I know usually the 12volt battery is charged through an alternator but how is it charged in a hybrid? I didn't think there was a traditional alternator in our vehicle.

 

Also, the small battery is a type of battery called "Absorbed Glass Mat", or "AGM". In a regular lead-acid battery, the lead plates are separated by liquid, so when one of the plates warps or the battery gets a hard bump, the plates touch and the whole cell shorts out. But in an AGM battery, the stuff between the lead plates is a gel that is absorbed into a fiberglass mat. The gel is the acid, and the fiberglass mat prevents the lead plates from touching even if they warp or the units gets bumped hard. AGM batteries have been used in airplanes and military applications for many years. They never need water. And, they have an expected life of 10-15 years, even in heavy-duty commercial use.

I will be very surprised if I ever have to replace that battery.

 

coolshock1 — When the system is "on" the 12-V battery gets charged (and all 12-V power requirements are met) by the 245-V NiMH battery via a dc-to-dc converter module that's part of the high-voltage battery electronics. But this can only happen after the system is "started." So, if the 12-V battery is run down, it needs to be "jumped" in order to start the system, and then the NiMH battery takes over and charges the 12-V battery (assuming that your NiMH battery is in good condition).

Stan