The E-Ink Paper Display (EPD) on the M5Paper consumes power even when the screen is idle due to its underlying technology. E-Ink displays are different from traditional LCD or OLED screens in that they only consume power when the content on the screen is changed. Once an image or text is displayed on an E-Ink screen, it doesn't require any power to maintain that state. However, changing the content on the screen, such as refreshing to display a new image or text, does consume power.
In your case, when the EPD power is disabled, the display is essentially turned off, and it consumes very little to no power because it's not actively refreshing the content. When you enable the EPD power and the display is idle, it's likely in a state where it's ready to receive new content and is therefore consuming some power in anticipation of a refresh.
The power consumption you're observing is not necessarily a sign of a defective unit but rather a characteristic of E-Ink displays. If you want to optimize power consumption further for your low-power application, you might consider:
Reducing Refresh Rate: Minimize the frequency of screen refreshes. E-Ink displays are typically used in applications where the content doesn't change frequently. You can adjust the refresh rate to be as infrequent as possible to save power.
Full Screen Updates: Instead of partial screen updates, use full screen updates when you need to change the content. Partial updates can be more power-intensive.
Deep Sleep: Continue using the deep sleep mode of the ESP32 when the screen is idle. This will significantly reduce overall power consumption.
Optimize Code: Make sure your code is efficiently managing the display and other peripherals. Ensure that you are properly putting the ESP32 into deep sleep mode when it's not actively needed.
Hardware Modifications: Depending on your specific use case, you might consider hardware modifications to further reduce power consumption. This could involve custom power management circuitry or using different power supplies.