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

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.

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

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

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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.

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

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

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

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