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

4S LiPo is currently the standard battery cell count for FPV mini quads and racing drones. As we are all continuously striving for more performance in our quadcopters, 6S setups might be a better option, or not?

There are different opinions on what benefits 6S can give us over 4S. Note that we are still at the early stages of experimenting with 6S batteries on mini quads, a lot more testing is needed to verify these theories.

Further Reading: New to FPV Mini Quad? Take a look at our beginner’s guide.

Benefits of 6S Over 4S

A 6S LiPo battery has higher voltage than a 4S because of the two extra cells connected in series. This results in a higher maximum voltage of up to 25.2V in a 6S battery, compared to 16.8V of a 4S.

Potential improvements of running 6S compared to 4S are:

  • 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.

As we know, power is calculated by multiplying voltage and current, P = V x I.

To achieve the same amount of power, a higher voltage will reduce current draw. 6S is 25.2V when fully charged while 4S is 16.8V. For example, to produce 1000W, it takes nearly 60A on 4S while it’s only under 40A on 6S.

Lower amp draw translates into:

  • Less stress placed on your battery
  • Less “voltage sag” for the duration of the flight

Let me show some DVR of a flight on 6S.

 

Better For Your Battery

The lower amp draw will keep the battery at a cooler temperature during flight, which is beneficial for the health and longevity of your batteries. You could even use lower C rating packs and achieve the same power, therefore you might be able to select less expensive batteries.

However, to match the weight of a 4S battery we do tend to choose lower capacity batteries, which can limit your maximum discharge current… So you might still want to use higher C rating packs.

Max Discharge Current = C Rating x Battery Capacity

Agility

From my experience I have found a 6S mini quad to be highly responsive and has a great amount of control through the entire throttle range. It has the ability to accelerate almost instantly yet it doesn’t feel as “floaty” as I expected it to be.

The advantage of that agility and acceleration come from the higher voltage, and the lower voltage sag under load. These enable the motors to change RPM more rapidly and effectively, resulting in a more responsive and agile craft. 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 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 for a racing drone. 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.

Building a 6S Mini Quad is More Expensive

Not only our options for 6S compatible components are currently limited, they are also priced higher than parts designed only for 4S.

In order to support higher voltages, all supporting electronics need to be specially chosen in order to handle the higher voltage spikes, such as the capacitors, MOSFET’s, voltage regulators. 4S parts are cheaper because they are making them in a larger volume.

Choosing 6S LiPo Batteries

In this review I tested different 6S batteries.

The idea here is to pick a 6S battery of similar “watt hours” to what we normally use on a 4S quad. “Watt hour” (Wh) is a measure of how much energy is stored in the battery, which is calculated by:

Wh = Capacity x Voltage

For example a 6S 1000mAh should give you similar flight time to a 4S 1500mAh. Of course the weight should also be considered when choosing a battery.

This is not the most accurate calculation because we are assuming our 6S setup has the same efficiency and power to our 4S setup, but it’s good enough to get us started.

4S (16.8V) 5S (21.0V) 6S (25.2V) Wh (mAh*V/1000)
2100 mAh 1680 mAh 1400 mAh 35.28
1800 mAh 1440 mAh 1200 mAh 30.24
1500 mAh 1200 mAh 1000 mAh 25.2
1300 mAh 1040 mAh 867 mAh 21.84
1000 mAh 800 mAh 667 mAh 16.8

6S batteries are generally more expensive than 4S, and there are currently very few options, here are the ones I’ve tested so far:

  • Amax 6S 75C 1150mAh
  • Tattu 6S 95C 1300mAh
  • Genace 6S 45C 1050mAh

Further Reading: How to choose LiPo Batteries for Drones?

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 (e.g. 2500KV).

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 goes 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 (e.g. higher pitch) more effectively. 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.

Motor options:

Higher KV Motors

I have been suggested to try some higher KV motors with 6S and I was told the result is remarkable, such as 2000KV or even 2300KV motors, which is equivalent of 3000KV-3500KV on 4S.

These “high” KV motors on 6S should give brutal straight line speed and grunt in the corners. The downsides are obvious: terrible voltage sag and you lose any of the efficiency gain from running lower KV motors.

In order to avoid drawing too much current and maintain better efficiency, you can try to limit your throttle, for example using throttle curve in your radio, or set maximum throttle in Betaflight.

Update (April 2018) – I tested high KV motors on 6S and here is my finding.

Motor options:

  • Amax 2305 2350KV (Review (coming soon) | Buy: Amaxshop)

My 6S Build

Feb 2018 – Amaxinno 5″ Build Log

Parts List:

Test flight:

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 there are more components supporting 5S due to it’s lower voltage. I only tested 6S because it’s more “extreme”. :)

Conclusion

I am still testing as we speak so this is not the conclusion yet.

But so far I really like the performance of 6S! Slightly longer flight time for freestyle flying, and the quad’s response feels so such sharper and direct! I really think it has the potential to replace 4S if there are more component options, and prices drop to the same level as 4S.

Oh, how many people still remember the discussion back in 2014/2015 when we were still deciding between 3S and 4S? :)

20 thoughts on “6S vs 4S for FPV Mini Quad & Racing Drone

    1. Forrest Frantz

      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.

      Reply
  1. wuzzle

    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.

    Reply
    1. Tony

      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!

      Reply
    2. dan

      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.

      Reply
  2. Walkeer

    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.

    Reply
  3. Walkeer

    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

    Reply
    1. Walkeer

      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.

      Reply
  4. Arcus

    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

    Reply
  5. Romain

    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?

    Reply
    1. Oscar Post author

      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 :)

      Reply
  6. Jerrod

    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

    Reply
    1. Paul Hope

      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?

      Reply
      1. Walkeer

        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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