The MAX17043 fuel gauge uses a very small amount of current relative to the rest of the system. Typical usage is 50 μA which is 1/1000 of the 50 mA needed to keep the STM32F205 running.
It is possible to put it into sleep mode, which only uses 1 μA. In order to get out of sleep mode you need to send the Power-On Reset (POR) command. However, you’ll get much less accurate results if you keep resetting the fuel gauge as it needs to do a quick start after coming out of sleep mode.
In order to use these features you’ll probably need to dig into the MAX17043 data sheet, as it’s not otherwise documented.