Using TI Fuel gauge with a charger IC for Lithium batteries

Thread Starter


Joined Mar 5, 2020
I am using a TI fuel gauge (BQ27621YZFR-G1A) to meausre SoC of my Lithium battery.
I also have a charger IC (MCP73832T-5ACI).
My issue is, the fuel gauge works fine when the batteries are not being charged. SoC measurements are characterized and accurate.
But when the device is put into charging more, the charger IC charges the Lithium batteries, In this situation when the fuel gauge is polled for SoC, it returns values lower than they were before charging.

For example, in following steps
1. Battery not charging, Fuel gauge estimates SoC to be 90%
2. Battery connected to charger, fue gauge estimates SoC to be 60%
3. Disconnect charger IC (disable it by plugging out V-USB), Fuel gauge estimates SoC to be 90% again.

I have a EEMB lithium batteries with 4.2 mac voltage and 500mAh. I follow the datasheets for the design and usage.
What could lead to this undeterministic behaviour? I cant see a flaw in the design. Is it not allowed to do a fuel gauge estimation when we are charging? Do I need to control the two ICs and switch them on/off in firmware to keep charger IC off when fuel gauge estimates the SoC?