File:Battery-powered preamp - power management.png

English: Power management in a battery-powered, arduino-controlled preamp. There are three 3S 18650-type lithium batteries (11.1 to 12.6 V in each stack) - one for the controller and relays, one for -12V audio rail, one for +12V audio rail. Each pack is wired through its own battery management (BMS) board.
In the off-state, all three batteries are connected to the charger via a common groundplane and three diodes (a Schottky diode steals no more than 100 mV of charging voltage per cell, so it won' noticeably disrupt charging). The MOS relay is off, so there's no power to the Arduino
When the user presses the on/off button, the button engages the MOS relay and powers on the arduino and any downstream digital electronics. The arduino sends a permanently high ENA signal, locking the MOS relay in on-state, and engaging two electromechanical relays that disconnect audio batteries from common rails and connect them to audio circuitry. The outgoing SENSE signal goes high (two LEDs to the ground = appr. +3.5V). Any contact bounce must be handled by a software timeout.
Analog ground is now separated from digital ground; they should be connected at a single point elsewhere. This connection should not carry any current between digital and analog halves of the circuit. If such currents are inevitable, they should be kept as low as possible.
The second press of the button forces SENSE to go low (one LED to the ground). The arduino detects a power-down condition, executes power-down housekeeping routine (mute, store settings in EEPROM etc.) and sets ENA port to low (off) state. Electromechanical relays disengage. The arduino is still powered because the pressed button shorts the MOS relay LED to the ground. However, as soon as the user releases the button, all power to the arduino is cut off completely.
The circuit has been built in 2019 and is working for two years by now.
The EXTERNAL RELAY channel is optional, it's not engaged in power management.
Values of resistors marked RS1, RS2 must be typically twice the DC resistance of respective relay coils. In the as-built config, two relays have a combined (parallel) 180 Ohms resistance, and RS1=360 Ohm in parallel with a 3kOhm+LED indicator. Thus, in steady state the relay coils draw only 22..24 mA, compared to 60..70 mA without RS1.