Obviously ICE are powered by was ist kobalt the gasoline you buy when you fill up at a gas station. The electric motor/generator is powered by a large (and by large, I mean large and heavy) battery pack. Most hybrids on the road today have large Nickel-Metal Hydride (NiMH) battery packs somewhere near the trunk, or under the seats. (Read about the Lexus Hybrid battery pack).
Deterioration is a function of temperature and charge level. Don't make the mistake of storing the battery fully charged, as that is the state where it breaks the fastest due to heat. Charge it to take it out and put it like that on the back seat of your car avoid direct sunlight and preferably put the mini lithium facts notebook on a firm flat surface while operating it.
There are some concerns when it comes to these high powered batteries and how good they are for the environment. But all hybrid batteries are set to be recycled (Toyota has a $200 bounty on each battery pack), and so unlike some lead batteries from conventional cars, they shouldn't end up in landfills.
The mating lithium bettery stock process between the two engines can be in series or in parallel. When the two powertrains are mated in series, only one of the motors controls the cars movement. The other one simply assists the first, dominant engine. This occurs most often in mild or assist hybrids. The Saturn Vue Green Line Belt Alternator System (BAS) is a good example. The concept vehicle named the Volt from GM is another good example of a series (but not a mild) hybrid.
In case you are using new nickel-metal hydride (NiMH) battery, remember you should not charge the battery for too long or it will overcharge it and end up shortening the life of the battery. However, it is important to charge and discharge the battery three times just like the lithium-ion battery to ensure that it has a long a life.
Recharge the battery to full again and repeat the above steps a few more times. This will allow you to use the battery for a longer time and you can do this no more than once a month. Replacement batteries can be expensive and stretching out their use when you are on a budget can help.