6S vs 4S for FPV Mini Quad & Racing Drone

by Oscar

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.

banggood

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.

Here’s a list of my favourite 6S and 4S LiPo batteries.

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

Responsiveness

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.

Higher Efficiency??

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)
2300 1840 1533 38640
2400 1920 1600 40320
2500 2000 1667 42000
2600 2080 1733 43680
2700 2160 1800 45360
2800 2240 1867 47040
2900 2320 1933 48720
3000 2400 2000 50400
3525 2382 2350 59220

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.

Here are my motor and props recommendations.

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.

Conclusion

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 :)

Edit History

  • Feb 2018 – article created
  • Jan 2020 – added info about running 6S on 4S motors

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22 comments

Ucok haha 13th October 2020 - 11:34 am

think your theory only applies if the 6s vs 4s both have the same drone weight, the same motor, the same mah, the same prop.
Especially the theory that says the use of low amperage for 6s, you don’t calculate the large momentum due to the weight of the heavier drone, maybe you try to just fly around, not in race mode … I think in 6s race mode it will drain tons more ampere than 4s (same drone, difference in lipo weight)

second, why do the 6s beginner pilot racer always use an esc of at least 40a? because the heavy load on the esc for 6s …. unless you are a pro, because pro pilots are able to compensate for the momentum of the weight quad
Btw, in sorry if i wrong, and Iscar liang alwats ny favorite…

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CWD 6th May 2020 - 11:47 pm

As usual, you make some great points. But there is one thing that really confuses me about motor selection. FYI, I am in the process of building a S500 size camera drone. I really want to use 6s. But it’s tough to find low enough Kv motors that can handle 6s. But my question is. Why do the motor test data sheets always seem to point to 4s being more efficient than 6s? Even though we know that 6s should be more efficient, theoretically.

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Mike P 19th March 2018 - 11:54 pm

Great work!

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Forrest Frantz 16th August 2018 - 8:10 pm

The 6S versus 4S debate is correct in thought but uses the wrong words. Specifically:
– 6S is less efficient than 5S, which is less efficient than 4S
… proven in all tests using 4″ props to 48″ props
… proven in all tests using 10A to 80A ESCs
… proven in all tests using motors from 20 grams to 600 grams
– 5S and potentially 6S might, however, give longer flight times. What? Why?
… we just set the world record for electric powered multicopter at 4 1/4 hrs
o beating the old record by more than two hour
o more than doubling the old record
… using a 6S which was less efficient than 5S which was less efficient than 4S
… but the ship drops out of the sky when below 15 volts
o so a 4S would only use a small portion of its 16 to 10 volt range
o and a 5S would use a portion of its 21 to 12.5 volt range
o but uses all of the 6S range
o so in some cases, a 5S or 6S battery ends up with more usable watt capacity than a 4S
o despite being less efficient

Tests show that efficiency drops about 1.5% for every 1 increase in S.

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wuzzle 13th March 2018 - 2:02 pm

Actually I am not convinced that 6S is more efficient than 4S. As you said, for the same amount of Energy to be translated into Speed you’d have less amp draw. Though there are additional factors to take into account here:
First: to have the same power to weight ratio as the 4S setup (and we’d have to assume exactly the same components to have the same test conditions. Same Frame, etc.) we need to have smaller capacity cells. While I think the added Material such as more balancer cables, separating material and so on are negligible there is another effect. You’d need a very high C-Rating on the smaller cells for them to be able to sustain the Amps needed. While you don’t need as much current, it would still heavily tax the batteries. The best currently available Cells are in the 1300~1800mAh range. Everything below 800mAh starts to greatly decrease in performance/real C-Rating.
Second: For a lower kv Motors you need more windings. More windings result in thinner copper wires and thus in an increased internal resistance. A higher internal resistance then again results in more losses to heat.

I have made the reverse test. I built a 3S quad with handwound 4100kv Motors and while Amp draw really skyrockets, the results in flight performance are almost equal to the 2600kv 4S variant I built before that. In the end it’s more an issue of optimization of the several parts rather than the numbers of cells. That’s my observation at least.

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Tony 21st March 2018 - 4:27 pm

As current draw is lower when running at higher voltage, less heat should be produced due to resistance in the battery? I’m not sure that this is true it’s more of a questions…
I suspect there are so many variables at play it’ll be hard to answer efficiency of a real quad in flight.

FWIW – I’m interested in 6S to be able to punch harder without battery alarms – hence why I’m looking into this and found this page. That should be easier to verify (I hope)

@Oscar – great article as always! Thanks for your contributions to the hobby!

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dan 29th May 2018 - 2:06 pm

A lot of pilot’s running 6s are running 1300mah packs. That would be running close to a 1900mah+ pack on 4s. The quality of the cells are a lot lower on 6s and when voltage drops to low, puff (damage) happens.

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Walkeer 7th March 2018 - 8:57 am

I believe the only difference in efficiency would be less heat generated in the wires and ESC (both depend on current only), but motors and battery efficient would be the same. overall, the difference would be very small in my opinion.

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Walkeer 6th March 2018 - 4:07 pm

Hi Oscar,

I disagree with following statements:
less stress placed on your battery
less “voltage sag” for the duration of the flight

They would be true only if the both batteries would have same capacity, that is the 6S would be much heavier than 4S. In real world, you should compare batteries with the same weight, lets say 1000mAh 6S vs. 1500mAh 4S , where above statements would not be true. Both have cca 25kJ of stored energy and for 65C cells, produce about 1600W. In fact, its almost irrelevant for the battery if you have 1S 6000mAh or 6S 1000mAh, they will have the same output power at the same load (C-rating). In fact, the more S you use, the more wires/welds/separators must be used, so purely from the battery perspective, 6S would be slightly more heavy than less 4S battery with the same output power.

Check my spreadsheet to check output power for 3S and 4S: docs.google.com/spreadsheets/d/10v60jHFL6-ocdkE_nku2GTTjnM14L6x5lH2uh5teTAI/edit?usp=sharing

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Walkeer 6th March 2018 - 4:16 pm

What I wanted to say is that power density [W/kg] is not depended on number of cells, but only on C-rating and cell weight.

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Sebba 8th September 2018 - 10:21 am

Wow your spreadsheet is sick dude! Nice work there!

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Apple 24th February 2018 - 9:41 pm

Hey Oscar, nice blog.

Do you know of anyone who has used a high kv motor on 6s?

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Arcus 24th February 2018 - 6:23 pm

6S is more efficient than 4S because as you need less Amps for the same power, you loose less energy in heat in the whole system. The system run cooler at the same amount of power. For the increase responsinevess it is due to the fact that you reduce your Kv, so you moteur have more torq.
Pth= Ri²

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Romain 24th February 2018 - 10:47 am

Hi Oscar, thanks for this very interesting article. I just noticed on my latest built that all my electronics can handle 6s. So it is tiggling me a bit to try it. I have T-motors 2204 2300kv there. So I was wandering if they would handle the load. The setup is a 4inch speed addict 180, Kiss Esc 24A RE, Kiss FC v1. 0, Matek PDB, Tramp Hv VTX, FOXEER Arrow V3 cam, XSR… What do you think?

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Dano 24th February 2018 - 8:13 am

What happened to 5S? We just stepped over 5S’s cold lifeless corpse to 6S?

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Oscar 6th March 2018 - 5:48 pm

5S has the same properties as 6S, but just not as extreme :) I only wanted to try 6S because I want to see the extreme :)

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Jerrod 24th February 2018 - 12:56 am

Oscar, great article. If you do keep the weight the same, and go from 4s to 6s pack, and you draw the same power out of the battery, you are using the same percentage of the C rating of the battery. I’ll put the math in a link here:

docs.google.com/spreadsheets/d/1kQpCmVJGBKKjdaE0T78ketV4vH1u47xri0Im9LDtmzE/edit?usp=sharing

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Paul Hope 28th February 2018 - 1:25 pm

This is exactly what I was thinking. You are effectively plugging different values into an equation to reach the same figure. There has to be a reason why it is more efficient… if it actually is. I’m wondering if gains are found by there being less heat loss (heat loss = wasted energy) due to lower amperage?

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Walkeer 6th March 2018 - 4:37 pm

I believe the only difference in efficiency would be less heat generated in the wires and ESC, but motors and battery efficient would be the same. overall, the difference would be very small in my opinion.

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Oscar 6th March 2018 - 5:47 pm

Yes I agree, I did mention that in the article as well :)

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Wojked 23rd February 2018 - 12:23 pm

Great entry! Thank you for the valuable insight and collecting the knowledge in one place again!

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Adam McKnight 22nd February 2018 - 7:05 pm

What was the cost of that build?

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