BQ25756: Charger re-starts charging even after healthy termination - after toggling CE pin

Hello Ali,

Thanks for working with this. Yes, this is normal behavior. If you set EN_CHG=0 and then set EN_CHG=1, the BQ25756 will go through another charge cycle regardless of the batteries discharge.

If you leave EN_CHG=1 and the BQ25756 terminates, the BQ25756 will start recharging only after the battery voltage drops to the recharge threshold.

Do you have an MCU toggling and then un-toggling the CE pin?

Let me know if you have any questions about this

Best Regards,
Ethan Galloway

Hi Ethan,

Thank you for your prompt response. Yes, the MCU is toggling the charger off/on repeatedly. I take it you are suggesting that we need to keep it on to allow it to terminate and naturally resume charging upon 93% VBatt?

Hi Ethan,

We will try to find ways to keep it on then. This answers our question.

Thank you!

Hello Al,

Your welcome. Let me know if you have any questions about this.

Best Regards,
Ethan Galloway

Thank you. Since this termination state seems volatile on this IC, may I know if there's any other way to stop charging at will, while still maintaining the termination state?

Asking as we're trying to avoid offloading this task (monitoring battery voltage and enabling charger accordingly) to the MCU as much as possible.

Hello Ali,

I'll look into this. Can you tell me why the system needs to stop charging at will? Maybe I can find another way for the system to do this function.

Best Regards,
Ethan Galloway

Hi Ethan,

The system houses a motor controller to operate a 24V motor that would deal with 1000s of Newtons of torque, so we deal with very high motor starting currents (transients) that repeat 100s of times a day. To have the motor suddenly start while the charger is operating would be to expose the charger to excessive instantaneous loads repeatedly and wish for its internal protection to kick in every single time without failing down the road (15-20 years).

In order to protect the charger from that, we let the FW handle it by disabling the charger shortly before the motor starts, then re-enabling it again shortly after it stops. The charger currently handles a max of 2.2A at the input and 1.4A at the output after boost converting 18V to 29.2V. (The source feeding the charger cannot supply currents beyond 2.2A @ 18V / 40W anyway).

This method was used with the previous charger with success. We are simply doing the same with the new BQ25756. Then we realized that it is causing the charge cycle to repeat, and over time the battery gets over-charged, indirectly affecting how the Fuel Gauge looks at the capacity and also keeping the LFP battery floating at full-charge indefinitely, until the sun sets, which is known to be an unhealthy practice and impacts battery life.

Let me know what you think.

Regards,

Ali

Thank you for your response, Ethan.

Indeed. We can confirm through experiments that the charger would not output beyond 1.4A but not yet sure what happens when consumption spikes up instantaneously, hence asking.

To your question, if we run the motor while the charger is ON, it would likely always guarantee resuming charging due to the dip in battery voltage below 93% at the start. We have not tested this out yet and whether it would eventually cause over-charging.

Q/ Do you think we can lower the recharge threshold below 93%? It seems to be the lowest option on BQStudio.

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We have not yet tested keeping the charger ON as the motor starts since we fear the same unpredictable/random failure in the field due to this transient behaviour down the line, which we already resolved with the previous charger. It used to indirectly trigger the motor power gate into an "open" state after it senses a possible short or very low voltage event when the charger gets overloaded, ultimately shutting down the motor quickly after starting. However, the issue was solved after disabling the charger before the motor operation. Tested over many months already.

Given it is usually not a recommended practice for us to revert this FW fix as it would need long-term field testing to guarantee stability,

Q/ Could you guarantee that operating the charger this way with motor currents varying suddenly, widely and very often, is going to be completely normal? (Asking to emphasize - I need to be sure before advising our FW team to go down this route)

Let me know your thoughts.

Ali

Hello Ali,

Ali Al-Sheikh said:

To your question, if we run the motor while the charger is ON, it would likely always guarantee resuming charging due to the dip in battery voltage below 93% at the start. We have not tested this out yet and whether it would eventually cause over-charging.

Q/ Do you think we can lower the recharge threshold below 93%? It seems to be the lowest option on BQStudio.

93% is the lowest recharge threshold.

Ali Al-Sheikh said:

Q/ Could you guarantee that operating the charger this way with motor currents varying suddenly, widely and very often, is going to be completely normal? (Asking to emphasize - I need to be sure before advising our FW team to go down this route)

Thanks for clarifying your concern here. Let me check on this.

Can you have the MCU remember the termination state and then set EN_CHG=1 once the battery reaches the recharge threshold? For example, the motor activates and the MCU disables the charger. Once the inrush current is over, the MCU then sets EN_CHG = 1 or EN_CHG = 0 depending on if the charger was in the "Charge Termination Done" state or not?

Best Regards,
Ethan Galloway

Hi Ethan,

Thank you for your quick response.

Ethan Galloway said:

Can you have the MCU remember the termination state and then set EN_CHG=1 once the battery reaches the recharge threshold? For example, the motor activates and the MCU disables the charger. Once the inrush current is over, the MCU then sets EN_CHG = 1 or EN_CHG = 0 depending on if the charger was in the "Charge Termination Done" state or not?

Yes, that is the solution we thought of, but we figured maybe we reach out first to hear from you should there be another simple and more straightforward solution.

Looking forward to hearing your input on my earlier question "How normal it is to keep the charger enabled in the presence of extreme and spikey loads?"

Regards,

Ali

Hello Ali,

Got it. Thanks for the clarification.

Ali Al-Sheikh said:

How normal it is to keep the charger enabled in the presence of extreme and spikey loads?

It's relatively normal. I haven't seen any problem with load transients as long as the battery is connected to the BQ25756.

Let me know if you have any other questions about this.

Best Regards,
Ethan Galloway