When fired point blank, plastic BBs can hurt as much as rubber bullets, however most people engaging in military simulations with airsoft guns wear thick military garb. Additionally, precautions are taken to protect the neck, face, and eyes. Many people chose to wear a balaclava around their face and neck and cobalt ontario ANSI approved eye protect is a must. The only place that plastic BBs can really cause damage is to your eyes.
I run a PC repair business in Vancouver. I always tell people who call with clock problems to check their motherboard battery first before calling me round. For most people it's easy to lithium facts check and easy to replace. I don't want to make money from fixing something so simple.
ONickel Cadmium (NiCd) - The oldest battery type used in cell phones. They are the heaviest and lowest capacity battery type. These can be somewhat hard to find.
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!
For jobs where the batteries are required to be light weight as well as being reusable, then lithium batteries are ideal. But if a battery is needed to be very reliable or to remain unused for a long time then they might not be the right batteries for it.