Batteries or rather cells in batteries are components that are known to fail. An average LiPo pack lasts 300 charges, which translates to about 2 years of average use. While this may seem a long time, most other components on your aircraft - when used properly - easily outlive your batteries. They are removed from the plane during daily use, charged (or forgotten to be charged) and then added back to the plane again, always causing stress on critical failure points (connectors). Also, batteries tend to fail in use, when the plane is in the air, while mechanical parts or servos either break in crashes or during handling on the ground. Therefore special focus should be devoted to choosing the correct batteries layout to maximize the safety of your system, as the battery lies at the heart of your FPV system and most of the components you purchase depend on the choice of battery size and voltage.
The goal for a battery system must be to provide the maximum amount of operation time of the whole system (R/C and video). As such, a single battery system is always the best as it guarantees that regardless of the electricity usage pattern, all systems always have the same amount of battery remaining. With a single battery, however, special attention must be paid to choosing the right components. When using a 3S system, 12V video transmitters or video cameras may not get sufficient power to operate when the battery power gets low or a cell fails. Relying on 5V gear and/or voltage converters to negate this problem is therefore necessary. Additionally, noise in the electronic circuitry caused by your speed controller (ESC) or servos needs to be filtered. For this, an LC filter is needed, or a device like the TBS CORE.
The downfall of a singe battery system is if one battery fails, the plane is lost. Therefore a second battery connected in parallel can be a desirable solution. (Parallel means connecting the power and ground of both batteries together or in laymans terms: red to red, black to black). This ensures that a mid-air battery failure will still keep your plane and video system running.
Careful though, reverse currents will quickly charge the remaining cells of the damaged battery and can cause fire. This is very crucial in setups using 2S batteries and still important on 3S batteries. With more cells, the reverse currents going into each cell are divided among more cells and should not cause havoc. Reverse currents can be prevented by using diodes. The problem here is that adequately sized diodes are heavy. If there is a way to monitor sudden voltage drops in your system and start a quick return home, it is possible to fly safely without such a diode.