The red LED on the MCU block is the low battery indicator, however, if you're using the BITalino Plugged the red LED may be always on, in which case a little tweak on the PWR block is required to "activate" the low battery indication.
The low battery function takes up analog input A6. To allow BITalino Plugged users to choose what they want to use the 6 analog inputs for, by default the battery voltage output pin (ABAT) on the PWR block is not connected to A6 (hence the red LED always on).
In order to have the low battery function working on your BITalino Plugged, you need either to shunt the two pads to the right of the JST connector on the PWR block, or to run a wire from the ABAT pin on PWR to the A6 pin on the MCU.
Note that this procedure only applies to BITalino Plugged users in which the low battery LED is always on, and it renders analog input A6 unusable to sample data from other analog sources, namely from the sensors.
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The BITalino Team