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Hello Shannon/@shicks – thanks for the detail.I missed that it was an internal connection on the mega1284 to the 3.3V
The issue with the 3.3V, is that it is connected to the LiIon through the regulator SPX3819.
The regulator has a dropout that is variable depending on the load, temperature etc – so from the datasheet at load of 500mA it could be a potentially max of 700mv under some extreme conditions, but more likely to be the typical 340mV.
So assuming that the LiIon V is measured before any load is applied ie less than 50mA, the max regulator dropout is 250mV, but more likely to be 125mV…
So I am reading the datasheet, (and sometimes let out the smoke for getting it wrong 🙂 )…
Taking worst case analysis – measurement of the LiIon V with std settings, below 3.5V could be distorted.
Practically speaking I’m seeing a distortion of LiIon V when it drops below 4.1V, measured by BattV compared to an ExtVolt with 100K/100K. Still investigating – I’m probably got something wrong, but trying different options.