UPS Shield for Raspberry Pi

General Information
June 3, 2019 by
UPS Shield for Raspberry Pi
Serzh Ohanyan


1. Introduction
2. UPS Shield
3. Technical Specifications
4. The Real-Time Clock
5. The RS-485 interface
6. Discharge supercapacitors
7. Connecting two Industrial Arduino PLCs using RS-485 to open and close a relay


In this post, you will find general information about the UPS Shield and the configuration that has to be done to the Raspberry Pi. 

UPS Shield 

The UPS Shield 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 in step-up mode. The second DC/DC converter is a Front-end step-down converter. The only function is converting a high input down to 5.1V for the 5V rail. 

Technical Specifications


Max. Total Backup time
Min. 57 s
Typ. 81 s
Operation until supercapacitors not discharged.
Tamb = 25 ºC. 
Backup time for Sort-term power failure
Min. 22 s
Typ. 31 s
Operation until supercapacitors not discharged. 
Tamb = 25 ºC. 
Charge time (fast charge)
= default configuration
Typ. 25 sOperation until supercapacitors not discharged. 
Tamb = 25 ºC. 
Operating time (slow charge) Typ. 60 sOperation until supercapacitors not discharged. 
Tamb = 25 ºC. 
Operating temperature range (module)-40 ºC to +85 ºC
-25 ºC to +70ºC
Assembled PCB
Including Push-Button and Screw terminals
Operating temperature range (supercapacitors)-40 ºC to + 60ºCSupercapacitor type: 22F / 2.7V / DxL = 16 x 26 mm


Input Voltage Range7V DC ... 28 DCat screw terminal
Average Input Current1 A
0.5 A
Charge mode+RPi3 Booting at Vin = 12 V
Charge mode+RPi3 Booting at Vin = 24 V
Current consumptionTyp. 15 mANormal operation (No charge mode, no Shutdown mode), Tamb = 25 ºC


Output voltage range
4.75 V DC ... 5.25 V DC

Output current range 
0 Aavg ... 1 Aavg
Typ. output ripple
10 mVppAt shutdown mode, Tamb = 25 ºC, ripple at fSW = 1200 kHz

Supercapacitor Interface

Working voltage at supercapacitor contacts1 V ... 5.25 V

Peak current through supercapacitor7 A
Charging method7.5 W (fast charge, default) = 1.5 A at 5 V
2.5 W (slow charge) = 0.5 A at 5 V
Constant power charging
Constant power charging
Balancing+/- 300 mAActive balance circuit for two supercapacitors in series


The Real-Time Clock

The board contains a DS3231 integrated circuit communicating via I2C-bus with the RaspberryPi. A Lithium Coin Battery supplies the RTC chip when the Raspberry Pi is not powered on. We recommend using the CR2032 battery.

RTC chip typeDS3231
Supported by the Raspbian OS
Battery holder CR20323 V, Lithium
Battery Lifetime> 20 yearsat capacity 200 mAh
CommunicationI2C-busPins 3 and 5 of the GPIO interface (SDA and SCL), requires RTC driver installed on the RPi

In the file /boot/config.txt uncomment the line


and at the end add the line


The RS-485 interface

The UPS Shield module contains a 2-wire RS-485 (or RS-422) transceiver. The transceiver is driven by the Raspberry Pi UART interface on the GPIO14 and GPIO15 pins. We send and receive data by /dev/ttyS0.

In the file /boot/config.txt add the line


RS-485 Interface Communication Type 2-wire, half duplexTransceiver chip ISL8483E
RX/TX direction controlControlled by GPIO24Diver enabled by a low TX data bit
Driver disabled 25 us after the end of a low TX data bit
Failsafe biasing390R to GND
390R to 5V

Line Termination120R390R//220R//390R+2*10R
ProtectionESD to +/- 15 kVNo Surge/Burst protection
Echo-featureRX always enabled
TX enabled at data
Data transmission speed 0...250kbps
slew rate limited
25 us transmission pause
between TX and RX required

The driver-enable line follows the data with a short delay (25 μs) before disabling the driver. There is no need to adjust the time constant for different transmission speeds. The automation direction control works from zero to maximum transmission baud rate. The only requirement is a 25 μs transmission pause between sent and receives data. Thi short time interval corresponds to a time interval of about 3 data bits at 115 kbps. At 9600 bps the 25 μs time interval corresponds to a fraction of one data bit. 

Instead of keeping the transmitter enabled for the entire transmission, the driver is enabled on the leading edge of the start bit or any logic low at the drivers in out. It also disables the driver 25 μs after the leading edge of the stop bit or any logic high at the driver's input. When the driver is disabled, the biasing resistors ensure the receiver's output is a logic high. 

Discharge supercapacitors

The on-board discharge load should be used before unplugging the module from the carrier PCB. It helps to avoid sparking and excessive current flow when discharging the super-capacitors. 

Discharge load2 Ohm
Meander PCB trace used as shunt monitor
at 25 ºC
Typical Discharge Time ar default 2x22F supercapacitors 1 min.Short the pads R83 with a tweezer 

Discharge supercapacitors

Connecting two Industrial Arduino PLCs using RS-485 to open and close a relay


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UPS Shield for Raspberry Pi
Serzh Ohanyan June 3, 2019
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