If you are just starting out with FPV drone, you might have a hard time to decide whether to use 4S or 6S LiPo. In this post I will explain the differences and help you make an informed decision.
Further Reading: New to FPV Mini Quad? Take a look at our beginner’s guide.
Update (Feb 2021): Betaflight has a new feature that makes it possible to power your 4S quad with a 6S LiPo battery by reducing the motor output.
Benefits of 6S Over 4S
A 6S LiPo battery has 50% higher nominal voltage than a 4S because of the two extra cells. 6S’s maximum voltage is up to 25.2V, while a 4S is only 16.8V.
These are the possible improvements from running 6S compared to 4S, though some are more controversial than others:
- LOWER amp draw
- LESS voltage sag
- LONGER flight time
- MORE responsive and agile performance
Lower Amp Draw?
One of the most important advantages of 6S over 4S is the lower current required to produce the same power.
Power is voltage multiplied by current: P = V x I
To achieve the same amount of power, increase voltage will reduce current. For example, to produce 1000W, it takes nearly 60A on 4S (at 16.8V) while it’s only under 40A on 6S (at 25.2V).
Lower amp draw translates into:
- Less stress placed on your battery
- Less “voltage sag” for the duration of the flight
- There’re less losses in the wires too due to resistance, but it’s negligible
Here’s some DVR footage of a 6S quad.
6S is Better For Your Battery?
The lower amp draw means your battery are cooler during flight, which is beneficial for battery longevity. The lower amp draw also allows you to use lower C rating packs for achieving the same amount of power, therefore you might be able to select less expensive batteries.
However, to match the weight of a 4S battery we tend to choose lower capacity batteries, which can limit your maximum discharge current, since battery capacitor is a factor of max discharge current. So you might still have to use higher C rating packs.
Max Discharge Current = C Rating x Battery Capacity
Because of the less voltage sag under load, the motors have more consistent acceleration and it enables the motors to change RPM more rapidly and consistently, results in an overall more responsive and agile aircraft. The faster rate of RPM change is probably one of the most significant factors when it comes to the handling of your quad.
One controversial theory suggests that a 6S quad gives longer flight time than a 4S.
This is not necessarily true since we try to select 6S batteries of the same weight, or Watt hours (Wh) as 4S batteries. To achieve similar speed on a 6S, the motors have to be spinning at similar RPM and therefore it will require a similar amount of power. So in theory, flight time should be similar to a 4S as well if we aim to fly at the same speed.
You can have longer flight time on a 6S by either reducing the power of your motors (e.g. lighter props, or less aggressive flying), or using a larger battery pack that can store higher energy. But either way will inevitably impact the performance of your aircraft.
I think the benefit with flight time can only relate to an AP platform (aerial photography) where motor speed is more consistent and you are not spinning motors up and down so rapidly like a racing drone.
6S is Faster?
Without a doubt a 6S quadcopter with specially selected components can outperform a 4S (that’s why one of the fastest drone world records is set by a drone using 10S battery and not 4S). But in my opinion, 6S is not necessarily faster if you intend to build it having the same or better efficiency than your 4S quad, where the props are spinning at the same RPM.
When it comes to drone racing, race tracks are often setup with lots of turns and gates which require just as much pilot skills as raw speed, if not more.
But lower current draw and voltage drop give the racer more headroom to experiment with motor and prop combinations. This flexibility allows more strategies by finding the best balance between efficiency and performance based on the specific race track.
Choosing Motors for 6S
There are two sides of the argument when it comes to selecting motors for a 6S racing drone. Some recommend to run lower KV motors and match the theoretical max RPM of 4S-equivalent motors. Others suggest we can use motors of similar KV to what we use on 4S and take full advantage the extra power from 6S batteries.
I will experiment with both ideas in my testing :)
Lower KV Motors
For a safer and more efficient setup, you can find a very low KV motor that gives you a theoretical max RPM in the same range as an equivalent 4S setup using a common KV rated motor.
|4S – KV||5S – KV||6S – KV||RPM (VxKV)|
Because you are using more volts in the system, you do have the ability to execute the KV more effectively. So in fact you actually need even slightly lower KV than you would calculate to achieve the same RPM in flight. That means we might be looking at 1600KV on 6S = 2500KV on 4S.
As you move up in KV on the motor, it’s said the torque rating would go down, but from my observation the more influential factor is voltage sag that prevents you from actually reaching the theoretical max RPM on 4S.
With the 6S setup I am also noticing the ability to swing a heavier prop more effectively (e.g. higher pitch props) . If I were using a 4S LiPo, most high KV motors just don’t have the grunt to swing a heavy prop at the high RPM I see on the 6S setup.
Running 6S LiPo on 4S Motors
I tested high KV motors (2350KV) on 6S.
These “high” KV motors on 6S gives brutal straight line speed and grunt in the corners. The downsides are obvious: incredibly bad voltage sag and terribly inefficient. The motors also get burning hot after a short flight.
In order to avoid drawing too much current and maintain better efficiency, you can try to lower motor outputs in Betaflight, e.g. to use 6S LiPo on a 4S build, set motor output to 66%.
How about 5S?
Well the cell count and voltage of 5S are in between 4S and 6S, so you can expect the performance is also in between the two. But 4S and 6S batteries are just more widely available, you can rarely find 5S batteries.
I prefer 6S mostly because of the less voltage sag, apart from that i don’t feel it’s that much different from 4S
Oh, how many people still remember the discussion back in 2014/2015 when we were still deciding between 3S and 4S? Seems we are still moving up in voltage, maybe in the future 8S or even 12S will be a thing who knows :)
- Feb 2018 – article created
- Jan 2020 – added info about running 6S on 4S motors