Hello!
I am working on a low-power esp32 camera project that is supposed to fit in a gopro case.
It uses 2 18650 batteries for power. It also has a mosfet and real time clock that can be used to put the device in an ultra low power state until an interrupt occurs from the rtc.
However, in an effort to keep the footprint small I am not using a separate coin cell battery, and rather I am using the power from the first cell of the battery pack to suply power to thep backup pin of the RTC.
You can take a high level look at the schematic below:
And here you can see the rtc power schema:
Here is the sheet for INA219 "PowerMonitor" portion of the circuit:
Here you can see the physical layout of the board for context:
Here is the thing, if I use my power supply or a 2s lipo to power the device WITHOUT hooking up the RTC backup voltage (ie, the leaving the voltage from the lower battery cell unconnected) everything seems more or less ok.
If I plugin both 18650 cells as intended (with the voltage of the lower battery cell connected to the battery backup line) the both batteries start to smoke and get very hot.
The backup voltage goes through a diode and a small resistor before the rtc backup pin which I thought seemed reasonable.
When I measure for a short between any of the battery positives and ground I dont find anything suspicious.
Any ideas people? Thanks in advance!