Battery Monitoring and charging system

I want to make a simple project, because i am facing a lil problem regarding charging system of battery… i am using GPS Tracking system in motorbike the system has a built in 3.7V 800mah battery and timing of fully charged is 12hr it comes with a charger.The problem is that i want to tun the Tracking system ‘on’ permanently. I attached the charger with 12v Motorbike battery permanently but the bike battery discharge within a 2 days if i don’t use it…

i want to make a circuit which check the battery voltage of Tracking system battery if the battery voltage is less the 3.3 then turn the relay ‘on’ and if the battery voltage is 4.2 or grater then relay will be turn ‘off’ … then i use this relay with bike battery and charger connection so the charger is only turn on when the Tracking battery voltage going less then 3.3V.

I need help to make this project… how i start it?..

my logic i this…


i don’t know how the voltage is sensed by arduino…?

Why reinvent the wheel ? Why not just get this.

i think its not gona work because i want to connect the battery and charger permanently and disconnect the charger using relay from motorbike battery.

Have you measured how much current your relay is eating? If you were using a solid state circuit, you could probably leave it permanently connected with no issues.

ok i will use a transistor instead of relay but tell me how i measure voltage for through Arduino to use if conditions... :)


Using one battery to charge another is very inefficient. Can you remove or bypass the 3.7V battery and run everything off the 12V battery? Use a DC-DC converter rather than a regulator to turn the 12V into the voltage needed by the tracker.

If this can't be done, the way to measure the voltage with the Arduino is to use the Arduino's internal voltage reference. Use a voltage divider on an analog input to get the battery voltage down to less than the reference voltage. Also put a zenner diode on the analog input to protect it from voltage spikes when the bike is running.


Thanks PaulRB i like your idea will try it... and also try to program using voltage divider as a voltage sensing device will let you know what happened.