bq34z100-g1 grounding question

I think I had this backwards in my mind. So the gas gauge chip ground is the same ground as everything else. The gas gauge chip actually measures the current sense Voltage negative to it's own ground (and everything else ground. ) Am I now correct? But the charger Voltage will still be wrong by the amount of drop across the current sense resistor?

We referenced the grounds on the bq34z100-G1 EVM to BAT-, but you can reference it to PACK-. There is a GNDSEL bit in the Pack Cfg register to allow either option. You can also reference the device and its external support components to BAT- and the I2C but to PACK-. The voltage drop across the sense resistor will not be large enough to cause communications problems.

Hi Thomas,

Thank you for the reply. Either way I do it, the sense resistor has to end up in series between the charger and the battery or the gas gauge can't measure the charge current. So when I enter the CV phase of charging, the actual charge Voltage across the battery will be the charger Voltage minus the sense resistor drop. So, will it mess something up if I toggle the GNDSEL bit during charging? I could measure Voltage with A/D referenced to pin 10, then measure Voltage with A/D referenced to VSS, subtract and know the sense resistor drop so I can increase the charger Voltage by that amount. I would need to do this every maybe 5 minutes as the charge current will be decreasing during this "absorption" CV phase of charging.

If you think that has the possibility of messing something up, the alternative is to just read the charge current and calculate the sense resistor drop if you think that is a safer bet.

This lead acid battery data sheet is very specific about charge Voltage being exactly right (adjusted for temperature) for this "absorption" phase of the battery charging.

Also, there isn't any true charge termination. After 6 to 8 hours of this absorption phase, I lower the charger Voltage and leave it charging at this lower Voltage continuously. This is to basically "break even" with the battery's self discharge. This charge current will probably be well below the relax threshold current but it won't be zero. Do you think this could be a problem for gas gauge accuracy?

Marty

The gauge is designed to declared charge termination after the current has dropped below the taper current, so it should work if you reduce the charging voltage and allow the current to drop below the taper current. The current cannot drop immediately to zero or it will not work. I should be allow to remain between zero and the taper current for at least 80s.

The voltage drop across the resistor should not be a problem when using the taper current technique. If the taper current is 250mA, then the voltage drop across the sense resistor will be less than 2.5mV. (10 mohm sense resistor).

Hi,
what if my BQ34Z100-G1's reference is to BAT- and I have a microcontroller communicating with it in my circuit? I mean, the I2C's GND will be directly connected to BAT-, but if I want to measure the current consumed by the microcontroller, its negative supply pin is going to have to be connected to PACK-, in order to this current pass through the shunt resistor, right? In this case, BAT- e PACK- will be connected by the I2C's GND and the supply pin of the microcontroller, resulting in a messed up reference and still without being able to measure the microcontroller's current correctly, since at least part of the current will go to BAT- directly, without passing through the shunt resistor. In other words, it doesn't look possible to me to use BAT- as a reference to BQ34Z100-G1 if both it and the microcontroller hav a battery as power supply. What do you think?

Thanks
Vítor Pereira

Sorry, read it again and saw the tip about referencing the I2C to PACK-.