In early 2018, MultiGP, a large American drone racing league announced a change in regulations derestricting the battery cell limit in races from the conventional 4 cell (4S) LiPO limit to open. This rule change has prompted some competitors to adopt 6S drone racing setups whereas others have elected to remain with 4S setups.
This article will discuss the technology behind 6S setups and outline the key differences between a conventional 4S high motor kV race setup and a 6S low motor kV race setup. As low motor kV 6S setups appear to be the most prevalent 6S racing setup at the time of writing this article, most examples will relate to the advantages and disadvantages of this setup unless otherwise specified.
Power and Batteries
The main obvious difference between a 4S and 6S race quad is the two extra battery cells. This changes the resting voltage of a LiPO battery pack from 14.8V to 22.2V. A key advantage of a 6S LiPO is its capability to supply a similar quantity of power as a 4S LiPO with reduced current draw. To help explain the differences between the battery packs, the following formula will be employed:
P = I x V
P = Power (watts/W)
I = Current (amps/A)
V = Voltage (volts/V)
Power is a measurement of an objects ability to do work. At full throttle, racing quads can use around 2,500 watts (2.5 kilo watts/kW) of power. That is equivalent to 3% the power of a Honda Fit (and that is the variant with VTEC)!
The issue with a 4S lipo is that it must be able to supply a large amount of current to provide the craft with such power. Using the 2.5kW power consumption example, a 4S pack would have to provide ~169A of current (2.5kW/14.8V= ~169A) to keep the drone hurtling down the straight. Although high end and/or large capacity 4S packs can supply this quantity of power and current, high current draw for extended time intervals can damage the battery cells which greatly reduces pack longevity.
The advantage of a 6S pack is that the voltage increase significantly reduces the current required to provide the same quantity of power to the craft. Running with the current example, the 6S battery would only have to supply ~112A of current (2.5kW/22.2V=~112A) to provide the craft with 2.5kW of power.
Reducing the current demand from a battery pack can increase the longevity. This is because each individual cell is required to draw less current which is less damaging to the whole pack. In a battery pack, the individual cells are in series (linked like a chain). When the battery is required to supply a current load of, for example 50A, every cell in the pack will need to supply 50A of current. Because there are more cells in a 6S pack, each cell is not required to supply as much current as the cells in a 4S battery. This is assuming that both packs are required to supply the same quantity of power.
If the power consumption of a 6S craft compared to a 4S craft is increased due to e.g. larger sized or high kV motors, the current draw of the 6S pack will increase. This will be similarly as damaging as drawing high current from a 4S pack. The 6S craft can gain performance with bigger/faster motors compared to a 4S quadcopter (assuming that the weight is similar) however the current speed and acceleration of a 4S race quad is subjectively enough for most racing scenarios.
Battery Capacity and Weight
An apparent disadvantage to 6S packs is the fact that an equivalent capacity battery to a 4S can weigh approximately 50% more. Whilst this is true, a 6S drone racing battery can be sufficiently smaller in capacity than its 4S counterpart and still supply the same quantity of power over a set unit of time. The following equations demonstrate this:
Battery voltage (volts) x Battery capacity (amp hours) = Stored electrical energy (watt hours)
For a 4S 1300mAh 4S LiPO: 14.8V x 1300mAh = 19.2Watts per hour (Wh)
For a 6S 865mAh LiPO: 22.2V x 865mAh = 19.2Watts per hour (Wh)
19.2Wh of energy means that the batteries could supply 19.2 watts for an hour, 38.4 watts for half an hour or 1185 watts for 60 seconds. The previous equation shows that both batteries can store the same amount of energy. This essentially means that a quad using either battery could achieve similar flight times. That is based on the assumption that both quad copters consume the same quantity of power, and have similar weights.
A disadvantage of using a smaller capacity 6S battery pack is that the decreased surface area of the pack’s cells reduces their current drawing capacity. This is why many 6S drone racing setups will use batteries with around 1000mAh capacities. To learn more about batteries, a more comprehensive article can be found here.
Motors are a key part of a quadcopter and running a 6S drone racing setup can be better and worse for motors. Due to the length, material and diameter of their (usually copper) windings, motors have a set internal resistance. Running a higher voltage through the motors will mean that they will draw more current compared to a 4S setup. This is demonstrated in the formula below:
V/R = I
V = Voltage (volts)
R = Motor internal resistance (ohms)
I = Motor current draw (amps)
This heats the motors at an increased rate and can eventually burn the insulation off the motor windings causing a motor failure due to a short circuit. Because of this, a 6S motor’s internal resistance needs to be lower than an average 4S motor’s to achieve similar longevity.
Whilst low internal resistance motors exist, they are currently quite expensive as higher-grade materials are usually required to produce them. Low internal resistance motors can also be used on a 4S setup to gain efficiency and flight times.
Commonly, 6S drone racing setups will use of kV motors with a battery of equivalent energy storage to a larger capacity 4S pack. The idea behind a low kV 6S setup is to achieve the same motor RPM as a 4S setup using a high kV motor. This is demonstrated using the following formula:
Voltage (V) x Motor kV (kV) = Revolutions Per Minute (RPM)
For a 4S setup: 14.8V x 2600kV = 38,480RPM
For a 6S setup: 22.2V x 1733kV=38,480RPM
An equivalent motor RPM essentially means that the top speed of both setups will be quite similar (depending of course also on the motor size and the propeller). Lower kV motors also have more windings around each pole providing them with more torque (or twisting force) than a similarly sized high kV motor with less windings. This allows the motors spin up with more force enhancing the responsiveness of the motors, and subsequently the craft, at low throttle positions.
The main issue with low kV motors on a 6S drone racing setup is the current lack of selection. They are significantly less common than high kV 4S designed motors. Motor manufacturers are however quite likely to improve the availability of low kV 6S drone racing motors if there is increased usage. On the other hand, there is an immense quantity of 4S motors to choose from with a large variance of prices, specifications and qualities. To learn more about motors, you can visit this article here.
6S drone racing also requires 6S capable electronics (speed controllers, flight controller, video transmitter, etc.). If you plug a 6S pack into non 6S rated electronics, you may get to experience the phenomenon of magic smoke! 6S drone racing electronics however can be prone to premature failure due to the higher voltages causing semiconductor breakdown. More and more reliable 6S rated electronics are coming into the market however there is still a much wider available selection of highly reliable 4S capable electronics.
To conclude, 6S drone racing setups do look like they will be gradually replacing conventional 4S setups however it may take a while before racing is 6S dominated. Personally, I will be building a 6S low kV racer in the months to come as it looks like the direction that quadcopter racing is heading towards.
As a bonus, here are some parts that I would recommend for general 4S high kV setups and 6S low kV setups: