Electronic – How to measure 6S LIPO battery juice in percentage from Raspberry Pi

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.

While measuring voltage is possible with an AVR, its non-trivial to calculate the amount of juice left in it. This is done using precise coulomb counting techniques. Maxim makes a variety of these ICs which can talk to the AVR over the Dallas 1 wire bus. These ICs are very low power and feed off the batter directly, which lets them stay on continuously even if the AVR is off. They generally include the coulomb counting algorithms which take into account the behavior of Li or LiPo cells, and can compensate for temperature and cell degradation and keep track of charge cycles.

You can look at the DS2780 for an example for such an IC, although it's only for 1S and can be used for 2S. There are other chips of the same family which work for larger arrays.

Note that if you want to be able to actually shut off the AVR entirely and prevent any leakage from the DS2780 via the 1 wire conection, you would need some sort of a level translating buffer sitting in between which you can tristate the output of. Ive used the TI SN74LVC1T45 for this (one in each direction) with success. . Opto isolation would also work but would be an unnecessary power consumer.

This is, however, not a DIP solution. Further, its generally best practice to keep this thing as close to the cell as possible - generally touching it - to make the temperature compensation work well.