For multirotors we exclusively use LiPo batteries. These are able to deliver high currents in short times and their charging is very fast.
Their best attribute is that there is no memory-effect and they are able to hold their capacity for a long time, with very low loss. The negative side is that in case of piercing they can catch fire and the outcoming gas can damage some metals. Plus, they need a special device called balancer, that can charge all the cells to the specified voltage level.
- discharge rate
- charge rate
- cell count/voltage
- internal resistance
The nominal voltage of one cell is 3.7V, fully charged it rises up to 4.2V. Nowadays, there are special batteries that can be charged even to 4.3V.
Note that the real voltage is 1.14 times the nominal voltage, so e.g. the nominal voltage of a 3cell pack is 11.1V and the fully charged voltage is 12.6V.
It is very important to never discharge the cells below 3.2V, otherwise the cells can get damaged.
A good quality charger is a must, choose one that can charge at least with 5A, so the charging times are shorter.
On multicopters we generally use little devices which are able to check/monitor the voltage, and in some cases they are even able to send the information back to the ground, by using telemetry.
The most important parameter of the batteries is the discharge rate. It specifies how much current the battery is able to deliver, without getting damaged. It is labelled by a “C” number on the packs, that means “capacity”. If the battery is rated e.g. 20C that means that it able to deliver 20 times its full capacity, for example an 1300mAh 20C battery is able to deliver maximum 26A continuous. Note that the manufacturer ratings are many times out of specs, so when planning, always buy a bit overrated batteries so you can be sure that the necessary current can be delivered. Also note that when the battery is discharged close to the max. C rating, it can be overheated, so always choose a higher C rating then needed.
Most of the batteries are manufactured in China and then rebranded for different companies. That means that if someone is selling a battery outside China it will automatically get more expensive. To be worse, not so long ago new rules has been introduced in air cargo, prohibiting LiPo batteries onboard. Upon this, many sellers had to choose new carriage methods and that increases the battery prices a bit more. So if you are buying a cheap battery, by the end it can be more expensive, then if it was bought in the EU, for example. Sometimes it is worth to buy batteries from the local dealers, and forget about shipping from China.
The best battery source in EU is Lindinger, they are not so cheap but sell quality cell batteries (brand Wellpower), and the capacity ratings are always in spec.
For special (racing) application, it is worth to consider buying from Singapore, search for the “Dinogy” brand. They have high C rating race batteries of excellent quality.