The Juice4Halt module is a supercapacitor based energy storage. It contains two independent DC/DC converters. The first one is a bidirectional step-up/step-down converter working as the interface between the stable 5V supply rail and the supercapacitor. During charging the converter works in step-down mode and transports energy from the external power supply to the supercapacitor. In case of a power failure the load device (Raspberry Pi or another SBC) is supplied from the supercapacitor via DC/DC converter working in step-up mode.

The second DC/DC converter is a Front-end step-down converter. The only function is converting a high input voltage down to 5.1V for the 5V rail. It is necessary to use the Front-end converter in case of an external power supply.

Modes of operation

Charge mode

• charging the supercapacitor using the step-down DC/DC converter when external power supply voltage is available at the input

• the load device at the output (Raspberry Pi or another SBC) stays disconnected from 5V power rail until supercapacitor charging is completed

Normal operation

• the Raspberry Pi (or another SBC) is powered from the external power supply

• the supercapacitor is directly connected to the 5V output to ensure reliable operation without short-term power failures or brown-outs

Brownout (or short-term power failure)

• starts when voltage falls below 4.9V

• external power supply will be disconnected

• Raspberry Pi or another SBC is powered from supercapacitor

• ends when voltage falls below 4.75V, at this point the external power supply voltage is being checked:

• when external power supply voltage is present reconnect to power supply and charge the supercapacitor back otherwise enter the Shutdown mode

For more detail: The Batteryless UPS for the Raspberry Pi