rootdev4 / batterymonitor

12V camper battery monitor with low energy bluetooth readout

Battery Monitor with low energy Bluetooth readout

Measure and display the

• capacity in Ampere hours (Ah) of a battery
• the voltage of the battery (V)
• the current flowing from/to the battery (A)

of a camper/mobil home 12V battery on an Android Smartphone.

The monitor is based on three modules (DC-DC converter, ESP32, ADS1115), which are placed either on a bread board or a PCB. The schematic and the layout of this base board can be found in the Eagle folder.

Hardware

• ESP32 board (Mini)
• Current sensor (Hall sensor to measure the magnetic field, or shunt resistor)
• Step-down regulator (DC-DC Buck converter)
  • Input: 10V-20V
  • Output: 5V (fixed or adjustable)
  • Max. current: 1A
• ADS1115 16 bit ADC

For a version utilizing the internal ADC, see the Readme

Current Sensor

A split core current sensor with a Hall sensor or a shunt resistor is used to measure the current.

Hall Sensor

Advantage: sensor works contactless. No need to mess with the cabling, since the sensor is just placed around one of the battery cables.

Utilized model: YHDC HSTS016L +-20A

• 2.5+-0.625V
• reference voltage 2.5V

Shunt Resistor

A commercial shunt suitable for a high current (e.g. 100A) is placed into the ground connection to the battery. The voltage drop is small (e.g. 70mV for 100A). The ADS1115 allows to change the voltage range down to +-256mV. This makes it suitable to measure the current with sufficient precision, without the need of an additional amplifier.

The reached resolution is for both cases about 50mA.

Analog to digital conversion

The ADC ADS1115 with 16 bit resolution is readily available as a module. It exhibits 4 channels, a wide adjustable voltage range, a build in amplifier and can directly measure voltage differences.

Android APP

Developed with the MIT App-Inventor2

• Low energy bluetooth (BLE) connection to the ESP32
• Main Screen with current, voltage, capacity
• Settings window

Software

The Arduino IDE is used for the code development. The code consists of a single file.

Requires packages

• ESP32 support with BLE and Wire
• ADS1115_WE by Wolfgang Ewald

Concept

• Settings: define time intervals and used channels in the #define section of the code
• Setup: reduce CPU clock to save power, setup timers and the external ADC
• Loop: interrupt every 1ms to determine the required action. Read the battery voltage and current, calculate the Ah and power and store current battery state to flash
• The BLE connection is initialized by the phone
• A sign-on message is expected by the ESP32. This is a hash value of the current time (salt) and a pre-shared pass phrase.
• If the message is not received, the BLE connection is terminated.
• For the first 60 seconds the connection is possible without the passphrase
• Data are send to the APP every second as a block of 20 bytes. The values are 16 bit integer values, which have been scaled to reflect the predefined number og significant digits.