I have a old R1 weathermote that I'm using to test the "
solar sketch" that Felix has up on his blog in 2017. I'm liking this supercapacitor idea for an outdoor node or two and it helps that I'm finding it all a bit of fun.
I've removed the two SMD resistors for the voltage divider and replaced with 2x1M. This was done so I could measure the voltage for the 5.4V 15F SuperCap. DMM reads the SuperCap got up to 6.1V on a sunny day, but the old 10k+4.7k resistors topped out at 4.86V.
Side node: this is really looking gloriously kludged now as I used regular sized resistors on the breakout section of the weathermote and connected these to the the old SMD pads with component leg off-cuts.
I've also changed the BATT formula to the below code. This was after a lot of DMM testing to get everything lined up. Now I get the correct Voltage measurement while connected to the FTDI programmer as well as when connected to the charged superCap.
#define BATT_FORMULA(reading) reading * 0.0032219 * 2.004
My query is around the values I'm seeing for the SuperCap voltage. It seems the lowest voltage I'm seeing is bottoming out at 3.32V. (I know it's still working as I'm still getting Temp readings. The recorded voltage level just gets stuck.
I'm just wanting to better understanding where this is being limited and what is preventing the measurement from going lower than this value.
Is this due to the p-MOSFET not having enough volts to allow power to the voltage divider section? (reminder that I'm using an old Weathermote R1) If so - I take it the only way around this is to get a weathershield R2 which doesn't have the MOSFET? OR is there another kludge I can do to hack it so it will read the VIN power until it brown outs?