GUEST CONTRIBUTOR: A 20 HORSEPOWER ESC?!!
Castle Creations new Mamba XL X2 ESC was announced just about 24 hours ago and one question keeps popping up. “How are people getting over 20 HP out of this thing?!” Bottom line: It’s all about the batteries... and a super low resistance ESC. Strap in. It’s going to be one of “those” posts.
Before I dive in to how the XLX2 is delivering such astonishing real-world power numbers, I need to define a few terms and explain a few concepts. (This is RC Physics after all.)
“Bottom line: It’s all about the batteries...
and a super low resistance ESC.”
All power systems start out with a bucket of energy that they use as their power source. In Internal combustion vehicles, that energy is stored in the chemical bonds in their liquid fuel. In electric power systems, that energy is still stored chemically, but as ionic bonds in its battery cells. It’s interesting to note that liquid fuel actually stores many times more energy per lbs. (or kg) than batteries do, but most of that energy goes right out of the tailpipe as heat in the combustion process. In fact, many small motors only convert ~20-30% of the fuel’s energy into actual work. By contrast, a well-designed electrical power system will only waste a few percent of its total power on the way to the motor’s output shaft (within its designed power range…. more on this later).
So, the point of any power system is to do “work” in the real world. Scientists and engineers define “work” as applying a force to an object over a certain distance. This is measured in units called Joules [J]. Power is then the amount of work done over a certain amount of time. It is measured in units of Joules per second (aka Watts) or good ol’ American horsepower here in the states. (1 HP = 745.7 Watts)
The interesting thing about power is that it actually exists in several forms in an RC system. There is Mechanical Power (as measured at the motor shaft or at the wheels), Electrical Power (as measured at the Electronic Speed Controller (ESC) or inside the motor coils), and there is Drag Power (all the power lost to aerodynamic drag, friction, and mechanical losses in the driveline).
Mechanical Power is the torque at any point in the driveline multiplied by the rotation rate of the components (aka rpm). (P_mech = torque * rpm)
Electrical Power is the current flowing through the batteries, ESC, and motor multiplied by the system voltage. (P_elec = I*V)
Drag Power is the sum of several drag forces in the system (the rolling resistance including the friction from the tires and bearings, and the aerodynamic drag) multiplied by the speed of travel. (P_drag = (Rolling_Res + Aero_Drag)*Velocity)
Since P_elec = I*V, Castle ESCs calculate system power for their data logs by multiplying their real-time current and voltage measurements. However, it is important to know that the ESC and batteries do not PUSH power. The motor actually PULLS power from the batteries through the ESC based on the mechanical load at the motor shaft. The ESC then acts like a “valve” regulating how much of the available power will be allowed to flow to meet the mechanical loading. Remember, P_mech = torque * rpm. Electrical motors are special because they can produce nearly constant levels of torque over a VERY wide rpm range. However, they need gears to match the rpms (and power band) of the motor to the expected loads on the driveline. If the gear ratio is too “tall”, you can overload the motor. If the ratio is too “short”, the motor will not load up to the output levels you need and you will not reach your speed goals.
So, to log an output power of over 20hp, you need to have gearing that will sufficiently load up the motor, and power system capable of delivering more than ~15kW of electrical power… which is not an easy task thanks to a nasty little thing called VOLTAGE SAG.
Ohm’s Law says that voltage in a system will drop (or sag) depending on the system's total resistance (aka the load) and the current flowing in the circuit. (V = I * R) The higher the current and resistance loads, the greater the voltage sag. Batteries and ESCs are not perfect conductors, so the amount of restriction they offer to the current-flow is call Internal Resistance (IR). Higher output ESCs like the Mamba XLX2 play a lot of sophisticated engineering tricks to drive down their internal resistance (and power losses) to incredibly low levels allowing them to pass enormous levels of current without producing much heat. Similarly, high output motors have incredibly low resistance windings capable of pulling enough current to literally melt any ESC not up to the task. Since high performance brushless motors and ESCs have such low resistance values, the IRs of individual battery cells usually determines the maximum current in a Castle-based power system.
Since P=I*V, we know that a 33V power system flowing 500A is greater than 15kW. Done! …not really.
A 33V power system flowing 500A in the real world will sag based on the IR of the battery cells and never actually get to 20hp. So, to get to 20hp, you actually need to figure out a way to flow quite a bit more current (without damaging the battery cells) OR you need to figure out a way to reduce the internal resistance of your battery pack(s) to limit the in-run voltage sag.
Now, at this point I would normally dive into the math and tell you exactly how to accomplish both of those goals, but out of respect for my team members and the guys who share their tricks with me, I will simply say that these are solvable problems with “current” technology (pun intended ) However, if you are not actively chasing a record, your time and money may be better spent making your rig more efficient.
I’ve written several articles about this, but just because your ESC is measuring a huge number like 20 Electrical Horsepower, it does not mean that your car is getting 20 HP to the ground. In fact, an average RC car is probably losing 35-50% of its electrical output to driveline drag and motor inefficiencies. Driveline loses are usually 10-20% depending on your bearings, gear meshes, and the quality of your lubricants. Motors are a bit more difficult though. “Good” motors typically convert 90 - 95% of the power they receive into “go”… within their designed output range. “Bad motors” may fall to 70-85% efficiency within their designed output range. However, brushless motors will often allow you to drive them to much higher output levels, but at the cost of efficiency. So if you pick a motor that is too small for what you are trying to do or over-gear a properly sized motor, you could actually be throwing away most of the electrical power in your system! This is because your motor just doesn’t have anything else to give and is just turning all that electrical power it is drawing straight into heat. So, a mighty 20 HP system could be reduced to 10-13 HP at the wheels (or less) if it was not built with efficiency in mind. By contrast, a well-tuned, efficient system might be 2-3X more efficient and deliver much better real-world performance.
And we haven’t even considered the effects of aerodynamic drag!
So a well-built 15 HP system might actually beat the pants off of a “more powerful” but poorly optimized setup. But what if I have a "decent" speed car and “just send it”? If you have a "decent" motor with proper gearing, it will probably all come down to your batteries.
“Unfortunately, there is no industry standard for C-Ratings based on IRs, so finding those cells or comparing batteries from different brands can be pretty pointless... unless you can find some actual test data. ”
Most battery makers (including my sponsor Venom Power) offer hi-end LiPos with IRs ranging from ~1-3milliOhms (0.001-0.003 Ohm) per cell. Unfortunately, there is no industry standard for C-Ratings based on IRs, so finding those cells or comparing batteries from different brands can be pretty pointless... unless you can find some actual test data. For example, I just tested my entire stable of batteries and discovered that some (but not all) of my Venom 50C 5,000mAh 3S and 4S LiPos have better IRs than other manufacturers' 100C (or higher) cells. They are all at least what you would expect from a "250A continuous" LiPo, but some can give a LOT more. It actually makes sense for high-volume manufacturers like Venom to do this because of their strong warranties. By charging a slight premium on each battery, these companies can deliver higher quality cells that will minimize real-world failures resulting in fewer warranty claims. At least, that's the idea.
So, if you limit yourself to .5V – 1V of sag per cell, these batteries can usually safely deliver 350-500A. Rearranging Ohms Law a little gives you I = V/R. If you are lucky enough to get a 1mOhm/cell LiPo set, it will deliver roughly 500A - 600A (at 21V-22V for 6S and 28V-29V for 8S). That’s almost 15 HP on 6S and around 19 HP on 8S without even working too hard. If your cells end up on the lower end of the high-performance spectrum (~3mOhms per cell), don’t get yourself too down. You should still be able to see roughly 300-350A of current and ~9-10 HP on 6S and ~11-12HP on 8S.
This sounds like a HUGE power difference to leave up to blind luck, but in the real-world, it’s usually careful tuning and technique that make the biggest performance differences. All things being equal, a 50% increase in wheel-power under the best conditions will only increase your top speed by ~15%. (Aero Drag is a funny thing.) But a bad bearing, a tight gear mesh, or an over-geared motor might make a “fast” car a LOT slower than it should be. Just remember that a skilled driver can often go faster with less power than a “better” car in the hands of someone with less skill.
I hope this was useful. :)
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