we are to construct a over discharge protection circuit for 12v battery using Arduino which can
protect 12V SLA battery against over discharge, and also protect the connected load from over
voltage in case of overcharged battery is connected.
We are to construct a farmer friendly GSM pump motor controller circuit which could turn on and
off the water system remotely from anywhere in the world via cellphone SMS and return you with
an acknowledgement message
Understanding Battery Charge/Discharge Rates
All the batteries have natural decline, but most of them get damaged due to ignorance from the
users part. The life span of battery will get shorten if the voltage of a battery goes below certain
degree, in case of 12V SLA battery, it must not go below 11.80 V.
This project could be accomplished with comparators, but here we are using microcontroller and
coding to accomplish the same.
This circuit is well suitable for resistive loads and other loads which don’t generate noise in the
supply during operation. Try to avoid inductive loads such as brushed DC motors.
Microcontrollers are sensitive to noise and this setup may read error voltage values in such case, and
it may cut-off the battery from load at wrong voltage.
The discussed over discharge protection circuit for 12v battery consists of a voltage divider which is
responsible for stepping down the input voltage and reduce to narrow range where arduino can
read the voltage.
The 10k pre-set resistor is used to calibrate the readings on arduino; these readings are used by
arduino to trigger the relay, the calibration of this setup will be discussed later part of the article.
A LED indicator is utilized for indicating the status of the relay. The transistor drives the relay on/off
and a diode is connected across the relay for arresting high voltage spike generated from relay, while
switching it on/off.
When the battery voltage goes below 11.80V, the relay gets turned on and disconnects the battery
from load and LED indicator also turns on, this happen same when the circuit reads overvoltage from
the battery, you can set the overvoltage cut-off in the program.
When the battery goes below 11.80V, the relay disconnect the load, the relay will reconnect the load
to battery only after when the battery voltage attains above nominal voltage which is set in the
program.
The nominal voltage is normal operating voltage of the load. The above stated mechanism is done
because; the battery voltage rise after disconnecting from load and this must not trigger the relay
ON at low battery state.
The nominal voltage in the program set as 12.70 V which is full battery voltage of typical 12V SLA
batteries (Full battery voltage after disconnecting from charger).
How to calibrate:
The calibration for this battery over discharge protection circuit must be done carefully; you need a
variable power supply, a good multimeter and a screw driver for adjusting the pre-set resistor.
1) The completed setup is connected to variable power supply without load.
2) Set the 13 volt on the variable power supply, verify this using multimeter.
3) Open the serial monitor and rotate the 10k preset resistor clock or counter clock wise and bring
the readings close to the readings of multimeter.
4) Now, reduce the voltage of variable power supply to 12V, the multimeter and serial monitor
must read same or very close value.
5) Now, reduce the voltage to 11.80 V the relay must trigger on and LED must light up.
6) Now, increase the voltage to 14.00V the relay must trigger on and LED light up.
7) If the above sets are successful replace the variable power supply with a fully charged battery,
the readings on serial monitor and multimeter must be same or very close to same.
8) Now connect the load, the readings on both must remain same and synchronized.
If the above steps are successful your circuit is ready to serve the battery.