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RC Car Theory: 2s vs. 3s, high kV vs. low kV

21/4/2021

37 Comments

 
Whether you realise it or not, your RC car has been designed to run quite a specific battery voltage, in this article I'll explore what happens if you put a 3s battery in a 2s car (and vice versa) and why jumping up in cells should only be done as part of a novelty or pissing contest for top speed. I'll be making particular reference to the WL Toys 144001, 124018 and 124019 series of cars (most notably to my drop in brushless upgrade) but conclusions here can be generalised.
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Like most smaller RC cars, the WL Toys 144001, 124019 and 124018 are designed to run on 2s. This is made for reasons of budget, practicality and availability. Once battery size is confirmed, this starts a cascade of specification decision making. Motor size and wind (kV), gear ratio, wheel size, drive train materials, thicknesses, battery capacity etc, etc. Some of these can be tweaked to give a specific feel, application or performance and a good example of this is non other that the 144001, which purposely has an atypically larger motor and tall gearing thus resultant short battery life. This was done with the sole focus of maximum speed at lowest price for bragging rights. Just take a look at the marketing - claims of top speed is always near the top of the spec.

In the old days when we used to use NiMH or NiCD cells these were 1.2v each meaning we have to have 6 cells to get to 7.4v. Adding another cell would increase this by 17% - A noticeable increase in energy potential but not crazy. With the move to Lipo tech we only need 2 cells to get us to a comparable 7.4v. Adding just one more cell with a 3s pack takes as to 11.1v which is a 50% increase. As you can probably start to tell, this is a huge increase and takes the carefully considered power train that was specced for 2s well out of it's designed operating zone.
Moving on from me being a grumpy old curmudgeon, there are cars the exist out there that have a conservative and/or over-engineered powertrain that can handle this increase. The 144001, 124019, 124018 are not conservative nor heavily over-engineered. Even if the ESC didn't immediately ignite on anything more than a plug in, the battery, motor, ESC and even drive train would be run well outside it's ideal operating window. Yes it'll sure as hell be fast but longevity on a 3s pack size is not something you could reasonably expect given we know how hot they run on a standard 2s pack.
The choices when upgrading to brushless
So when you make the decision to move to brushless you have the chance to rewrite the performance requirements of this car which is helped somewhat by the new gearing options I've discovered as a result of much testing (namely 15t, 16t, 17t as opposed to the stock 27t pinion).

I haven't mentioned it explicitly, but all of the brushless systems I've specified for my cars have been based around a 2s battery which includes the stock battery as I wanted to keep these cost-effective. Typically I've chosen 4500kv motors on a 15 or 17t pinion gear with varying motor sizes to give me a nice operating speed, acceleration and battery life. The only time I've moved from this formula is in my drop in brushless upgrade. In this case I didn't have a 4500kv option so went with a 5400kv 2445 due to ease of install. The speed increase was instantly noticeable but sadly so was the increase in motor and ESC temp AND reduction in battery life. I have mitigated this through changing timing and improving ventilation but clearly we are outside an ideal operating window.
Ok, so what if I put a 3s battery in this setup? So long as your ESC can handle the extra pain, it will be MUCH faster but think of it this way - running a 4500kv motor on 3s is equivalent to running a 6750kv motor on 2s. No, the current draw won't be quite as high as the 2s equivalent but all the other negative impacts are - heat, wear, damage etc. Fine if this is a one off for bragging rights or speed runs (same thing!) but not something you'd do regularly. Lets no forget that the 5400kv motor was already a compromise and that is only 20% more than my ideal 4500kv
Testing the theory using the Drop in Brushless Upgrade
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If you have been picking up what I've been putting down then you'll see running 3s on a 2s specced system is not sustainable except under very specific conditions. So how about running a 2s battery on a 3s setup? For this example I will be replacing my 2445 5400kv motor on 2s with a 2445 3600kv on 3s based on the setup for my 144001 124018 124019 Drop in Brushless Upgrade Guide. I chose this setup because I should be able to achieve the same peak RPM which should give the same top speed since I'll be using the same gearing and wheel size.
  • 5400kv x 7.4v (2s) = 39960rpm
  • 3600kv x 11.1v (3s) = 39960rpm

Theory - Ohm's and Power Law
So what is the point of this? This is where it gets interesting. Using a derivation of Ohm's law, power = voltage x current. In a perfect world where there are no heat losses or at least they are negligibly different then for the two motors as mentioned above, and increase of 50% voltage for a fixed power output will result in a current requirement decrease of 33%. Thus you'll have LESS load on the ESC and LESS requirement on heavy gauge wires and battery c rating or for a fixed battery C rating, a lower voltage sag under full throttle. These are all good things and have been discussed at painful length when 5 inch quads moved from 4s to 6s with and equivalent drop in motor kV as suggested above.
Sticking with quads, 6s is now the mostly accepted standards and something interesting popped out of this - if you run your 4s pack on a 6s quad of course it is slower but you get amazing throttle control for flying in tight spaces and crazy efficiency. This can also bee considered with RC cars - running a 2s battery on a car designed for 3s will be slower but will be very smooth and super efficient if you want to run on rough terrain or even a smaller backyard without fear of damage or overheating
Results 5400kV speed
As reported in my drop in brushless upgrade review, I achieved the following results using the 2445 5400kv motor:
  • 2s 5400kv 2445 on 15t pinion, stock wheels = ​78km/h max speed
  • ​3s 5400kv 2445 on 15t pinion, stock wheels = 109km/h max speed
As mentioned several times run hot and I needed to make a few changes to mitigate this including venting of the body and reducing timing on the motor. Even though I no longer hit thermal cutoff this is still more heat than I feel comfortable with.
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Results 3600kV speed
​I've since installed the 3600kv motor in the same size and got the following results:
  • ​2s 3600kv 2445 on 15t pinion, stock wheels = 50km/h top speed
  • 3s 3600kv 2445 on 15t pinion, stock wheels = 72km/h max speed
​​If the kV values were accurate then I would have expected the 3600kV motor on 3s to have the same top speed as the 5400kV motor on 2s as you can see from my RPM calculations above. As it stands it was slightly slower and my gut feel tells me that the 5400kv motor is probably a bit higher in kV than it states. At a guess, around 5800kV
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My thoughts on the 3600kV motor on 2s
Acceleration on this setup was very strong and in small areas it gives you the feeling that it is powerful as a result, but in wide open spaces the lack of top end becomes more noticeable. To be fair though, 50km/h is the same as stock which is no slouch. As predicted is it super efficient - I got 15 minutes on a 2s 1500mah pack that I only got around 10 minutes on for the 2s 5400kv run (and about 6 minutes on the stock brushed motor). The and ESC also runs VERY cool as expected - barely lukewarm when I gave it an onroad flogging, such a completely different result to the 5400kv motor. As a purely subjective preference I actually quite like the 3600kv motor on 2s here since, as I mentioned, it feels powerful due to the strong acceleration and it feels more appropriate for small spaces and off-roading. Because of the gearing and the relatively kv this is probably the only time I'd be happy to fit to a bigger 85mm wheels like the EMB-MT wheels the Geoffrey wrote about here.
My thoughts on the 3600kV motor on 3s
The numbers don't like here it is not quite as fast as the 5400kv motor on 2s more like due to an inaccuracy on the motor kv number than anything else. Acceleration is definitely much stronger and a lot less space was needed to wind up to top speed compared with that setup. 72km/hr objectively is still very fast on a car this size and in anything other than a flat road surface you would find it a handful to manage. Efficiency is right up there again. At around 20 minutes of running my 3s 2200mah battery I had to go and still had v per cell left - probably another 5 or 10 minutes. Most pleasing though was how cool the motor and ESC was though. Again, as the theory above had predicted, lower current draw using more voltage for the equivalent power has resulted in much lower temps for the ESC and motor. The ESC was cool enough not to cause my finger any discomfort for any amount of time and the motor I could hold for 3 seconds or more. Certainly cooler that the 5400kv on 2s, even when I backed the timing back to 3.75 degrees advance.
Conclusions
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Even though I suspect some inaccuracy in the kV rating of the motors I tested, this test has been a great way to demonstrate low vs high kv motor setups with varying battery voltage options. There are a few key takeouts for me:
  1. A lot of work goes into specifying power trains for RC cars. Up-volting to the next battery size has more disadvantages than advantages for anything beyond a one off top speed test -in my opinion.
  2. Taking advantage of lower kV motors and more battery cells (where the ESC allows) makes for less current draw resulting in greater efficiency and much less heat. This means you don't need a massive ESC.
  3. My preference in future will be to use lower kv motors and higher cell counts where practical. Using batteries with lower cell counts on these rigs makes for fun in small spaces and offroad, even with bigger wheels.
  4. So what about the 2445 3600kv vs 5400kv? If you are looking for reliability and a setup well suited for offroad then the 3600kv is for you. If you don't mind dealing with the heat and want to live on the edge of performance then the 5400kV is for you. Consider grabbing both as these are cheap motors at around $13 each

Hopefully you have found this article useful, I'm focusing on making my guides and testing more technical and exhaustive so the choice is made easier for you. If you purchase via the links here or on any of my pages I receive a small commission at no cost to you which is enough to help my cover my hosting fees and other expenses related to quadifyrc.com - thanks!
37 Comments
RCFREAK
27/4/2021 09:40:04 pm

Brilliant, this article made go straight out and buy this set up. Can't wait to run it.

Reply
QuadifyRC
28/4/2021 09:55:09 am

Awesome, I hope you enjoy it. It's certainly been fun testing to see how things like this impact performance and efficiency. Let me know what you think once it is all up and running :)

Reply
Gord
29/4/2021 05:13:06 am

Great article! This explains why I was hitting thermal shutdown when running the 5400kv motor going full throttle on and off road. I think I'll drop in the 3600kv motor and see how it feels. Love all your stuff. Keep up the good work!!

Reply
RCFREAK
29/4/2021 07:57:03 am

Can someone clarify how I can tell if I'm hitting thermal cut off. I know in the instructions it says the ESC will flash a certain color, but it's hard to see in daylight.

When I run my 144001 with the 5400kv set up, it will show in speed as the battery nears voltage cut off, but it's often possible to wait a moment and then full speed seems to return. It's at this point im not sure if I'm hitting thermal cut off (because the car is getting hot ish by this point), or in just hitting voltage sag?

Reply
QuadifyRC
29/4/2021 09:14:43 am

Sounds like low voltage cutoff when the voltage sags to that level. I'd recommend increasing to 3.4v to protect your battery a bit better.

Dimitris
5/5/2021 11:23:44 pm

Correct me if I am wrong but if you use the 3600kv 2S and want more speed I believe that you could also gear up to the 17t pinion (after you elongate the motor mount holes).

(I do not have the setup I am just guessing)

Reply
QuadifyRC
6/5/2021 09:12:40 am

Hi Dimitris

Yes that is also an option to use the 17t pinion for sure. I don't tend to recommend this as much because it is a little trickier to install but it is a good option.

Reply
RCFREAK
9/5/2021 05:56:43 am

I have just finished my installation of the 2445 3600kv set up with 15t pinion in a 124018.

Using 2s so far. It's excellent. Lovely smooth drive. Speed seems similar to stock without testing it. Acceleration lovely and prompt, makes a great sound. Runs luke warm at worst. The 124018 just generally seems a big improvement on the 144001.

What kind of improvement in top speed can I expect if I invest in a couple of 3s batteries? Also did I read somewhere that 17t pinion and 3s on 15t is roughly equivalent?

Reply
RCFREAK
14/5/2021 07:16:42 am

I just wanted to say, that after running both the 2445 5400kv and this 2445 3600kv, that the latter is the fault superior set up. With 3s batteries you have practically the same top speed (towards 50mph) which I find is more than enough for everything but bragging rights. You have much lower temps and longer battery time. On 2s you still hit 30mph which again is perfect for smaller areas or just popping off some jumps and still manages to turn people's heads when they see it speed past. I'm completely in love with my 124018 with 2445 3600kv set up.

Reply
QuadifyRC
14/5/2021 10:41:29 am

Yes that's exactly where I ended up too - I don't want to have to stress out about overheating, I just want to go out and play and have a car I can rely on which is why I prefer the 3600kv like you. Yeah, it's not slouch on 3s and only gets a little warm. Run time is nothing to sneeze at either. Have fun :)

Reply
RCFREAK
15/5/2021 10:07:40 am

Yeah the only downside has been having to buy 3s batteries that I didn't previously own! Hardly an issue for most in sure.
It's been so fun when running on 2s and someone says "wow that's fast" and then you tell them it's running in slow mode, watch this *inserts 3s battery* zooooooom! Lol.

Thank you once again quad. Great guides!

RCFREAK
15/5/2021 10:08:00 am

Yeah the only downside has been having to buy 3s batteries that I didn't previously own! Hardly an issue for most in sure.
It's been so fun when running on 2s and someone says "wow that's fast" and then you tell them it's running in slow mode, watch this *inserts 3s battery* zooooooom! Lol.

Thank you once again quad. Great guides!

RCFREAK
15/5/2021 10:08:13 am

Yeah the only downside has been having to buy 3s batteries that I didn't previously own! Hardly an issue for most in sure.
It's been so fun when running on 2s and someone says "wow that's fast" and then you tell them it's running in slow mode, watch this *inserts 3s battery* zooooooom! Lol.

Thank you once again quad. Great guides!

RCFREAK
15/5/2021 10:08:33 am

Yeah the only downside has been having to buy 3s batteries that I didn't previously own! Hardly an issue for most in sure.
It's been so fun when running on 2s and someone says "wow that's fast" and then you tell them it's running in slow mode, watch this *inserts 3s battery* zooooooom! Lol.

Thank you once again quad. Great guides!

RCFREAK
15/5/2021 10:08:46 am

Yeah the only downside has been having to buy 3s batteries that I didn't previously own! Hardly an issue for most in sure.
It's been so fun when running on 2s and someone says "wow that's fast" and then you tell them it's running in slow mode, watch this *inserts 3s battery* zooooooom! Lol.

Thank you once again quad. Great guides! 🙂👌

Andrea
8/8/2021 11:44:26 am

Ho letto quasi tutte le guide e quella sulla combo 2445 e esc hobbywing è quella che mi attira di più. Ma una domanda per capire.... Se invece del 2445 da 3600 o 5400, uso un 2440 da 4600kv che risultato avrei?

Reply
QuadifyRC
9/8/2021 02:09:11 pm

Hi Andrea. The 2440 has a little less power to turn the wheels and so is a little slower - keeping it light is important. I did a build with the 2440 4600kv here and got 59km/h on 2s: https://www.quadifyrc.com/rccarreviews/144001-brushless-upgrade-the-cheapest-setup-worth-having

Reply
John
16/8/2021 10:11:58 am

Hi Quad! Thanks again mate!!! So I installed the 2445 5400kv based on your "drop-in" recommendations and it was brilliant. That is until I tried 3s. Super fast for maybe six passes and then the motor burned up. Of course it did!!! So I'm looking at getting the 2445 3600kv motor but then I'm thinking of looking into the 3650 size which I believe will also drop right in, right? I know the motor mount will work and I'll have to drill out the 15t pinion to 1/8". So what would be the advantages, if any, of moving from a 2445 to a 3560?

Reply
QuadifyRC
17/8/2021 04:45:48 pm

My pleasure John! Yeah, a car with a 30a ESC has no right to go that fast. I only recommend the 3650 for straightline speed runs as they aren't great for weight or balance. If you are looking at this kind of setup you'd need at least a 60-120a ESC. With the 3650 motor you are really limited to just the 27t stock pinion.

I'm not sure which ESC you have but consider a 2845 if you are 60a or more and don't mind drilling a pinion. If you are 30a the 2445 3600kv is good - you can still get over 70km/h on 3s comfortably.

Reply
John
18/8/2021 01:28:48 am

Thanks Quad. I finally decided on the 2445 3600kv motor, 15t pinion, running 3s with a 10bl120. I'm sure it will be plenty fast!

Ron wilson
18/8/2021 05:16:09 am

I dont see how th ehigher voltage ran cooler, ive always thought higher voltage caused more heat and drew more amps

Reply
John
18/8/2021 05:22:14 am

Hey Ron. I'm gonna try to answer this and it's more of an exercise in my brain that I'm writing down. I think electricity can be visualized as water coming out of your faucet. The faucet doesn't care what temperature the water is - temperature equals volts. But it DOES care about the amount of water coming out as it can only handle a certain flow and that's it! Volume of water equals amps. The amount of heat energy in a tub of water would equate to the amount of water multiplied by its temperature. The motor needs X amount of energy, amps times volts. If it has more volts, it will require less amps.
Okay, so I'm sure my analogy breaks down eventually and I'm not totally convinced it's good. I just felt like talking.

Reply
Lou
5/12/2022 11:27:30 pm

Ohm's law is much like Moore's law. Its an innacurrate guideline that happens to fit perfectly within a range of variables. Look at high core count CPU, and think of that as your example of the ermergance of smaller, lighter wight 5200mah 100c 2s. The 'Law' wasn't.really meant to work at all within these previously unforseen range.of.variables. the further above that old spec we go, the further the law breaks down.

Science makes the laws, we just try to discover them and then find ways to break them. Case in point, brushless extra low turn light weight motors, turbo boost ESC, and parallel 2s that run the 3650 5200kv motor up to near 75k rpm, a previous impossibility at 700w peak.

QuadifyRC
18/8/2021 09:46:56 am

Hey Ron.

Heat is related to current rather than voltage. Since power = voltage x current then you need less current to achieve the same power output.

In the real world this means dropping the kv of your motor by the appropriate amount. Conversely, if you keep the same motor kv between 2s and 3s then the extra voltage causing the motor to spin to a higher peak RPM will actually need MORE current to get it there. Thus for a fixed kv motor, more voltage also needs more current.

A good example of ramping up voltage to reduce current is high tension power lines. Higher voltage, lower current to get the same amount of power. This allows them to run a lower gauge wire since wire gauge only cares about current, not voltage.

Reply
Stuart
24/11/2021 02:57:28 pm

Has anyone tried the 2445 3000kv motor on 4s? If so what Amp esc was needed?

Reply
Isaac
3/12/2021 05:49:41 pm

Hey Stuart, it's not Quad, it's me Isaac. I wouldn't recommend 4s on that motor, and you wouldn't be able to run 4s either, partially due to the fact that you're not going to be able to fit that battery, unless you do what the speed run guys do and make custom out-board battery trays that sit on either side of the shell, bad idea for bashing. I'm almost certain the 2445 3000kv would go up in smoke on 4s, it's really meant for 2s, you can run 3s but be careful, and keep an eye on the temps. If you want that much power, the closest I can think of is Quad's "ultimate brushless guide", that uses a 2838 motor I believe. Or the 2445 3000kv 3s. The Wltoys stuff isn't build for 4s extreme bashing, hate to say it.

Reply
Stuart
14/12/2021 08:51:51 pm

Thanks Issac. I'll think I'll go for the 3600kv on 3s. Might get the 3000kv for the kids 124018. Once again thanks for your help.

QuadifyRC
10/12/2021 11:04:08 am

The smallest ESC that I am aware of that can do 4s is the max10 I reviewed here https://www.quadifyrc.com/rccarreviews/wl-toys-104001-big-brushless-upgrade-and-installation-guide

Motor kV is still to high though, it'd be the equivalent on 6000kv on 2s. Would be a great option for 3s though.

Reply
Stuart
14/12/2021 08:55:06 pm

Thanks Quad, yes going to 4s is a big jump up in the esc department. I think I'll stick to 2s and 3s. Thanks for your input keep up the great work.

van Zeller
1/12/2021 11:55:27 pm

What would be the largest wheels you would feel comfortable if using the 3600 kV motor with the WP-16BL30 ESC? You mention 85m in your text, would that be the maximum you would recommend?

Reply
QuadifyRC
10/12/2021 11:01:57 am

The problem is when you go past 85mm you are dealing with extra weight and inertia as well so you put a lot more stress on the diffs and driveshafts. Personally I'd stop at 85 but you could go to 90 or even a 100 purely from a gear and kv perspective

Reply
van Zeller
29/12/2021 11:41:37 am

Hi once more. I am eying the wheels from the HBX 16889, available on aliexpress. How would you think those wheels would fare on a 144001 as compared with the remo wheels? I'm asking because I believe you own a 16889, so maybe you can give some qualified advice here. Many thanks once more!

Scott
5/1/2022 04:46:17 pm

Thanks for the nice write up! I'm curious to find out how regearing the lower kv motor to a top speed similar to the higher kv motor would affect battery life, acceleration, and temp. That would be really useful as well in my opinion.

Reply
QuadifyRC
13/12/2022 05:25:05 pm

I think it is safety to say that all motor have a practical operating window for gearing in a particular car. If you go to extreme at one end something will suffer. In the case of low kV and tall gearing this is cogging and can be seen in the Eachine EAT14 and version 1 of the WLToys 124017

Reply
ian
12/6/2022 03:31:18 am

Just had a bit of a surprise result when experimenting with different KV motors in a 144002. I've been running a 15T 2445 3600KV setup back to back with a 17T 540 3300KV.

I expected the higher power (880W vs 500W) heavier motor (540 is 50g+) to have a shorter run time, given that both setups give similiarish performance in terms of top speed. I've run several different battery packs (all Wltoys stock 1500mAh, 2200mAh & 2600mAh) and had the same results each time (heavy bashing with plenty of full throttle).

The 540 is consistently giving run times of 30% longer.

Is this just down to motor KV or there something else happening here? Has anyone else had similar results?

Reply
William
30/8/2022 02:19:07 am

I went from 53 km/h stock to 86 on 2S and 100 on 3S. It's not about bragging rights, just love speed runs. I upgraded to a 3650 - 3900kv brushless motor and generic v2.3 esc 120A. Run the 2S brushless run with a 1 year old stock battery. My 3S battery as old but less used. Respectively 2200 and 1500 mah.
I now, thanks to this article understand the problem in my system. Definitely buying a lower A esc... Thanks.

Reply
QuadifyRC
30/8/2022 09:01:35 am

Glad it is working for you, have fun!

Reply



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