How to use 6s Batteries with 4s High-kv Motors for FPV Drones

6s batteries are becoming very popular, but the problem is that everybody’s FPV drones still have 4s rated motors.

The higher-kv 4s motors are not designed to handle a higher voltage than 4s. If you’ve ever tried using 6s batteries with your 4s motors, the RPM’s skyrocket, and the motor most likely burns up.

However, there is a couple ways to put limit the motors so that they can handle 6s batteries without negatively affecting your drone. Before trying this, make sure your electronics are 6s capable otherwise they will be destroyed!

Here are two ways that you can use 6s batteries with 4s, high-kv motors!

Why using 4s Motors with 6s without Motor Output Limit is Dangerous

A motor’s theoretical rotations per minute is calculated by multiplying the KV of the motor with the voltage of the battery.

6s motors are normally around 1750kv, and 4s motors are generally around 2300kv. This puts the average rpm for each motor used with it’s correct voltage at around forty thousand.

If a 4s motor is used with 6s voltage, the RPM’s skyrocket to over sixty thousand, a 150% increase.

Running 4s motors on 6s voltage makes the drone very dangerous, because not only can it go faster than before, but a motor could burn out and the drone can become a 100+mph falling projectile, but it can also start a fire.

And when LiPo batteries catch on fire, they’re hard to put out.

So please, use a motor output limiter.

Use 6s by Limiting the Motor Output in Betaflight

Setting up a motor output limit in Betaflight is pretty easy, and the best method for using 4s motors with 6s voltage.

A nice feature that Betaflight has is automatic cell count recognition, so you can make Betaflight automatically detect 6s batteries, and lower your motor output limit for you!

Step 1: Make all your PID profiles are Reasonable

The motor output limit in betaflight assigns a certain motor limit to profiles, so you’re going to need to have at least two matching PID profiles

To do this, click on copy profile values in Betaflight’s PID tuning tab, and hit copy to copy your PIDs to Profile 2.

Step 2: Set up Auto Voltage in CLI

Next, you need to set up automatic voltage detection in Betaflight’s CLI tab.

For this step, you need to enter each profile and then assign it to a specific voltage. Betaflight starts at 0 and counts up, so instead of Profile 1 you need to type Profile 0.

1. Type profile 0 to enter profile 1.
2. Type set_auto_profile_cell_count = 4
3. Hit enter

If you’ve followed those steps, you should see this on your screen. You’ve successfully assigned your 4s batteries to profile 0! These same steps can be used for other scenarios as well, such as using 4s battery on 3s motors, etc.

You still have to assign your 6s batteries to profile 2.

1. Type profile 1 to enter profile 2
2. Hit enter
3. Type set_auto_profile_cell_count = 6
4. Hit enter again
5. Type in save and hit enter yet again

You’ve successfully assigned your 6s batteries to profile 2!

Step 2: Assign Motor Output Limits to 6s Profile

Profile 1 (or what Betaflight likes to call Profile 0 in the CLI) is assigned to 4s voltage. Since you’re using 4s motors, you don’t need to set an output limit.

Profile 2 is assigned to 6s voltage, and needs a motor output limit. To get the same theoretical RPM’s as 4s voltage, the output limit needs to be set to 66%.

1. Type profile 1 to enter profile 2
2. Hit enter
3. Type motor output limit = 66
4. Hit enter
5. Type save, and hit enter again

Test your Motor Output Limit

There’s two methods to test if the motor output limit you just set up works.

First of all, go into the CLI again in Betaflight and type diff all. Here you can verify that all your commands were saved.

For the first method, enter your PID profile tab in Betaflight. If you plug a 6s battery into your drone, it should switch to PID profile 2. If you plug a 4s in, it should switch back to PID profile 1.

The second method is to listen to the motors. With your propellers taken off, arm your drone and listen to the difference in the motor sound between max throttle on 4s, and max throttle on 6s.

You should be able to hear that they’re spinning at about the same RPM

Use 6s by Setting a Throttle Limit

A good way to use 6s batteries for planes, fixed wings, and brushless motors that can’t use Betaflight is to set a throttle limit of 66% on your radio.

This is not recommended to use on a drone with Betaflight, since this only affects the total throttle limit.

With the throttle limit method, the drone can still use the full power of the motor to correct itself. If the drone clips a branch and starts spinning, the motors can still use 100% power and potentially burn up.

Final Thoughts

Using a motor output limit to use 6s batteries on 4s motors works really well! My main freestyle quad has been using 4s, 5s, and 6s batteries for the last year and flies great.

If you want to use 5s batteries as well, set your second profile to 5s, and then on the third profile set the auto_cell_count to -1. This will assign any battery except 4s and 5s batteries to this profile, and you can assign a motor output limit of 66% to it.

Before trying any higher voltage, double check that your electronics can handle 6s voltage. Make sure to check all your electronics; including the camera, flight controller, esc, and vtx.

Make sure to comment with any questions, and let us know whether you use a motor output limit on your drone!

Happy flying!