Obviously ICE are powered by lithium stock 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).
The battery will over-charge if left attached to the charger. The battery stops charging automatically when full lithium facts so it will not over charge. If the battery is left on charge too long it is possible that heat will damage the battery.
The last thing to start tinkering on is the rate of fire. The biggest thing that will determine your ROF is the battery voltage and the amount Milliampere-Hour rating (mAH). Some of the best batteries for increasing ROF are the LiPoly batteries (Lithium Polymer) but you should not use these types on out of the box guns. When preparing to use the Lipoly battery you need to make your internal components are reinforced and upgraded where they need to be.
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.
Briefly relating back to overall size... many lithium 18650's will differ ever so slightly in length (measured in millimeters). This is mainly due to an added protection circuit. The circuit helps to prevent the battery from over-discharging, since Li-ion cells can easily become damaged if their power is drained below a particular voltage. The protection circuit also prevents the cell from being over-CHARGED as well. These cells are very sensitive. They must be handled carefully, gently, and as described above, must never run-down too far, or be charged over and above their threshold. They can actually explode if severely mistreated!
Using these easy tips to keep the battery in the best condition won't do wonders, but will let you use it to the maximum number of recharges. Ever wondered why some laptops keep their maximum capacity for 3-4 years, while others dive under an hour of runtime in one? This is why.