltage and decreasing current (image below). This does not exactly match either the CC or CV phases, but it seems to provide close to full charge to the cells. It will even (with some NiMH packs) terminate when it reaches 0.1C.

I hope that you can confirm that our approach will work, and that the results we are seeing are as expected.

Thanks,

--Jackson

0 Jeff F over 5 years ago in reply to Jackson Wilson

TI__Guru**** 166626 points

Hi Jackson,

The charger's safety timer isn't going to know if you have LiIon or NiMH connected. If the safety timer doesn't appear to be stopping charge, it is because it is getting reset, most likely due to termination from CV mode/ITERM or toggling ISET. Setting CV to a higher OVP setting should prevent termination before the timer expires unless you a highly resistive connection to the NiMH batteries or the batteries themselves are high impedance.

If you are using your MCU to terminate, then I see no need for the safety timer for termination, unless you want it for redundant protection. If you tie TTC pin to ground, the safety timer and termination are disabled. If your MCU has accurate enough ADC, you might be able to use the termination algorithm for -dV/dt and dT/dt as explained in BQ2000 datasheet. Keep in mind NiMH batteries voltage drops slightly when they are full charged.

The only recent data that I have on NiMH charging is from the BQ24105-Q1 app note that I sent you. The plot you show above implies that the voltage at FB is closer to termination than the voltage at the batteries themselves; therefore, the charging is entering CV mode early. Is possible that your MCU GPIO pin that is monitoring is pushing up the FB pin voltage?

Regards,

Jeff

0 Jackson Wilson over 5 years ago in reply to Jeff F

Intellectual 330 points

Jeff:

Thanks for clarifying the safety timer function. I feel pretty confident that we can use the BQ24172, with our MCU, to charge the NiMH pack effectively, but we would like to implement backup protection (safety timer) in case our MCU fails.

Is there a way to tell that the safety timer has been reset? I am not toggling ISET. Does the TTC pin change behavior?

Can you clarify how I can tell if safety timer is resetting "due to termination from CV mode/ITERM"?

I am not sure how to go about "setting CV to a higher OVP setting". Can you explain this in a bit more detail?

Would it be helpful to forward a schematic of how we have implemented the charging circuit?

Thanks again!

--Jackson

0 Jeff F over 5 years ago in reply to Jackson Wilson

TI__Guru**** 166626 points

Jackson,

Regarding how to know when the safety timer expires, the only way of which I am aware is to monitor the STAT pin for blinking, which indicates a fault.

Regarding setting the FB resistors for OVP, LiIon batteries charge to a fixed voltage (CV) and stay there when charged. So a LiIon charger simply enters CV regulation and waits for the charge current to taper down to termination current (ITERM) and reports charge done. When a NiMH battery reaches full charge, its voltage decreases slightly, hence the recommendation to terminate with -dV/dt. Also, the voltage at which -dV/dt occurs varies as the battery's impedance changes with age. Therefore a LiIon charger waiting for a fixed voltage may never terminate or may terminate too soon for high Z batteries. Therefore, we recommend to only use a LiIon charger for constant current charging of NiMH batteries and use the safety timer or processor with ADC to terminate. To prevent termination by CV/Iterm, you must set by the FB resistors higher to an OVP point in order to prevent termination by CV and termination current. In other words, you can't rely on static NiMH battery voltage for termination.

Regards,

Jeff

0 Jackson Wilson over 5 years ago in reply to Jeff F

Intellectual 330 points

Jeff:

Thanks for the clear information! I think we're in pretty good shape with using the BQ24172 to charge our NiMH packs now.

--Jackson