11.Real-time clock（RTC）

The Raspberry Pi 5 includes an RTC module. This can be battery powered via the J5 (BAT) connector on the board located to the right of the USB-C power connector.

The J5 battery connector

You can set a wake alarm which will switch the board to a very low-power state (approximately 3mA). When the alarm time is reached, the board will power back on. This can be useful for periodic jobs like time-lapse imagery.

To support the low-power mode for wake alarms you should edit the bootloader config:

sudo -E rpi-eeprom-config --edit

adding the following two lines.

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POWER_OFF_ON_HALT=1
WAKE_ON_GPIO=0

You can test the functionality with:

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echo +600 | sudo tee /sys/class/rtc/rtc0/wakealarm
sudo halt

That will halt the board into a very low-power state, then wake and restart after 10 minutes.

The RTC also provides the time on boot e.g. in dmesg:

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[    1.295799] rpi-rtc soc:rpi_rtc: setting system clock to 2023-08-16T15:58:50 UTC (1692201530)

Note：The RTC is still usable even when there is no backup battery attached to the J5 connector.

Lithium-manganese rechargeable RTC battery

The official battery part is a rechargeable lithium manganese coin cell, with a pre-fitted two-pin JST plug and an adhesive mounting pad. This is suitable for powering the Raspberry Pi 5 RTC when the main power supply for the board is disconnected, with a power-off current draw measuring in single-digit µA, giving a retention time that can be measured in months.

Enabling trickle charging

Trickle charging of the battery is disabled by default. There are sysfs files that show the current trickle charging voltage and limits:

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/sys/devices/platform/soc/soc:rpi_rtc/rtc/rtc0/charging_voltage:0
/sys/devices/platform/soc/soc:rpi_rtc/rtc/rtc0/charging_voltage_max:4400000
/sys/devices/platform/soc/soc:rpi_rtc/rtc/rtc0/charging_voltage_min:1300000

If you add rtc_bbat_vchg to /boot/firmware/config.txt:

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dtparam=rtc_bbat_vchg=3000000

and reboot, you’ll see:

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/sys/devices/platform/soc/soc:rpi_rtc/rtc/rtc0/charging_voltage:3000000
/sys/devices/platform/soc/soc:rpi_rtc/rtc/rtc0/charging_voltage_max:4400000
/sys/devices/platform/soc/soc:rpi_rtc/rtc/rtc0/charging_voltage_min:1300000

The battery will be trickle charging. Remove the dtparam line from config.txt to stop the trickle charging.