If you are just starting out with 5-inch FPV drones, you might find it challenging to decide whether to use a 4S or 6S LiPo battery. In this post, I will explain the differences and help you make an informed decision.
Further Reading: New to FPV drones? Take a look at our beginner’s guide.
Pro Tip: Betaflight has introduced a new feature that allows powering a 4S quad with a 6S LiPo battery by reducing motor output.
Table of Contents
Benefits of 6S Over 4S
A 6S LiPo battery has a 50% higher nominal voltage than a 4S due to two extra cells. The maximum voltage of a 6S is up to 25.2V, compared to 16.8V for a 4S.
The potential improvements from using a 6S over a 4S include (some of these points might be up for debate):
- LOWER amp draw
- LESS voltage sag
- LONGER flight time
- MORE responsive and agile performance
Lower Amp Draw?
One of the most significant 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
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 results in:
- Less stress on the battery
- Reduced “voltage sag” throughout the flight
- Less energy loss in the wires due to resistance, though this is often negligible
Check out some DVR footage of a 6S quad:
Is 6S Better for Your Battery?
The lower amp draw means your batteries run cooler during flight, which is beneficial for battery longevity. This also allows the use of lower C rating packs for the same power, potentially saving costs by selecting less expensive cells.
However, to match the weight of a 4S battery, we often choose lower capacity batteries, which can limit the maximum discharge current, since battery capacity is a factor in maximum discharge current. So, higher C rating packs might still be necessary.
Max Discharge Current = C Rating x Battery Capacity
Due to less voltage sag under load, the motors maintain consistent acceleration, enabling them to change RPM more rapidly and consistently. This results in an overall more responsive and agile aircraft. The faster RPM change rate significantly affects the handling of your quad.
There’s a controversial theory that a 6S FPV drone offers longer flight times than a 4S.
This isn’t necessarily true, as we often select 6S batteries of similar weight or watt-hours (Wh) as 4S batteries. To achieve similar speeds on a 6S, the motors need to spin at comparable RPMs, requiring a similar amount of power. Therefore, theoretically, flight times should be comparable to those of a 4S when flying at the same speed.
Longer flight times with a 6S are possible by reducing motor power (e.g., using lighter props or adapt a less aggressive flying style) or using larger battery packs with higher energy capacity. However, either approach will inevitably impact aircraft performance.
The benefit in flight time is more likely to be seen in AP (aerial photography) platforms, where motor speed is more consistent, and rapid RPM changes like those in racing drones are less frequent.
But why are there people claiming they are getting longer flight time on 6S than 4S? Maybe it’s due to the lower current in the system, which leads to less overheat and overall higher efficiency. However this benefit might not be noticeable in most cases.
Is 6S Faster?
A 6S quadcopter, with specially selected components, certainly have some advantage over a 4S (as demonstrated by most professional racers have now switched from 4S over to 6S batteries). However, in drone racing, race tracks often feature numerous turns and gates, requiring as much pilot skill as raw speed, if not more.
The lower current draw and lower voltage drop of 6S give racers more room to experiment with motor and prop combinations. This flexibility allows for more strategic choices in finding the best balance between efficiency and performance based on the specific race track.
Choosing Motors for 6S
There are two sides to the argument when selecting motors for a 6S FPV drone. Some recommend lower KV motors to match the theoretical max RPM of 4S-equivalent motors, while others suggest using similar KV motors as on 4S to fully utilize 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, choose a low KV motor that provides 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
With more volts in the system, you can execute the KV more effectively. In fact, you might need even slightly lower KV than calculated to achieve the same RPM in flight. For example, 1600KV on 6S might equate to not 2400KV on 4S, but actually slightly higher such as 2500KV.
As KV increases, the torque rating is said to decrease. However, from my observations, voltage sag is a more influential factor, preventing you from reaching the theoretical max RPM on 4S. That might be why some people claim 6S feel “more punchy”.
With a 6S setup, I also noticed 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 wouldn’t have the power to swing a heavy prop at high RPMs like on the 6S setup.
Running 6S LiPo on 4S Motors
I tested 4S motors (2350KV) on 6S batteries.
These motors are considered ultra high KV for 6S, and they did provide brutal straight-line speed and impressive cornering. However, the downsides were significant: severe voltage sag and terrible inefficiency. The motors also became extremely hot after a short flight. Personally I would avoid using 4S motors on 6S batteries, motors could easily get damaged.
However, Betaflight offers a solution to reduce motor output, which allows you to power 4S quad with 6S LiPo by setting motor output to 66%.
How About 5S?
Well, the cell count and voltage of 5S are in between those of 4S and 6S, so you can expect its performance to also be intermediate. However, 4S and 6S batteries are more widely available, making 5S batteries relatively rare.
Personally, I prefer 6S mostly for the reduced voltage sag. Apart from that, I don’t feel it’s significantly different from 4S in other aspect. Oh, how many people still remember the discussion back in 2014/2015 when we were deciding between 3S and 4S? It seems we are still moving up in voltage. Maybe in the future, 8S or even 12S will become common – who knows? :) Update – Here’s a new article discussing the possibility of using 8S Lipo in FPV drones in 2023: https://oscarliang.com/8s-fpv-drone-2023/
- Feb 2018 – article created
- Jan 2020 – added info about running 6S on 4S motors
- Dec 2023 – updated article