BQ25798: Bq25798

Hi MJ,

The ADC looks at the BATP pin voltage.  Where are you measuring 3.6V?  If there is a large resistive connection (thin trace or wire) from the BATP sense point to the battery positive terminal, there will be a voltage drop = Resistance *ICHG.  If you disable charge does the ADC measure correctly?  If so, that is the issue.

Regards,

Jeff

Hi Jeff,

Where are you measuring 3.6V Ans: At battery terminals.

As you see with in the attachment with  BATP we have connected 100ohm Reg, is this causing an issue?.

We have few query's.

1. Can we supply voltage from an adopter rated 5V–3A, 20V–3.25A? Is the BQ chip able to switch to 5V where 1 cell is used? In the attached circuit, we are using MP8759GD to set 5V. Is MP8759GD really required here?

2. Using your earlier input to read battery: 0x3B = 0x10 and 0x3C = 0x63. Combined 0x1063 equals 4195 in decimal. What does the 0x3C register represent?

3. How can we accurately measure battery current and temperature?

4. The issue we face is that battery readings are incorrect — as soon as power is connected, it directly shows 4.1V.

5. Do we need additional fuel gauge to measure the battery ?

Hi Jeff,

Where are you measuring 3.6V Ans: At battery terminals.

As you see with in the attachment with  BATP we have connected 100ohm Reg, is this causing an issue?.

We have few query's.

1. Can we supply voltage from an adopter rated 5V–3A, 20V–3.25A? Is the BQ chip able to switch to 5V where 1 cell is used? In the attached circuit, we are using MP8759GD to set 5V. Is MP8759GD really required here?

2. Using your earlier input to read battery: 0x3B = 0x10 and 0x3C = 0x63. Combined 0x1063 equals 4195 in decimal. What does the 0x3C register represent?

3. How can we accurately measure battery current and temperature?

4. The issue we face is that battery readings are incorrect — as soon as power is connected, it directly shows 4.1V.

5. Do we need additional fuel gauge to measure the battery ?

Hi MJ,

Jeff is out of office for a while, so I can help answer some questions while he is out.

M j said:

As you see with in the attachment with  BATP we have connected 100ohm Reg, is this causing an issue?.

100 Ohm should be fine. Any issues Jeff anticipates would be parasitic from thin/long traces.

M j said:

1. Can we supply voltage from an adopter rated 5V–3A, 20V–3.25A? Is the BQ chip able to switch to 5V where 1 cell is used? In the attached circuit, we are using MP8759GD to set 5V. Is MP8759GD really required here?

From my understanding, 20V and 3.25A are within the ratings for the input of the BQ25798 device, but it will likely reach thermal shutdown before it can get to 60 W of power.

M j said:

2. Using your earlier input to read battery: 0x3B = 0x10 and 0x3C = 0x63. Combined 0x1063 equals 4195 in decimal. What does the 0x3C register represent?

Register 0x3C is an additional register to 0x3B. The registers on the device are 8-bit, but the ADC is 16-bit. Using 2 registers allows for the storage of the full 16-bit ADC reading.

M j said:

3. How can we accurately measure battery current and temperature?

Based on Jeff's reply, it seems that the ADC is most accurate when charge is disabled.

M j said:

4. The issue we face is that battery readings are incorrect — as soon as power is connected, it directly shows 4.1V.

What does it show before power is connected?

M j said:

5. Do we need additional fuel gauge to measure the battery ?

The BQ25798 can accurately sense the battery voltage, but the ADCs may less accurate than what the control loop determines. If an accurate voltage reading is needed externally, within an MCU or otherwise, a fuel gauge or monitor would be helpful. These ADCs should be much more accurate than the ADCs on the BQ25798.

Best regards,
Michael

Hi Mlchael,

Thank you 

• 4. before connecting power it tells 3.6, by reading 3b&3c.