I'm not sure how this chip brings anything to the game.
"The on-board analog-to-digital converter (ADC) has 10 bits of resolution and will perform a conversion
when the DS2438 receives a command protocol (Convert V) instructing it to do so. The result of this
measurement is placed in the 2-byte Voltage Register. The range for the DS2438 ADC is 0 volt to
10 volt; this range is suitable for NiCd or NiMH battery packs up to six cells, and for lithium ion battery
packs of two cells. The full-scale range of the ADC is scaled to 10.23 volt, resulting in a resolution of
10 mV."
It is not doing anything the ATTiny and its multiple analog inputs cannot do, it still needs some smarts to control it, it still needs isolation from the other batteries.
If the 12V sealed lead acid batteries were to fall to 2.4V, I would think there would be a bigger problem to deal with.