In this post we will have a look at the differences of mini quads running 3S and 4S lipo batteries. Many who are building their first mini quad might struggle in deciding which setup is best for them, I hope this post can answer some of those questions. I will also talk about how to move from a 3S mini-quad setup to a 4S setup, as I have been asked by FPV’ers who are looking for more power and speed.
* 3S and 4S refer to the number of cells in series in a LiPo battery. Learn more about the basics of LiPo batteries in this guide.
I am just beginning, should I start from 3S? Or go straight to 4S?
4S has now become the standard voltage level for 4″, 5″ and 6″ mini quads to use, so you should really just start with 4S. There is just too much extra investment, both time and money goes to waste when migrating from 3S to 4S later on.
If you think a 4S mini quad is just too powerful and hard to control, you can always adjust your throttle output range to 75% or even 50% to tame it down. This effectively reduce the power of your quad to a more manageable level. You can crank it back up as you gain confidence. You can do this in the Mixer page in Taranis TX, reduce weight to 75 or 50, and adjust the Offset so the range starts at -100.
Difference between a 3S and 4S mini quad setup
Obviously, the biggest difference is the voltage delivered to the motors. A 3S Lipo has a nominal voltage of 11.1V or 12.6V when fully charged, while a 4S Lipo has a nominal voltage of 14.8V and 16.8V at full charge.
Motors can spin faster when the applied voltage is higher. The RPM (rotation per minute) of the motor under no load is determined by the KV value of the motor – RPM = voltage x KV. (Read this post for more motor basics)
Let’s say we have a 2300KV motor, when powered with a fully charged 3S lipo (12.6V), RPM would be 2300 x 12.6 = 28900 in theory. That means the motor spins approximately 28,900 times a minute without any propellers. When you power the same motor with a 4S lipo (16.8V), the RPM is 2300 x 16.8 = 38640, and as you can see that’s a significant increase in RPM.
This increase in RPM means the motor can generate higher thrust, more power, faster flight speed and agility. But the throttle stick also becomes more sensitive and harder to control.
Note that 4S setups are usually a little heavier than 3S’s due to the additional weight of the voltage regulator, heavier batteries etc.
Upgrading from 3S to 4S
In case you have already built a 3S mini quad, and you are ready for a faster and more responsive setup, here is how you can upgrade it to 4S. If you can afford it, you might also consider building new quad purely designed for 4S power. It’s not a bad idea to have two quads when one is down for maintenance.
Motors and ESC’s
First question to ask: can your motors and ESC’s handle 4S for the increased voltage and current?
Most modern 2204, 2205 and 2206 motors are compatible with both 3S and 4S. Some 1806 motors might be only rated for 2S-3S, so it would be ideal to upgrade your motors to 22xx class which are rated for 4S.
Be aware of the current draw increase too, with the same setup, higher voltage would actually increase the current draw because of the higher RPM. When current gets higher than the safe limit, the motors will get hot as the winding starts to gather heat. If you keep pushing it, the enamel on the motor winding will burn off, causing electrical shorts inside the motor between poles. Eventually it will lead to fatal power loss, and the motor will fail while your quad is travelling at high speed. And BANG! Your quad crashes into pieces.
As for ESC’s, most ESC’s these days supports 4S some even up to 6S, but it’s always worth checking first. Make sure the current draw of your motor and propeller would not exceed the current rating on your ESC. In some cases it’s necessary to upgrade your ESCs from 12A-18A to 25A-30A or higher.
Other Electronics – VTX, FPV camera, OSD etc
Depends on what input voltage your electronics support in your quad, voltage regulators might be required to convert 16V at 4S to something lower such as 5V or 12V. Fortunately there are many options these days, some FPV cameras and VTX support wide voltage range up to 6S, while there are PDB that provides 5V and 12V outputs.
To sum things up, 3S has moved to the back seat as the hobby has started to standardize things around 4S builds. Similar to an F1 flight controller, they work and fly well but F3/F4’s have become the norm, with F7’s probably to follow. Some pilots are now even using 5S and 6S LiPos in their mini-quads!
- 2015 April – Article created
- 2017 June – Article revised