Battery concern (inside light dims when operating power windows)

[ahintofpepperjack]

So, the question is, would the second battery pack be the reason why the battery on the engine bay is small... I wonder...

The secondary lithium battery in hybrid vehicles is high voltage for the electric drive motor. The gas engine and all of the normal 12v electronics run off of the small 12v battery. So no, the second battery pack shouldn't have an affect on the 12v battery under the hood.

[MetroMPG]

Re. the hybrid 12v battery: remember in a hybrid, the 12v doesn't have to start the engine like a conventional car's 12v battery does. The hybrid battery pack handles that demanding job through the big starter/generator/motor.*

The hybrid's 12v battery only has to serve as a buffer between the DC-DC converter (hybrid version of an alternator) and the car's conventional 12v system (lights, computers, sound system etc.).

(* With the exception of Hondas in sub-zero temperatures, if the hybrid battery is too cold the computer sometimes decides the backup 12v starter should handle the task of starting the engine.)

[Ares]

So no, the second battery pack shouldn't have an affect on the 12v battery under the hood.

I doubt that is 100% accurate.

The fact that there is a secondary pack is what enables the use of a smaller battery.