BQ27742-G1: Low voltage operation

Hello,

Could you provide the .gg and log file from this test? If it's done on a custom PCB are you able to recreate it on the EVM?

It will be helpful if we can see how the gauge is operating during the gap with the logging feature of bqStudio.

Sincerely,

Wyatt Keller 

Hello. We have Gauge Development Kit (GDK) ans we use BQStudio, but unfortunately, communication with the fuel gauge board via i2C is only possible when the battery voltage is 2.25V or higher.
In the range 1.7..2.25 there is no communication via i2c and it is impossible to write log data using the GDK.

Sincerely, Yuri Lagoydich

Hello Yuri,

The low voltage charge mode should be enabled when you're charging above the V_LVDET and lower than the V_UVP. What voltage are you applying to charge the battery?

Sincerely,

Wyatt Keller

Hello, Wyatt

In the range of 1.7V (V_LVDET) to 2.13V the charge voltage from an external power source is 3V - 3.3V, the charge current is about 20-40mA.

But in the range of 2.13V - 2.25V, the charge stops. We tried to raise the charge voltage to 4.2V in this range, but there is still no charge current. In this range the CHG mosfet is closed and it is not clear why.

Starting with a battery voltage of 2.25V, the charge current starts again 100-150mA with a charge voltage of an external power source of 3V - 3.3V.

Sincerely,

Yuri Lagoydich

Hello Yuri,

Could you clarify the test conditions? The gauge has an external power input and should be able to communicate with a charger present. If the gauge s not powered during this region the FET won't be controlled.

Do you charge the battery separately to get to the 2.25V before it starts charging again?

Sincerely,

Wyatt Keller 

Hello, Wyatt

Below I give an schematic diagram for clarity. There is no communication with the sensor in the range 1.7..2.25V. The charge, as I said above, is only in the range 1.7..2.13V. If the battery voltage is above 2.25V, there is also a connection via i2C

I do not charge the battery separately to 2.25V after a forced discharge. Since the battery is in good condition, its voltage rises very slowly to 2.25V on its own, without a charging current in the range of 2.13..2.25V. But this will not happen with a battery in poor condition.

Hello Yuri,

During this region it will be performing a trickle charge of the battery because the CHG FET will be in the ohmic region, have you tried lowering the charging voltage as well?

Can you outline your testing procedure and battery specs?

Sincerely,

Wyatt Keller

Yes, I tried to lower the charge voltage too. Charge voltage varied from 2.8V to 4.2V

The testing procedure is as follows:
1. Forced battery discharge up to 1.7V. The load connects directly to the battery terminals
2. Turn off the load from the battery. After that, we connect the charging voltage from the external power source to the FG board, as shown in the schematic diagram above.
The charge voltage from the power source is 2.8-3V, the charge current is 40-50mA.
The voltage on the battery during this period gradually increases.
3. When the battery voltage reaches 2.13V, the charge stops. We tried to change the charge voltage from 2.8V to 4.2V,
but there is no charge current in this range. And yes, during this region it should be performing a trickle charge of the battery because the CHG FET will be in the ohmic region, but it is not - the CHG mosfet is completely closed.

4. We wait until the battery voltage rises to 2.25V on its own, without charge. In this case, the voltage from the external power source remains connected to the FG board and is equal to 3.3V. After reaching the battery voltage of 2.25V, the charge starts again and the charge current is about 100-150mA. In this range, there is a connection with the sensor for i2s.

The performing of the device does not depend on the type of battery. The attached file contains the specification of one and the corresponding battery types

18650 datasheet.pdf

Hello Yuri,

Just to confirm, you are seeing the gate of the charge FET connected to the pack voltage before 2.13V is reached, then at that voltage it is no longer connected and drops down to a lower voltage? (voltage goes low again?)

I am talking with another apps engineer about this issue, I hope to have an answer for you tomorrow.

Sincerely,

Wyatt Keller

Hello, Wyatt

I measured the voltage to the gate-source of the CHG mosfet and on the CHG pin of FG

In the range of 1.7V to 2.13V:

Gate-source voltage is 0.9-1V, mosfet in the ohmic region, the charge is on.
The voltage on the CHG pin of FG relative to the GND is 2.5-2.8V, the charge voltage from an external power source is 3-3.3 V

Starting at 2.13 V:

The voltage on the CHG pin of FG relative to the GND becomes 0 V , the charge voltage from an external power source is 3-4,2 V (no matter)

Gate-source voltage of the CHG mosfet becomes negative (-2.13 V), of course. The mosfet turns off, the charge is off

Sincerely,

Yuri Lagoydich

Hello, Wyatt

Yes, it seems mosfet turn off during the transition from low voltage charging mode. But this is not brief and the duration of switched off mosfet depends on the rate of increase of voltage on the battery without charging current. If the battery is in poor condition after self-discharge, then the voltage on the battery will not overcome the gap of 2.13-2.25V without charging current and mosfet will be closed indefinitely. The voltage on the battery will remain no more than 2.13V. We have many such batteries, although after recovery they can still work many cycles. If nothing can be done in the range of 2.13-2.25V, then in our applications  the minimum voltage will be set at 2.25V instead of 1.7V.

Sincerely,

Yuri Lagoydich

Hello Yuri,

If you are not seeing a time delay and it is a voltage gap I'm not sure what the problem could be, I will check to see what is meant by briefly turning the FETs off.

Have you tested it with multiple chips or is it only occurring to one chip?

I will let you know when I hear back from my colleague next week.

Sincerely,

Wyatt Keller

Hello Yuri,

With batteries that are used or older you observed the charging never begins again after entering this voltage range? The gauge should be able to eventually get out of the mode because when the charge stops the voltage drops and starts the low voltage charge again, this should loop until you are out of the 2.13v to 2.25v region. Is this not happening for your batteries?

You can also try increasing the capacitance on the REG25 pin to 2.2uF or a little higher to see if this helps.

Sincerely,

Wyatt Keller