I am working on a project which is essentially a car battery monitoring and reserve management system. I am monitoring the cars 12V lead acid battery using an Atmega328P, a voltage divider to monitor voltage, a temp sensor and a current sensor. With these three elements I can determine the health/state of the battery.
This will all be powered by a secondary lithion-ion battery which will be the reserve battery. If the car's main battery doesn't have enough 'juice' the secondary battery will kick in as essentially a built in power pack to jump start the car. This secondary battery will also be charged off of the alternator/main battery.
What I am finding problems with is:
1 - Sourcing an appropriate second battery.
2 - How to switch over to the the secondary battery to jump start the car (perhaps a relay of some sort which is activated by the Atmega328P when it determines the battery will be unable to start the car. I can't seem to find much info anywhere.).
3 - How to charge the secondary battery when the car is running but not when the car is off (perhaps some sleepmode code and another relay?)
Why do You need this? A car running now and then will keep its starter battery in order. If the car has been sitting idle for a long time, apply a charger a day before starting the car.
Your project looks like overdoing things for the wrong reason.
HI,
Jump starting a vehicle is not something that should be done even slightly autonomously.
Buy a battery jump pack and do it safely and manually.
A flat or low battery could be due to a number of problems, check you battery system BEFORE you do any jump starting.
Most jump packs have charger ports that can run off the DC power socket that most vehicles have these days, cigarette lighter in the olden days, the socket is only powered when engine is running so safe charging.
That is because different terminology is used to describe such systems. Contactors, not relays, are used to start cars and switch high current industrial loads. They are not designed to steered by Arduinos.
I agree with @TomGeorge. Backup batteries designed for emergency car starts are not expensive and would be a far better and much safer choice than trying to build your own.
I will also add that the sensors mentioned will not work for determining the batteries ability to start a car. It requires testing under load. You could try to incorporate a load tester in your project but that would be a waste since the starter itself is the best load test. If the voltage drops below 10-11v while cranking the battery is weak.
Next as to relay, yes, the terminology is wrong for what you want. A Google of "automotive starter solenoid" will get you plenty of high current 12 VDC coil contactors. You can also add DPDT as a prefix and see plenty of Double Pole Double Throw examples.
Now consider that an automotive charging system designed around charging a 12 volt sla battery is not going to charge your secondary battery chemistry. You may get a fire or explosion.
If you have a vehicle that sits for long periods you may want to consider a simple inexpensive battery tender. My motorcycle winters in the garage and I just leave a battery tender connected to it. My bike starts right up and even sub zero F temps.
Here is my insight. I have had two 4X4 Diesel pickup trucks in the last 30 years. Each had dual 12 volt batteries in parallel for both charging and engine starting. Never had a problem.
Have to agree with others . A dead battery is a rare occurance and adding all your controls is likely to make things less reliable .
Why are you wanting to do this , what problem are you experiencing and trying to fix ?
