Running FC and ESC on 8S vs. 6S Battery

ArduCopter Copter 4.3
1

I’d like to replace the single 6S1P pack on my 5" FPV Quad with an 8S pack (2x 4S1P packs in series) for packaging reasons. Specs for the setup are:

FC: Kakute H7 Mini, 2-6S input voltage
ESC: Tekko 4in1 45A - 2-6S input voltage, 45A max cont. current

Current Battery Config: Single Tattu R-Line 6S1P, 1300 mAh, 130C Discharge (22.2V, 169A , 3.8kW)

Desired Battery Config: 2x Series Tattu R-Line 4S1P, 850 mAh, 130C Discharge (29.6V, 111A , 3.3kW)

Although the FC and ESC are rated to 6S, can I use the 8S series config given that both the current and power consumption is lower than the 6S config? From a thermal perspective, joule heating should be lower on the 8S setup so seems like this should be ok.

Would appreciate a sanity check to make sure I’m not missing any other considerations.

2

The problem is the higher voltage - some components and insulation wont be able to tolerate the higher voltage without being damaged. Also the voltage reading will be wrong (limit exceeded) and likely be damaged.
You would definitely need a 8S rated ESC
There’s some tricks you could do to drop the supply voltage for the FC to 6S level - but is it worth it??

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3

Thanks for the input - makes sense. Do you happen to have any recs for 8S mini FC/ESC stacks (20x20mm mounting holes) that are Ardupilot compatible?

My desire is to keep the stack as small as possible for packaging reasons

4

http://www.mateksys.com/?portfolio=h743-slim

I dont know of a 4in1 ESC that will tolerate more than 6S

5

Looks like this would work:https://www.getfpv.com/t-motor-mini-f60a-3-8s-blheli-32-4in1-esc-20x20.html?utm_source=google&utm_medium=cpc&utm_campaign=DM+-+B+-+PMax+-+Shop+-+SM+-+ALL&utm_content=pmax_x&utm_keyword=&utm_matchtype=&campaign_id=17881616054&network=x&device=c&gc_id=17881616054&gclid=CjwKCAjwv8qkBhAnEiwAkY-ahuWEZ6uqG2Jjv1WTbb55Yw_NUSbK5HN6oqayQ3WbaH9b4QdCCjwqjhoCIXsQAvD_BwE

I’m looking for an 8S FC with 20x20mm mounting holes. The one you have is 30x30

6

Would this also be true for the motors? Do I need 8S rated motors or can I just use the 6S motors given the lower power/current draw?

7

Don’t have to look far. The Matek H743-mini board is 8S capable. There are probably others, look around…

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8

You have the concepts wrong.
The components are rated for certain voltage level. Ex: 4S components are usually rated ~18V and 6S ~28V.

Just because the current draw will be low, doesn’t mean the components will work with a higher voltage setup (ex: 8S).

Additionally, it’s neither drawing lower power. P = V x I, you’re increasing the voltage and reducing the current, and keeping the power draw same.

With that said, you can run 6S motors on 8S - considering you set a motor output / RPM limit to avoid motors from burning out. We often do this with betaflight for FPV quads.

Watch this video for a short explanation: How To Scale Motors Based on Battery Voltage in Betaflight - FPV Questions - YouTube

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9

Looks like another option is just running lower Kv motors.

At max discharge, the 8S config will draw slightly less power due to lower pack capacity.

Power, 6S = 22.2V *1.3Ah * 130C = 3.8 kW
Power 8S = 29.6V * 0.85Ah * 130C = 3.3 kW

10

You aircraft doesn’t draw power based on whatever your battery can output lol.

11

Yup I can attest to that. Plugged in a 12S pack into electronics rated for 10S, instant magic smoke! Some electronic components aren’t rated for that high of voltage, regardless of how much lower current your motor draws. As soon as you pass higher voltage than rated, it will fail before motor draws even 1 amp.

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12

Would appreciate your explanation for this. I’m relatively new to the space and my understanding so far has been that power output to the motors is limited by the battery and power consumption of FC.

13

Mate, power consumption is NOT calculated by the battery capacity.
Each device on your drone is rated for a specific power consumption.

For example, look at the motor specification below.

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Capture865×1016 319 KB

In the last table, you can see a column named Power (W).
When you fly your quad, each motor takes up a certain amount of current, depending on the load (prop size, weight of the quad, internal resistance, etc.).

Whether you use a 4S battery (4.2*4 = 16.8V) or a 6S battery (4.2*6 = 25.2), the motor will consume the same amount of power, but the current draw will be lower (P = V x I).

Also, the battery capacity is calculated this way.
Volts per cell * Number of cells * Capacity
6S 1300mAh will have a capacity of 4.2 * 6 * 1.3 = 32.76 Wh when fully charged.
By using 2 x 850 mAh 4S cells, the total capacity will be 4.2 * 8 * 0.85 = 28.56 Wh when fully charged.

When the battery capacity is lower, it simply means your quad will fly for a shorter time. Not that the power draw is lower. (In that way, if you remove the battery, the power draw is 0; your quad should fly forever.)

This is referring to the C rating of the battery. The battery links you have provided have a C rating of 130.
Which means they are capable of providing 130 * Capacity Amps of current draw.
1300 mAh at 130 C = 130*1.3 = 169 A continuous power draw.

From the reference motor above, it consumes 35 A at 100% throttle on 6S (24 V). 4 such motors at 100% throttle can consume a total of 140 A, but it is highly unlikely that you will fly at 100% throttle anytime unless you are racing.

You are spending way too much time worrying about things that you do not understand. I suggest that you spend more time reading about the components.
You can read this blog: How To Get Started With FPV Drone - The Ultimate Beginner's Guide - Oscar Liang
Or watch Joshua Bardwell on youtube.

14

Appreciate the detailed response, but yes I know most of this. My point was that the maximum power delivery to the motors is limited by the maximum power available from the battery.

Multiplying the continuous current draw by pack voltage gives the power output of the battery at full throttle (169A * 4.2V * 6cells / 4 motors = 1 kW per motor). Lower in reality due to internal resistance voltage drops.

In your example, my understanding is that the 4S will config will only draw the same power as the 6S if the 4S battery can supply it (dependent on pack capacity and C rating)