I am working on a project that involves monitoring a cars battery. I can measure the voltage of it using a voltage sensor, a display and an arduino but that doesn't tell me much about the health of the battery.

After researching, what seems to be the most popular way is putting a load onto the battery for a few seconds and measure the internal resistance. I need to read the open circuit voltage and make a reading of the voltage under a load. I would like to determine if the battery has enough 'juice' to start the car. If not then a reserve battery will kick in to help start it. Each car batteries cold cranking amps differ and I am having troublewrapping my head around it all.

I'm looking for ideas in order to load test the battery and display the results to the arduino in which then I can determine the battery health

 

The starter is a good load to check the battery. If, during crank, the voltage falls off, you could relay in the second battery.

 

Oh, this thread is about an old 12V lead-acid battery, not a modern Lithium-Ion Electric Vehicle battery?

 

Oh, this thread is about an old 12V lead-acid battery, not a modern Lithium-Ion Electric Vehicle battery?

yes my bad i should have specified

 

The old-fashioned Durite discharge tester measured the voltage drop at 275A.
So, you're going to (briefly) have to dissipate 3kW.
And the relay that switches in the auxiliary battery isn't going to be small either!