You actually don't need a sophisticated setup (like bq78412) for measuring the SoC of a starter battery in a car. Your second approach with measuring the battery through a voltage would work just fine.

The lead-acid batteries can be split into two (2) groups:

1. Starter batteries, such as the ones found in common cars and motorcycles. These always operate high SoC under normal conditions. They are discharged by the starter for just a few seconds. That takes may be 5% of the charge. Afterwards, they are recharged by the alternator in just a few minutes. An important requirement for starter batteries is the ability to deliver high current.
2. Deep cycle batteries are designed to discharge down to 20%. Probably, bq78412 was made with these batteries in mind.

If you know the battery capacity within 20%, you can still detect the faults, which you're describing (lights left on, defective alternator or battery).

P.S. This question have been up for a day, there are no responses, we've got use cases from @fceconel in his comments. I feel that I can provide a systems-engineering answer.

You actually don't need a sophisticated setup (like bq78412) for measuring the SoC of a starter battery in a car. Your second approach with measuring the battery through a voltage is adequate.

The lead-acid batteries can be split into two (2) groups:

1. Starter batteries, such as the ones found in common cars and motorcycles. These always operate high SoC under normal conditions. They are discharged by the starter for just a few seconds. That takes may be 5% of the charge. Afterwards, they are recharged by the alternator in just a few minutes. An important requirement for starter batteries is the ability to deliver high current.
2. Deep cycle batteries are designed to discharge down to 20%. Probably, bq78412 was made with deep cycle batteries in mind.

If you know the battery capacity within 20%, you can still detect the faults, which you're describing (lights left on, defective alternator or battery).

P.S. This question have been up for a day, there are no responses, we've got use cases from @fceconel in his comments. I feel that I can provide a systems-engineering answer.