BQ24073: Charging current problem

Hello,

Thanks for reaching out.

The pre-charge to fast charge transition threshold (VLOWV) is 3V (typical). When VBAT < VLOWV, the battery will charge at the pre-charge current level, which is 1/10 of the fast charge current. With a fast charge current of 350mA, it's expected to see 35mA of pre-charge current.

Per Sennström said:

When I charge under 3V it charges with the correct value 350mA.

If I understand correctly, you're measuring 350mA of charge current when VBAT < 3V? Is this correct? Could you please share the value of RILIM in your design?

Per Sennström said:

Above 350mA it my battery simluator goes into current limit (although i have a limit set to 1,2A).

Could you please clarify whether you mean "above 3V?" Does your battery simulator hit its current limit when VBAT > 3V?

I'd recommend measuring VBAT as close to the BAT pin of the charger IC as possible, such as on the BAT pin capacitors. While charging, there will be IR drop between the BAT pin and the positive terminal of your battery, causing the BAT pin to see the battery voltage plus this IR drop. Could you please confirm how much current you're measuring when VBAT is below 3V?

Best regards,

Alec

Hi Alec

Alec Lehman said:

If I understand correctly, you're measuring 350mA of charge current when VBAT < 3V? Is this correct? Could you please share the value of RILIM in your design?

Yes correct. RILIM is set to 2,55k

Alec Lehman said:

Could you please clarify whether you mean "above 3V?" Does your battery simulator hit its current limit when VBAT > 3V?

From around 3V up to 4,5V the battery simulator hits limit, I have set limit to 1A. Below 3V it doesn't hit current limit on the battery simulator.

Alec Lehman said:

I'd recommend measuring VBAT as close to the BAT pin of the charger IC as possible, such as on the BAT pin capacitors. While charging, there will be IR drop between the BAT pin and the positive terminal of your battery, causing the BAT pin to see the battery voltage plus this IR drop. Could you please confirm how much current you're measuring when VBAT is below 3V?

Below 3V I have measured 350mA.

I have also today removed the battery protection since it shuts off charging due to high currents and see similar issue (without the symptoms of the battery safety shutting down charging and resuming.

Thanks

Per

Hello Per,

Thanks for following up.

RILIM=2.55k will set the fast charge current to 350mA. If VBAT > 3V, I'd expect the charge current to be 350mA. If VBAT < 3V, I'd expect the charge current to be 35mA. When you see ICHG=350mA while VBAT < 3V, are you measuring VBAT as close as possible to the charger IC?

When your battery simulator hits its current limit, is the current flowing into or out of the battery?

Do you have a system load connected to OUT? If so, how much current is it pulling?

Could you please remove RILIM from your PCB and measure its resistance?

Are you seeing this behavior on multiple boards?

Best regards,

Alec

Hi Alec, thanks for the reply.

Alec Lehman said:

RILIM=2.55k will set the fast charge current to 350mA. If VBAT > 3V, I'd expect the charge current to be 350mA. If VBAT < 3V, I'd expect the charge current to be 35mA. When you see ICHG=350mA while VBAT < 3V, are you measuring VBAT as close as possible to the charger IC?

Exactly, me too and VBAT is not measured, I am using the value on the battery simulator Keithley 2306. However I have removed the battery safety and connected GND and VBAT close to the charger IC.

Alec Lehman said:

When your battery simulator hits its current limit, is the current flowing into or out of the battery?

Into the battery.

Alec Lehman said:

Do you have a system load connected to OUT? If so, how much current is it pulling?

Yes and around 20mA.

Alec Lehman said:

Could you please remove RILIM from your PCB and measure its resistance?

Are you seeing this behavior on multiple boards?

Done and measured RILIM when on board measured 1,4k, when removed measured 3,4k and yes, we see this behaviour on all boards,.

Thanks!

Hi again Alec

Update from my side. We have solved the with the current running high but have noticed a voltage drop over a transistor in the battery safety while charging. It occurs up to 3.1V then all is good. Transistor is EFC4627R. This gives the effect that the pre-charge function doesn't work.

Best

Per

Hi Per,

Thanks for the update. I'm glad to hear you found the issue! I'll follow up with you on Monday, 7/21.

Best regards,

Alec