When it comes to drones, batteries can get quite confusing. If you're looking at batteries for your aircraft and don't know what to get, then this page should hopefully have you covered.
Looking at batteries (commonly called LiPos, for Lithium Polymer, so if I keep switching between LiPo and battery, that's why), the first thing that will jump out at you is that many of them are labelled with an 'S' rating, and this stands for the number of cells (1S = 1 cell, 2S = 2 cell etc.). It may seem odd that this isn't called the 'C' rating, but there is a good reason for that that I will talk about later.
Choosing the correct cell count for your aircraft is very important as all the cells are hooked up in series, so the number of cells dictates the voltage the battery produces. For example, the nominal voltage for a LiPo cell is 3.7V, so a 4S battery outputs a nominal voltage of 14.8V (the nominal voltage is kind of like the mid-point of the cell voltage, so whilst a LiPo cell can hold a maximum voltage of 4.2V, giving a 4S battery a total voltage of 16.8V, this is at the high end of the voltage range so isn't representative of the average voltage of the LiPo).
Higher cell counts obviously output a grater voltage, giving you more punch and a faster craft (I'm thinking as a miniquad pilot, here, so if any fixed-wing pilots want to chime in with your experiences, feel free), but have the tradeoff of having more cells and so more weight, so it's really important to find the right balance.
Here is a little rundown of what might be right for you:
1S is obviously the lowest cell count available, because any less and you wouldn't be flying very far. This cell count is reserved mostly for brushed Tiny-Whoops and toy grade quads, such as the famous Hubsan X4H107 and Syma X5 series. If you are flying anything heavier than around 100 grams, you should really be looking for larger cell sizes. That being said, with a sufficiently light aircraft, 1S LiPos can still rip and do some very decent acro flying. These batteries output a nominal voltage of 3.7V.
Most Tiny Whoop style quads use a JST PH 2.0 connector:
And most toy grade quads use a micro-losi connector:
There are many other battery connector types, such as the new BT 2.0, but this page would be far too long if I were to name them all!
2S LiPos are where tings start to get a little more interesting. Because they have two cells, they have two leads: one only for charging/discharging, ususally with an XT30 connector on, and a second lead called a balance lead with a JST XH connector on. This balance lead is very important, as it means the LiPo can output some voltage back into the charger to make sure the cells have the same charge; if the cells have a different charge, then they are 'unbalanced', and this can damage the battery.
You will usually find a 2S LiPo in higher-end toy quads with brushless motors, like the Hubsan H501 series, or in brushless Tiny Whoops and a class of drone that I've not mentioned before - the toothpick class, which is like a stripped back Tiny Whoop in with a carbon frame.
These provide much more punch than a 1S lipo, but also more weight, which is why they are mainly only used on quads equipped with brushless motors that are powerful enough to compensate.
3S LiPos are used less and less by the miniquad community, and are being used increasingly by micro pilots. This is because the voltage they put out isn't exactly enough to make us go fast on a miniquad, with its greater weight than the micro classes. Well, at least by today's standards: 3S used to be perfectly fine for a miniquad, even if it didn't produce insane amounts of thrust.
When it comes to micro quads, however, 3S is quickly becoming a favourite; it puts out enough voltage to make those tiny, light quads go super fast, especially the ultra-light toothpick class quads, which is, from what I have seen, the type of micro quad where 3S is most commonly used. Lots of Tiny Whoops still use 2S.
3S LiPos also need a balance lead and output cables, however the balance lead has an extra pin to account for the extra cell, and the output cable uses an XT60 connector instead of an XT30 to cope with the greater current draw. That being said, on 3 inch quads, it is common to use a 3S pack with an XT30 connector - there's not much point having an XT60 as the motors won't draw as much current as on a 5 inch and above.
4S is pretty much ubiquitous in the racing and freestyle world, and is what I run my current freestyle rig on. These deliver a decent amount of power (up to 16.8V with a nominal voltage at 14.8V) whilst still being small, relatively cheap and light, so are pretty much exactly what you're looking for in a racing or freestyle battery.
Obviously in the JST XH connector on these batteries, there are 5 pins: one for each cell and a ground. 4S packs are pretty much exclusively equipped with XT60 connectors because theu're pretty much exclusively used on larger rigs.
There's not much more to say about 4S packs - if you're starting in the hobby right now with a racer or freestyle rig, 4S is probably what you want (make sure your rig is designed to take 4S packs though, if your motors are below about 2000KV, 4S might seem a little bit lacking on the punchouts). Even quads that are designed to take 6S can absolutely rip on 4S - check out the Nazgul 5 by iFlight or the Diatone M540, and you'll see what I mean.
It is worth noting that some people are ditching 4S for 6S, and they will probably even go as far as to say 6S is the future of the hobby. Personally, I don't think so. 6S packs are quite a bit more expensive than 4S packs and bearing in mind that batteries are, for the time being at least, a consumable, I really can't see people shelling out that much money to re-fit their entire fleet with 6S unless they are a serious racer or need insane punch and speed to pull off some crazy stunts.
I don't know why, but you don't really see many 5S packs about, at least in my experience. These packs fall right between 4S and 6S, so you're no longer in the weight vs power sweet spot, and you don't have the blistering speed of a 6S pack. That being said, the DRL Racer 4 runs on 5S, so it definitely can still pack a punch!
There's nothing I've seen that suggests there's anything inherently wrong with using a 5S pack, and I personally have never flown one, so if anyone loves 5S, please do get in touch and let me know!
6S is the maximum cell count that your average multicopter user will be running. Yes, fixed wing, x-class and commerical pilots may run even larger packs, but for our purposes, these are more than big enough.
6S follows the same pattern as all of the other packs, it has a nominal voltage of 22.2V with a maximum voltage of 25.2V. As such, it is capable of putting out much more power at the same current as other batteries. Instead of using 'regular' motors in the 2300-2450KV range, we instead lower the KV of the motors. This means that, whilst we do still get a little bit of a speed increase, we draw less current from the packs, meaning less battery sag and longer flight times.
Many quads are starting to be optimised for 6S (there are specific hardware differences, and as such the motors need to have a lower KV, usually around the 1700 mark compared to the 2400 mark used on 4S packs), some very popular examples including the iFlight Nazgul 5 and the slightly older Diatone M540.
6S packs are heavier more expensive due to the increased number of cells, so whether or not the increase in performance justifies that is up to personal opinion.
The C rating of a drone battery is much less concrete than the cell count (I told you I'd cover why the Cell count wasn't called the C rating), and different people have different interpretations.
The widely accepted interpretation, however, is that the C rating is the multiplier for the maximum input/output for a LiPo (or any other battery for that matter).
For example, if I have a 1300mAh LiPo with a 75C rating, then the maximum current discharge will be 1.3*75, which is 97.5A. There are different ratings for different batteries, and some batteries may not have a definite C rating. Take the Turnigy nano-tech 1300mAh 45-90C LiPos (the LiPos that I personaly use at the time of writing). They have a maximum continuous discharge rating of 45C, with a maximum peak discharge of 90C.
The C rating is also used when charging the LiPos. In general, I tend to charge at 1C, which mean that I charge my 1300mAh batteries at 1.3A. Different batteries will have different maximum charge C ratings, so make sure to check what your batteries are rated for to ensure you don't damge the cells or worse, cause a fire.
The capacity of a drone battery (just like phone batteries) is measured in mAh, or milli-amp hours. This is the current that the battery can put out over the duration of 1 hour, and gives a good indicator of capacity. For example, if I have a 1300mAh battery, it can put out a constant current of 1300mA (or 1.3A) consistently for 1 hour. Obviously our drones draw much more current than 1.3A, but this gives a good indication as to how much energy can be stored in the battery (as opposed to Joules, which will igve very high numbers that are difficult to visualise or assign meaning).
Another way you may see the capacity of a battery written is in Watt-hours (this is especially common in airlines or shipping companies when you are trying to transport batteries). This metric gives a much more mathematical (in my opinion) method of measuring the actual capacity of batteries, as instead of measuring the current (which doesn't really tell us too much about the energy stored - a 1300mAh 3S battery stroes far less energy than a 1300mAh 12S battery), this gives us the energy stored in Joules (well, kind of).
If I have a 2300 Watt-hour battery (to pick a random number as an example), we know that this battery could put out 2300 Watts over the span of an hour, so contains 2300J*3600 Joules of energy. This, however, doesn't tell us about the current or voltage.
In reality, unless you plan on flying with, or shipping batteries, you will only really need to know about the capacity of batteries in mAh, as the voltage of the battery is given by the S rating, and most websites will list the mAh as the main indicator of capacity.
Is a larger capacity always better? No. The larger the capacity of a battery, the heavier it is. In general, most people agree that for a 5-inch quad, you should use between 1300-1500mAh LiPos, as this seems to be the sweet spot between capacity and weight, but feel free to try different capacities for yourself and share your experience!