Here is why you need to Buy the Highest C rating RC LiPo

It is fairly common that a new hobbyist to RC may think too high of a C rating would be bad for their RC. In fact the higher the C rating, the better off your entire system will be. Here is a quick review of how we use the C rating to calculate the maximum continuous discharge current the battery can output. C ratings can come in the form of either continuous or peak ratings. Be certain that you are using the continuous ratings.

Significant Voltage Drop Under Moderate Load. Buying the Highest C rating Batteries would greatly reduce this.
Significant Voltage Drop Under Moderate Load

Continuous Discharge Current = Continuous C rating x Battery capacity (Ah)
Discharge Current = 35C x 4.5Ah
Discharge Current = 157.5A

There are three primary reasons that you will want to get the highest C rating possible for your RC, let’s go through them.

Get the Highest Performance Possible

When a LiPo battery is under load a voltage drop occurs. It is this voltage drop that can hurt performance. If your power system is experiencing a significant voltage drop under load, the motor will not be able to hit maximum RPM. Lost RPM is exactly where we lose performance. Your RC vehicle will have a reduced top speed. However, batteries with a higher C rating will be able to maintain voltage under load. This will mean your power system will be able to produce more power output in watts leading to a higher top speed.

Another area where you will see the greatest benefit is under heavy acceleration. A higher C rated battery will be able to sustain higher voltages but also at higher current output. This will allow an RC car to accelerate out of a corner more aggressively, or for an RC boat to get out of the hole faster.

For these reasons, it is a great idea to select a pack with the highest C ratings that you can.

Reduce the Temperatures on your LiPo Battery

Temperatures effect all electrical components and it is in our best interest to keep them as cool as possible. For a LiPo battery, temperatures below 140F are acceptable. Above 140F should not be acceptable and will cause permanent damage. To maximize the lifespan of the battery reducing temps much below 140F will help.

Moving up to the highest C rating that you can will reduce the temperatures of the LiPo battery. This is due to the fact that the battery will be able to sustain a higher amount of output. For example, consider a load that you need to satisfy at 70A. Two battery options for simplicity allow for a 100A and 200A maximum continuous draw. The 100A pack would be loaded at 70% of its maximum capability. The second battery pack would be loaded at 35% of its maximum capability. The reduced amount of stress on the second battery is going to result in lower temperatures and longer life.

Reduce Ripple Voltage within Your ESC

Ripple voltage is described in detail within this article. A higher C rating will be able to reduce the amount of ripple voltage that the ESC will see. This is due to the fact that the voltage under load would be reduced. Furthermore, if an adequate C rated battery is not selected for the power system, ripple voltage can be excessively high. If the amount of ripple voltage present in the ESC is too high, the capacitors on the ESC can fail. Once this happens the ESC will ultimately fail.

Ripple Voltage vs Current in Green. Going with higher C rated batteries would improve the very concerning issue.
Very Concerning Ripple Voltage vs Current (Drawn in Green)

It’s quite obvious that we would want to avoid this scenario at all cost. To reduce the ripple voltage within the ESC and maximize reliability of the ESC, select the highest C rated batteries that you can.

In the image above, ripple voltage is produced by a 4s 4000mAh 35C battery. The current draw is around 80A where the battery is rated for 140A. Ripple voltage is over 10% of the batteries nominal voltage which is very concerning. This must be addressed before it takes out the ESC. This LiPo battery is no longer performing at 35C due to many possibilities. Read more about LiPo batteries and what ages them here.

What can Limit C rating?

Size of the LiPo Battery

A major factor that can limit the C rating that you can use in your RC, is the size of the battery pack. As you increase the C rating of an RC LiPo battery, the physical size tends to increase. In addition to the increase in size is also the increase in overall weight. I have a battery pack that has one of the highest, accurate C ratings, that you can buy. However, it is far to heavy to place in to the RC plane that I bought it for. As a result, I have decided to use another pack instead. The new pack is lighter, improving the low speed flight characteristics of the plane. **A lighter plane will allow a less stressful landing for the pilot.** The pack is also smaller allowing an easier fitment upon strapping the pack within the planes battery bay.

Cost of the Higher C rating

Another factor that comes in to play is of course the almighty dollar. (Or which ever currency you have in your country – we use the dollar in Canada) Higher C rating batteries tend to get very pricey very quickly. Biggest thing here is that your budget should be met. If you can’t grab the 65C LiPo battery but the 45C fits within budget and your RC’s requirement, then buy it!

LiPo Battery Pack vs Pack Age

Since LiPo battery packs have been used in RC vehicles, performance levels have jumped up significantly. LiPo batteries have a lot of advantages such as being light weight, having high power output, high charge rates, low internal resistance and more. However, there are some disadvantages too. In particular LiPo batteries only perform their best for a short period of time relatively speaking.

LiPo Battery Pack vs Age
A LiPo Battery Pack Degrades in Age

How do LiPo Batteries Age?

Age of Battery Pack

LiPo batteries age for a number of reasons. One of the most obvious reasons a LiPo battery will age is due to time. Typically you can expect a LiPo battery to perform its best for the first year of use. The battery has an average lifespan of about 3 years. At the end of the 3 year mark, it doesn’t mean that the LiPo is completely useless, but it does mean that the performance of the battery is not going to be the same as when it was new.

Cycle Count

Another reason a LiPo battery pack ages is due to cycle count. The more times you charge and discharge a battery pack, the more wear that battery will experience. A cycle is considered a cycle when ever you are discharging and charging the battery. It doesn’t matter if you are only charging the battery pack 20% from 80% charged. Expect your high performance LiPo battery pack to provide about 500 cycles on average. After this point, performance drops off.

Battery Maintenance and Care

Here are a bunch of points that can help you avoid aging your battery quicker.

Maximum Discharge or Run Time

Using 100% of the packs capacity will significantly degrade the lifespan. What you want to do instead is time your run so that you end up with at least 20% of the batteries capacity remaining. Doing so will allow you to reach maximum lifespan of the battery pack.

Maximum Discharge Rate

Your batteries will get very hot if you plan to discharge the packs at the exact specification they are rated at. For example drawing 100A from a battery that can support a 100A load. What you must do instead is give yourself at least a minimum 50% headroom to a recommended 100%. This would be a battery that can support a minimum 150A or 200A recommended. Moving up to a pack that can support additional load will decrease the temperatures and increase lifespan.

Storing a LiPo Battery

When there is no plan to use your LiPo batteries for than 10 days, it is recommended to store them correctly. Not placing your batteries in to storage mode will significantly impact the lifespan of the packs.

The recommended storage voltage for a LiPo battery pack is 3.80 to 3.85 volts. This voltage is a resting voltage. Battery capacity will be approximately 40-45% at the correct storage voltage. Many chargers on the market are able to automatically bring the LiPo batteries to this storage voltage. If not, you will have to do this manually.

Leaving your battery pack at 100% charged is one of the worst things you can do to the battery when you are not using it. At the same time leaving the battery near empty is also far less than ideal. Be certain to leave your battery pack at the storage voltage specification to maximize your LiPo battery lifespan.

Operating Temperatures

Temperature in any electronic component is important. Operating a LiPo battery pack outside 140F or 60C will decrease the lifespan. Make certain to stay under these specifications for maximum reliability.

Summary and Conclusion

The 3 main issues we can face for battery packs aging is:

  1. Age of Battery pack in years. Typical lifespan of a High Performance LiPo battery is 3 years.
  2. Cycle Counts. LiPo Batteries have an average cycle count of about 500.
  3. Battery Pack Maintenance and Care

There’s not much that we can do about the age of the LiPo. Try to maximize the total amount of RC vehicles that you can use with that particular LiPo battery and only buy the pack when you plan to run the vehicle.

As for cycle counts, again not much at all that we can do here. We don’t want to let the LiPo just sit around unused. Throw them in your RC and don’t worry about cycle counts.

The most important item here is maintenance and care for your packs. Make certain that you are following the proper guidelines to maximize the life of your battery.

What is the Purpose of a Slipper Clutch on an RC Car

A slipper clutch is a mechanical assembly that transfers excess energy in to heat. The heat created is from friction between clutch pads and a clutch hub. Slipper clutches are not found on all RC cars. In fact almost all RC on road cars will not have one. Common types of RC cars to find these on include Rock Crawlers, Off road buggies, monster trucks and a few more. Correctly tuning your clutch for the conditions that you run in is important for the longevity of the clutch.

Slipper Clutch Exploded View
Slipper Clutch Exploded View

How Does the Slipper Clutch Work

A slipper style clutch is typically placed on the spur gear that meshes with the pinion gear found on the motor. It utilizes what are known as slipper pads to act as braking material for the clutch. These pads are forced under pressure by an adjustable nut and spring to maintain pressure against the slipper hub. The slipper hub would serve the same purpose that a disc/rotor would on a brake assembly. The slipper hub is typically locked on to either the spur gear or the shaft on the input side of the transmission. In the image above, it is located on the far right hand side. The clutch pads on the clutch above, are mounted directly to the spur gear. It is these few parts that allow the slipper clutch to function.

The clutch is loaded with torque from the motors pinion gear. The spur gear is forced to rotate with the slipper hub. If the torque is too large for the setting of the slipper clutch, the clutch will begin to slip. The slippage occurs between the slipper hub and the slipper pads. Over time, the slipper pads eventually will wear out requiring replacement.

The Goal of the Slipper Clutch

The goal of the slipper is as mentioned above, to turn mechanical energy in to heat energy. It would not be beneficial to turn all energy in to heat energy, however only when there is a “shocking” amount. Yep, I just did that. Any type of driveline shock can be damaging to the weakest link within the driveline. The slipper clutch is put in place to help dampen these shocks. Imagine sending your RC off of a large jump. Then you need to hit the throttle to rotate the car around to land flat. But as you are on the throttle the wheels touch the ground and load the entire driveline. It is this exact scenario or similar that leads to a spike in driveline load. All the inertia built up in rotating the wheels now comes to a halt when the tires hook up on the ground. If the RC that was jumped used a slipper, the clutch could slip at this exact time and save the rest of the driveline from any spikes in torque.

How to Detect Slippage

When the slipper clutch is slipping, it makes a higher pitched whining noise. It is this noise that can help you to determine if your slipper clutch is set correctly.

A slipper clutch that is set too tight will never function as intended and allow all forms of energy to be absorbed by the driveline. Too much driveline shock can be damaging leading to failures.

If the slipper clutch is set too loose, excessive amounts of heat can damage the slipper quickly leading to burnout.

Tuning the Slipper Clutch

Tuning your slipper is actually quite easy. To get started review the manual for your specific RC car. In your manual the manufacture will provide the procedure to set your clutch to the factory recommended settings. If your clutch is too loose and causing excessive slippage, tighten the slipper in steps of 1/8 turn. If your slipper is not slipping at all, loosen your clutch in steps of 1/8th of a turn at a time.

Conditions that may require adjustment

Running your RC car in extremely loose surfaces will require a more loose slipper setting. Opposite to this, running your RC in extremely grippy surfaces will require a tighter slipper setting.

Using your RC in an environment that causes the drive train to load up will result in requiring a tighter slipper tune. The perfect example here is if you are trying to crawl up a rock fairly slowly. The driveline can spike load if a tire were to get wedged stuck on something.

Upgrading your power system to provide more output power will require a tighter clutch setting where the opposite is true for having less power.

Note that it can be more problematic for your clutch when it is excessively slipping. Excessive slippage can cause the clutch to overheat and fail prematurely.

Three Misconceptions in RC – Brushless Motors & more

A Lower (vs higher) Kv Motor Produces More Torque

It is a common misconception that a lower Kv motor automatically produces more torque then a higher Kv motor in the same size class. What we do know is that the Kv constant of the motor is directly related to the torque constant of the motor, Kt. The relationship of Kt is the inverse of Kv. The lower a motors Kv value, the higher its Kt value will be. The torque constant of the motor tells us the amount of torque output per amp that we can get out of the motor.

Lower Kv motors do have a higher torque constant. However, by nature they are not able to sustain the same amount of current output as a higher Kv motor. Therefore a higher Kv motor can actually develop the exact same amount of torque as a lower Kv motor just by pulling more current.

Let’s take a look at an example.

Motor A – 1200Kv, 60A Max Current, Kt = 7.974 mNm/A
Motor B – 1650Kv, 83A Max Current, Kt = 5.799 mNm/A

The Maximum amount of torque output for motor A will be the Kt value multiplied by the max current. Motor A will be able to deliver 60A x 7.974 mNm/A = 478 mNm. Maximum torque output for Motor B is 83A x 5.799 mNm/A = 481 mNm. The difference of 3 mNm is negligible and discovered through rounding error.

Other Considerations when using a lower Kv motor in place of a higher Kv motor

If you are comparing a high vs low Kv motor operating at the same input voltage, the lower Kv motor will have a harder time making the same power output. This is due to the maximum current being lower on the lower Kv motor. The way that you would be able to make up for the loss in max current is to increase the voltage.

Cogging in a Brushless Motor

Cogging of a brushless motor is commonly regarded as the relationship of the motor and ESC being out of sync when starting from zero RPM. However, this is not the definition of cogging.

Cogging is a result of the iron core in a brushless motor reacting with the permanent magnet within the motor. As you rotate a motors output shaft, you can feel the “steps” of the motor through one full revolution. These steps require a certain amount of torque to overcome the resistance. Once the resistance is overcome, the rotor begins to accelerate as the torque requirement swings in the opposite direction. This creates an unbalance of torque at low speeds. Luckily for us, we can not feel the unbalance or even really notice is when operating our RC vehicles at low speeds. The only disadvantage of cogging is that it does tend to make the initial ESC to motor synchronization process a bit more difficult. To eliminate cogging, one can simply use a slotless rotor. A slotless rotor does not utilize an iron core. Visit this link to learn more on how to improve cogging.

Voltage Destroys Electrical Components

Voltage does indeed destroy electrical components. This is definitely not the misconception here. If you simply have a 6 cell LiPo ESC and run 7 cells to it, surely it will have smoke pouring out of it in no time. This misconception comes from the idea of adding a cell to a power system and having a speed control or motor blow, thinking it is directly related to the voltage increase.

Consider an example where we use the same 6 cell ESC as above and we go from using 5s LiPo to 6s LiPo. We then discover that our motor burned out. What caused the issue? Well when adding a cell extra of voltage, we are expecting our motor to turn quicker as RPM output is equal to Kv x voltage. Since our motor has the potential to spin faster and our gearing nor other RC car specs has not changed, load has increased. We know load must increase as it will take more torque for the motor to maintain a higher output speed and top speed of the car. Torque is directly related to the amount of current that our motor will pull. Increasing the voltage of the motor without decreasing the load or gearing of the RC car, will increase current. Since current is exactly what contributes to an increase in heat, this is how our motor will overheat and burn out.

Surely, Voltage Will Contribute to Motor Burnout?

When you look at why a motor would fail electrically, it really only is because of current. Not voltage. Now, a motor can fail mechanically. A mechanical motor failure can occur in the following areas. The first being the bearings that support the shaft of the motor. Mechanically over spinning the bearings can lead to bearing failure. At the same time mechanically over spinning the rotor of the motor can lead to a very similar failure. In fact a rotor failure is usually catastrophic. All motors have a rated maximum RPM and that is based on the limiting factor between either the rotor and the bearings.

The maximum RPM can be broken down in to Kv and Voltage. If you increase the voltage high enough, you will over spin the motor causing mechanical damage. The formula to determine the maximum voltage of a motor is:
Vmax = Max Motor RPM / Kv

Cavitation in RC Boat Propellers

Woops, yep, this is a fourth. We will call this a bonus!

I hear this misconception all the time not for just RC boats but even for full scale boats as well. Cavitation is the general term used to describe slippage that occurs when the prop sucks in a bunch of air causing the engine or motor to unload. However, this is not what cavitation actually represents.

Cavitation occurs when a rapid change of pressure in a liquid leads to formation of small vapor pockets in places where the pressure is relatively low. When subjected to higher pressure, the vapor pockets collapse and can generate a large shockwave. This shockwave can lead to pits found on the surface of the propeller ultimately damaging the propeller.

Now the way to reduce cavitation is by simply introducing aeration. Aeration is the process where you introduce air in to the propeller as defined in the actual misconceptions definition above. Introduction of air in to the propeller will simply not allow low pressures to build eliminating the potential for cavitation to occur.

This means that in our RC boats utilizing a surface driven prop, we will never really see the potential for cavitation to occur. Our more significant problem is we get too much aeration and are unable to get the boat on to plane quickly. The simple solution here is to move up to a larger propeller. Just make sure the larger propeller does not place too much stress on to your power system.

 

Ripple Voltage in an ESC Explained and How to Improve it

The ESC in a radio controlled vehicle is responsible for delivering power from the battery pack to the motor. Input power to the ESC is in the form of direct current. Output power is in the form of alternating current that is in perfect synchronization with the motor. The complex process of converting DC to AC power at the time a motor coil requires it, while managing motor speed is all handled by the ESC. However, the ESC is not perfect and in some cases requires help. Ripple voltage is one area that can get out of hand within your ESC. In this article we will understand what ripple voltage is, what would be an acceptable amount of ripple voltage and how it can be improved.

What is Ripple Voltage?

Ripple voltage is best defined as varying voltage at the source of power. Variation in voltage is measured at the battery input side of the ESC. It is possible to log ripple voltage within the ESC, however, not all ESC’s have this data logging capability.

You may have noticed that on the input side of the ESC are large capacitors. It is the job of these capacitors to smooth out any ripple voltage at the ESC. If the capacitors are unable to smooth out variations in voltage, the variation in voltage is felt by components further down in the circuit. FET’s also known as Field Effect Transistors are the components sensitive to ripple voltage. If it gets out of hand, these will burn up and render the ESC completely useless.

What Creates Ripple Voltage

During partial throttle usage, the ESC switches on and off the power that is sent to the motor. As the ESC goes through this cycle battery power is pulled at the same rate. The rate that the ESC is switching power on and off during partial throttle is the PWM rate. In many ESC’s, this value is programmable. The standard value or typical value that you would see is 12,000 Hz.

When the battery pack is loaded with an on cycle of the ESC, voltage sags. The amount of voltage sag that the ESC experiences depends on a few factors. One of those factors is the batteries ability to dump power quickly. As this process repeats itself multiple times per second the voltage bounces between the unloaded voltage and loaded voltage. In conclusion, it is this exact difference in voltage that is known as ripple voltage.

What is an Acceptable amount of Ripple Voltage

Now on to the important stuff. An acceptable amount of ripple voltage is less than 5-6% of the battery packs nominal voltage. Around 7-9% is OK. Around 10% is where you can get in to some permanent ESC damage which ultimately means that the lifespan of the ESC is reduced. Above 10% and your ESC is at risk. To determine the ripple voltage review a data log of your ESC while recording ripple voltage. Determine where the peaks are occurring within your run. Take the highest ripple voltage value that occurs as a peak within your run.

To calculate ripple voltage as a percentage, divide the ripple voltage by the nominal battery voltage (# cells x 3.7v/cell) and then multiple by 100.

Example:
Ripple Voltage – 1.27v / 6s Nominal 22.2v % Ripple Voltage = ( 1.27 / 22.2 ) x 100% = 5.72% This would fall in the category of good and acceptable

How to Improve or Reduce Ripple Voltage

There are a few different ways to improve ripple voltage. Let’s take a look at them.

The first area of concern is to check if the ESC to Battery leads have been extended. Having extended leads can “lead” to higher ripple voltage created within the ESC. It is important to not extend the leads of the ESC without compensating for this.

The second way to reduce ripple voltage is to add a capacitor bank. A capacitor bank is able to compensate for extending the leads on the battery side of an ESC. The capacitor bank AKA cap pack is also able to help for those beast of a power system. Cap packs are highly recommended for high powered EDF jets, high powered boats and speed run cars. You can read more on capacitor packs by visiting the Why Use a Cap Pack on an RC Car page.

One of the easiest ways to improve ripple voltage is if you are using undersized battery connectors. Check the rating of the connectors and make certain that the connectors are rated for the amount of power you plan to draw.

Lastly, using the highest rated C rating and battery pack capacity as possible will make significant improvements. The improvement comes from the batteries ability to dump more current. (amps) Under load a battery that can deliver more power will have less voltage sag. With less voltage sag there will be less ripple voltage.

Conclusion

The ESC is a very complex component found within our RC power system. However, when driven too hard or over any of its limits, the ESC can respond by turning in to a bunch of smoke. To avoid this it is highly recommended to monitor by using data logs the performance of your ESC. If you notice concerning data it is best to take action using the methods described above. Therefore, doing so will maintain maximum reliability within your power system.