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DIY UPS VRLA Battery Controller for ESP32C3, ESP32, M5Stack Core
Network devices and servers connected to USB ports will continue to operate even in power outage.

DIY UPS System Example
Fig. DIY UPS System Using M5Stack Core and 12V 7.2Ah VRLA Battery

DIY UPS System in Power Outage
Fig. Server Continuity Plan in Power Outage


I do not take any responsibility for safety. For example, mishandling batteries may result in danger to your life or loss of your property.


The instruction page for this system is on my blog written in 日本語 and some languages translated by Google Translate:

Supported MCUs


Monitor on Web Service Ambient
Fig. Web Service the Ambient for Monitoring Sensor Values

LINE Notify
Fig. LINE App Gets Message from the UPS

DIY UPS Schematic for M5Stack Core

Fig. Schematic of DIY UPS Battery Controller for M5Stack Core

Hardware Adjusting CV Charging Voltage

This charging system use CV; constant voltage charging, with current limit resistor 1.8 Ohm.
The CV Charging Voltage is set using a 50k Ohm variable resistor by manually. Please adjust the output voltage is NOT over 13.8 V or under 10.8V.
For the example setting from 12.0 V to 13.8 V, if it is set 12.0 V, the backup time in outage might be shorter. But if sets 13.8V, the battery life might be shorter than 12.0 V setting.

Software Definition ADC_CHG_DIV and ADC_BAT_DIV

The values of definition ADC_CHG_DIV and ADC_BAT_DIV on the source files need to be adjusted.
These are the voltage divided factors for the voltage measurements.
ADC_CHG_DIV is for the output voltage of NJM317F regulator for battery charging, and ADC_BAT_DIV is for the battery voltage.

If you put the resistors 100k, 10k, and 12k which have enough tolerance to measure, these values are calculated as below.

#define ADC_CHG_DIV 12./(110.+12.)  
#define ADC_BAT_DIV 12./(110.+12.)  

But generally, resistors have an error tolerance of +/-5% or +/-1%. So, you need to adjust the definition values.

  1. Measure the output voltage of NJM317F, the value is for the denominator of ADC_CHG_DIV.
  2. Measure the divided voltage of NJM317F by resistors at ADC_CHG. The value is for the numerator of ADC_CHG_DIV.
  3. In a similar way, measure for ADC_BAT_DIV at BATT and ADC_BAT.
  4. Edit the values like a below example.

    #define ADC_CHG_DIV 1.353 / 13.78
    #define ADC_BAT_DIV 1.363 / 13.72

How to Start

  1. Adjust Charging Voltage
  2. Set Definition
  3. Write the software to your target MCU.
  4. Check the voltage of a VRLA battery. If it is under 10.8V, please use another new battery.
  5. Connect the battery to the controller board.
  6. Plug the AC Adaptor.

Note: If you plugged the AC supply before connecting the battery, this system isn’t going to be started, please disconnect the AC, and connect the battery before the AC.

Overview of Assigned GPIO Pins

2 of ADC Input Pins used for Voltage and Current Measurement

One of the ADC is for battery voltage measurement.
The other is for measuring the charging or discharging current of the battery via a small resistor.

Circuits for ADC Inputs

Series R=110k Ohm and R=12k Ohm resistors divides the battery voltage to 1/10.
On the above circuit schematic, 110k Ohm are structed by 100k Ohm one and 10k Ohm.

Input on a GPIO used for Detecting Outage

This GPIO input is used to detecting power outage in your home. Series R=47k Ohm and R=12k Ohm resistors divides the 16V output of AC adapter to 3.3V.

Outputs of 2 GPIOs used for Switching Charging or Discharging

Two of the series FETs switches the connection modes of the charging, the discharging, and disconnected mode.
In the charging mode and the discharging mode, the both of FETs are turned On.
And if the voltage of battery reached too high voltage for CV charging, turned Off the charging FET.
Both FETs are turned off to prevent over-discharge when the voltage drops below the end voltage.

Pinout for M5Stack Core

GPIO Direction to
2 Output Charging FET
5 Output Discharging FET
26 Input Outage Detector
35(ADC1_7) Input Charger Voltage
36(ADC1_0) Input Battery Voltage

Pinout for ESP32C3

Pin 割当 接続   Pin 割当 接続
1 3V3 Power(3.3V)   18 IO 0 NA(or GND)
2 EN Pull Up, RESET SW   17 IO 1 Outage Detector(USER SW)
3 IO 4 Charging FET   16 IO 2 Charger Voltage(ADC1_2)
4 IO 5 Discharging FET   15 IO 3 Battery Voltage(ADC1_3)
5 IO 6     14 IO19 (I2C SCL)
6 IO 7     13 IO18 NA(or GND)
7 IO 8 Pull Up, RGB LED   12 TXD Serial RX
8 IO 9 Pull Up, BOOT SW   11 RXD Serial TX
9 GND Power(GND)   10 IO10 (I2C SDA)

Functional Design for 12V VRLA Batteries

12V VRLA Batteries are structed series 6 cells, and each cells work if the cell voltages meet to 2.0V or between 1.8V to 2.3V. This is CV charging system for UPS, so the charging voltage is set to 13.8V. And the stopping voltage for charging is set to 14.7, it’s protected from unexpected higher voltage, such as breakdown circuits.

Mode Battery Voltage Function Condition
Charging 2.3 x 6 = 13.8V Stop Charging Over 14.7V
Fatal 1.8 x 6 = 10.8V Disconnect Under 10.8V

by 国野 亘 Wataru KUNINO

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