bq24079 / Charging terminated immediately

Hi,

Our customer found that this problem is happened due to higher impedance into battery pack. They tried to add 1ohm resistor in series to normal battery pack, then they could duplicate the same problem. Now I need to explain the mechanism of this problem so can you give your comments for following question?

1, Why is the charging not started and terminated with almost charged around 70% and higher impedance battery pack?

2, Is this problem also happened on all of other TI's charger devices like bq2403x or others?

Best Regards,

Sonoki

Hi,

Following is the mechanism I think.

- Unexpected voltage will be added if the impedance in battery (Rbat) is higher than normal one and flow the pre-charge current to it.

- The charger may detect VBAT(REG) wrongly when the cell voltage (Vcell) is almost VBAT(REG).

- The charger will be into CV phase but the current can be flowed as (VBAT(REG) - Vcell) / Rbat.

- As the result the charger will detect the termination by detecting Iterm wrongly.

I'm waiting for your comments. Your quick response will be appreciated.

Best Regards,

Sonoki

Based on the description, I think what is happening is actually expected behavior. The way the battery charger terminate charge is by looking at the charge current when the battery voltage is above the VRCH threshold. In this case, it seems what is happening is that the large impedance is causing the battery voltage at the charger pin to be right at the CV value. Once the charger gets to CV and the charge current drops below the termination, the charge process starts. In order to confirm this is the case, we can do a few experiments and calculations.

It seems they are using 300mA charge current. With a 1 ohm impedance in series, when the battery is 4V, the voltage will try to go to 4.3V (4V + 0.3A * 1 ohm). However, the CV loop of the charger will prevent that from happening and the charge current will drop as expected. We need to see the battery voltage on both the actual battery and the VBAT pin to see how the signals look during this condition. I will also expect to not see this if the battery voltage is lower as the impedance is not enough to cause the VBAT voltage to rise to the CV level.

Please let me know if you have any questions.

Regards,

Miguel

Hi Miguel-san,

Thank you for your response.

I attached the additional waveform from our customer which include voltage at BAT pin. Please refer the page 2 of attached. I informed in previous post that there may be some impedance on charging pass but they confirmed this problem with out any additional impedance. Therefore the voltage on BAT pin is almost stable.

Now I found additional concern about their circuit. They are using 3kohm as RISET so the calculated IPRECHG and ITERM are almost the same. I suspect this may be cause of false detect the termination. Can you verify about this?

Following are their setting.

RISET : 3000 ohm
KISET : 890 Aohm
ICHG : 296.7 mA

KPRECHG:  88 Aohm
IPRECHG : 29.3 mA

ITERM : 29.7 mA (CE = LO, (EN1, EN2) ≠ (LO, LO))
                9.79 mA (CE = LO, (EN1, EN2) = (LO, LO))

Best Regards,
Sonoki

Hi Miguel-san,

Now I'm requesting our customer to change the charging current higher and confirm if this problem will be solved. I'll update when I will receive the result.

Best Regards,

Sonoki

Hi Miguel-san,

Now I'm waiting for the additional customer's evaluation result, but I'd like to have your comment about possible supposition if the fast charging current was set to low suck as 300mA. I should try it by using EVM but I'm sorry that I do not have it in my bench...

Best Regards,

Sonoki