BQ24040: SN2040: Detecting battery FULL event through CHG pin.

Just found out by reading some threads (e.g. this one) that the CHG pin can remain LOW if the following conditions happen:

1. TS (NTC) pin indicates an over-temperature condition. The charge is suspended and the CHG pin remains LOW.
2. If the battery is discharging slightly, it could trigger a recharge condition.

We replaced NTC with the fixed 10K resistor. Moreover, by monitoring the current that goes to/from the battery, we can clearly detect:

• Constant Current (CC) charging mode with the charging current being constant and equal to ~40mA (as expected)
• Constant Voltage (CV) charging mode when charging current starts dropping from ~40mA to ~0.1mA. CHG pin remains LOW when the charging current drops below termination current (~4mA) for some reason.

Hello

A couple of things to check:

1.) If the CHG pin is staying low, indicating charge is in progress then unit is not terminating charge.  Note that the CHG pin will only go low for the first charge cycle, not during a recharge.

2.) Unit not terminating charge. Try increasing termination from 10% to 20% ( 10k to 20k), problem may be an tolerance stack up at low current.

------The termination Deglitch time is 29ms, current must remain below termination level for this length of time.  We have seen pulsed loads prevent termination on other systems.

Hello, Bill Johns!

Thanks for your reply.

Concerning your points:

1. We don't have a problem with the CHG pin going LOW. Every time we connect a charger, the CHG goes LOW as expected.
2. It's quite difficult for us to change the resistor that is defining the termination current. We are currently using 0201 resistors on our board. What we can do is buy the BQ24040EVM module and play with it. However, before we do it, we would like to know if BQ24040 and SN2040 are fully compatible. What do you say?

Thanks for lettings us know about 29ms termination Deglich time! We are currently not using any pulsed loads. Our LED driver is turned OFF and the Host MCU is in sleep mode.

Regards,

Bojan.

Hi Bojan,

The BQ24040 should serve as a pin to pin replacement for the SN2040.

Best Regards,

Juan Ospina

Thanks, Juan Ospina!

Any idea why our CHG pin keeps LOW even though the charging current is below the termination current?

Last night, I've left the charger to charge the battery overnight. I noticed in the morning that CHG went to HIGH but this is probably due to 10 hours safety timer. 

Regards,

Bojan

Hi Bojan

The above over night test point to the unit not detecting a charge complete condition, current dropping to termination level and setting charge complete.

Termination is set by the resistor at PRE-TERM pin, the pin is a current source that will develop a voltage proportional to resistor value.  

When voltage at ISET pin equals voltage at PRE-TERM charge complete is set. 

The voltage at ISET is proportional to output current, the ratio is about 1 to 350.  But as charge current get lower the accuracy decreases reducing accuracy of termination point. 

I suspect this is the problem you are seeing and increasing the termination point may resolve the problem.

Hello, Bill Johns.

We have been playing with the charger for the past few days. It makes sense to increase a little bit termination threshold current as you suggest. We are gonna do it for the next batch of our PCBs.

However, it seems that something else is at the root of the issue. Even if we use a 40mA charging current and 10% termination current, simply power-cycling our board by disconnecting the battery and connecting it again seems to resolve the issue we have with the charger IC (SN2040). The charger becomes capable of detecting battery full events and toggling the CHG pin from LOW to HIGH.

Do you have any idea why SN2040 gets stuck sometimes and why power cycling the board recovers it to work properly?

Can we connect the TS output of the charger to our Host MCU and disable and re-enable charging by pulling the TS pin LOW for a short period of time every time we detect the charger IC getting stuck?

Sincerely,

Bojan.

Hi Bojan

Bojan Jovanovic said:

Do you have any idea why SN2040 gets stuck sometimes and why power cycling the board recovers it to work properly?

I suspect the battery voltage is above the Recharge Detection Threshold (4.1V) when power is cycled and the SN2040 sets charge complete.  At this point CHG will go HIGH, charge compete.

Hi, Bill Johns.

We try to properly discharge the battery before we start to charge it again. Anyway, we detect that both PG and CHG lines go LOW before we sleep in our Host MCU. After that, the CHG line never goes HIGH to wake the Host CPU.

Would pulling TS line LOW for a short time help the charger to reset itself?

Hi Bojan

Yes pulling TS low will reset the device and releasing will start charge. The CHG pin will also behave as a first charge, the pin will go low when charging.

Thanks, Bill Johns!

One last question... for how long we should keep the TS line LOW in order to achieve the reset effect?

Regards,

Bojan.