I'm using a RaspberryPi Zero and RaspberryPi B+ with a SunAirPlus 2. I was just testing the INA3221 without battery or solar using the grove connection powering and with an I2C connection to the B+. The B+ didn't start up, so I connected a Zero and powered the B+ separately keeping the I2C connection. The I2C didn't show up in the bus with sudo i2cdetect and I noticed the Zero wasn't powering up completely (just a red LED flashing). Testing the voltage on the USB out VCC shows 2V. I put this project down for the moment. I don't see why this would be the case, but you don't need the battery connected, right? Any help would be appreciated.