bq34z100 circuit diagram

I am a beginner in electronics. I generally code and write software. This is for one of my projects.

We are trying to use the Texas Instruments BQ34Z100 fuel gauge IC to measure state of charge (SOC) for our battery power supply.

After reading through the datasheet I understood that an external voltage divider is needed for battery voltages greater than 4.5V, but I don't understand why they are using MOSFETs with the resistors for voltage translation from higher to lower.

  • \$\begingroup\$ Read the description of VEN in table 5-1. \$\endgroup\$
    – CL.
    Commented Jun 30, 2022 at 9:10
  • 2
    \$\begingroup\$ The mosfets are used to switch the resistor divider when needed, otherwise the resistor divider would cause unwanted discharge of the battery. \$\endgroup\$
    – Kartman
    Commented Jun 30, 2022 at 9:11
  • \$\begingroup\$ Thank you,so when the switch connected to the VEN is closed the internal divider is disabled and the external divider circuit is used right?,sorry if am bothering with question but i am little new to this field and trying to understand and adapt these into my own projects which i am currently stuck on. \$\endgroup\$
    – IMEI
    Commented Jun 30, 2022 at 9:18

1 Answer 1


The MOSFETs disconnect the voltage divider from the battery.

The voltage divider draws a tiny bit of current all the time. If you want to get the longest possible run time out of your battery, every milliampere counts.

A 1 milliampere constant drain would take 24 mAh out of your battery every day. That would discharge a 250mAh battery in less than 10 days.

The MOSFETs are arranged to connect the voltage divider only when needed. The voltage divider then only draws current for a few minutes (or seconds) a day instead of all the time.

There's a note under the circuit diagram that says the MOSFETs are optional.


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