Originally Posted by Tripod
There is also the intrinsic limitations of having two different sized batteries on a single circuit(unless both batteries have their own alternator). If one battery goes bad, it will reduce the charging efficiency of the second "healthy" battery, as the charge will leak from the bad one.
There is only one alternator of course. The batteries are not directly connected, charge current to the smaller battery is regulated based on its state and isolated completely at times ("state" being sensed by the so called "Intelligent Battery Sensor"). I think this came to be the way it is in our vehicles for one or both of the following reasons:
1) In other FCA cars with ESS that came before this car the second battery was not in the engine bay, apparently due to space constraints. I imagine they more or less "copy-and-pasted" the existing ESS design for this car but found they had plenty of room for placing both batteries in one location, but choose to still use the two battery design to avoid engineering costs of re-designing and testing.
2) Two smaller, lower quality batteries are possibly cheaper than one large battery that meets all the specifications served by the two individual batteries.
As a team, if properly implemented and playing to their strengths, the two battery system can probably perform as good or better than a single really expensive battery for a longer period of time. As the primary battery ages, its capacity diminishes as well as its ability to maintain desired voltage under heavy load (ie, cranking the starter in cold weather). The electronics do not appreciate big voltage dips, they can potentially use some capacitors and voltage regulation to continue normal operation throughout a range of input voltage but at some point if the voltage is too low (say, dropping to 8.5 volts while the starter is cranking for 3-10 seconds) it will cause things to go haywire. Thats where having a small isolated battery that can maintain 12.x volts to the electronics at low amperage keeps that side happy while the primary battery swings wildly on startup between whatever its voltage dips too during cranking (10 or 11 volts for a somewhat aged battery) and then bouncing up to 14-18 charging volts as the alternator spins up.
Still, one really high quality large battery could maintain 12 volts while under a heavy load for most of its life I would think. A really big capacitor would be better in place of the secondary battery but that would be much, much more expensive than a cheap little SLA. It all comes down to cost I'm sure.