With the high performance applications that we are all after today, power consumption is about as great as it’s ever been in RC. Whether you are piloting an EDF jet, driving an RC Car or racing an RC fast electric boat a 100 amp power draw is not uncommon. With this much power being drawn, a single charge will be spent quite quickly. The easiest way to combat this is more batteries and faster methods to get them all fully charged. This is exactly where many use the concept of parallel charging for RC LiPo batteries.

The Benefits of Parallel Charging

The concept of parallel charging is not anything new, people have been charging this way for decades. However, using this charging method for LiPo’s has changed the way we have been charging in the past. We are now able to get many battery packs safely up to a fully charged state relatively quickly. The big impact that this charging method has is the ability to charge multiple packs within the same time frame as you may be able to charge one LiPo battery pack.

The Charger you will need for Parallel Charging

Depending on the type of charger you have will ultimately depend on your parallel charging capability. Not all chargers will be able to parallel charge multiple packs within the same time as charging a single pack. In addition to the charger you also must have a parallel charging adapter that will allow multiple battery connections.  Let’s look at an example to get a better understanding of the charging power requirements.

Parallel Charging Board consisting of balance taps and main charge lead

Parallel Charging Example

Let us assume that we have four 6s 5000 mAh battery packs and we are happy with charging this battery pack at a rate of 1C. Charging this battery pack at 1C will translate in to a charge rate of 5 amps. Check out the C rating page for more information on C ratings and how they are used.

A fully charged 6s battery will have a voltage of 25.2 volts. While charging this battery pack at 5 amps, the maximum charging load will equate to 126 watts. In other words, to charge only one 6s battery pack, your charger must be capable of charging at a rate of 126 watts. What happens when we wish to charge four battery packs in parallel? Well, we must multiply our 126 watts by 4, giving us a rand total of 504 watts. The multiplication of 4 is actually applied to the charging current of 5 amps. The result is 20 amps of current will be fed to the 4 batteries being charged in parallel. 20 Amps x 25.2 volts results in 504 watts of power.

Charging Specification Requirements

Your charger must be capable of 3 specifications to charge as above. Firstly, you must be able to charge a 6s LiPo battery. Secondly, you must be able to charge at a rate of 20 amps. Lastly, the charger will have to have the capability of charging at 504 watts total power. These specification are quite demanding and many chargers will not be able to handle the load. If you plan to parallel charge, grab a charger that has the specification you require.

For other charging specifications use the RC Charger Wattage Calculator.

The do NOT do’s of Parallel Charging

Charging batteries can be dangerous when there is an insufficient understanding and rules are not followed. Make sure that you avoid doing the following:

Do not charge LiPo’s in Parallel with varying charge levels

It is highly recommended to charge only battery packs in parallel that have reached the same level of discharge as a percentage. Do not parallel charge battery packs that are at a high degree of varying charge levels. For example, battery packs at a remaining capacity of 70% and 20% must not be charged together. The general rule that I follow and would recommend is charging battery packs that are within 30% or 0.1 volt. Anything outside of this is too much.

Do not charge LiPo’s with varying cell counts

When charging LiPo battery packs in parallel, you can not have LiPo’s of a different cell count. One rule you must follow with parallel charging is being certain all LiPo’s batteries to be charged are equal in cell count.

Do not force the Balance Adapter to charge the batteries

Balancing LiPo batteries happens through a balance tap that is a separate harness on all LiPo batteries. All LiPo batteries that are being charged in parallel must have the main power plug plugged in to the parallel charging adapter. Forgetting to plug the main power leads in to the charging adapter will cause the charging to occur in the balance leads. The balancing harness is not designed to handle much more than one amp of current. Forcing more current through the balance leads will cause them to over heat, leading to a dangerous situation. Be certain to always connect the main power leads to your parallel charging board first. Do not connect the balancing plug first.

Do not use resistance data when parallel charging

Battery resistance information will only display when you have a charger that is capable of determining its value. Charging LiPo’s in parallel will throw off the calculation that the charger performs for resistance. For example, two equal batteries have a resistance of 10 milliohms. Placing these two batteries in parallel will reduce the resistance that the charger sees to 5 milliohms. Additional packs added in parallel will further reduce the total resistance that the charger is capable of determining. Do not use the resistance value provided by your charger when parallel charging. Otherwise, you will be lead down the wrong path when evaluating your LiPo’s overall health.

Do not immediately begin charging your LiPo batteries

When charging your LiPo batteries in parallel, it is critical to not begin charging right away. The point of this is to allow your packs to balance out among all the battery packs that are connected in parallel. The time that you must wait until starting the charging process is relative to the difference between the least discharged pack to the most discharged battery pack. There are many devices that will allow you to plug the battery in to a voltage / capacity meter in order to determine the state of charge.

The chart below identifies the amount of time required to wait prior to charging the battery pack.

Time to Wait before Starting the Charger for Parallel Charging

How a Sensored vs Sensorless Brushless Motor Works

A brushless motor receives power from the electronic speed control. (ESC) The speed control must know the position of the rotor in order to accelerate the motor from zero RPM smoothly. When it does not know the position of the rotor, the ESC must determine it. The ESC is able to determine rotor position by sending power to the motor and receiving back EMF (Electromotive Force – this really just means a voltage “signal” is produced and sent back to the ESC) as the rotor is in motion. It is this feedback loop that allows the ESC to understand the position of the rotor.

If you believe the term for the synchronization process of a brushless motor is cogging, check out the article on cogging.

In a sensored brushless motor, as the name implies, the motor contains sensors within it. These sensors are responsible for sending information back to the ESC about the position of the rotor. When the ESC knows the position of the rotor, immediate synchronization is established right from zero RPM.

Sensored Brushless Motor Setup

Advantages of a Sensored Brushless Motor

Sensored motors are able to accelerate smoothly and strongly right from zero RPM. They also are able to produce more torque at slow speeds in comparison to a sensored brushless motor. Sensored motors also are able to allow safe operation of the motor by monitoring its condition. In some ESC’s, the ESC will not operate if there is something wrong in the sensor signals. Another piece of information that is captured from sensored brushless motors is the motor temperature. This is great for data logging motor temperatures.

Advantages of a Sensorless Brushless motor

There seems to be a lot of advantages for a sensored motor, but sensorless motors have their place as well. Although sensorless brushless motors do not perform well at low speed, their performance at high speed is excellent. It is so good that most ESC manufactures rely on sensorless synchronization of a sensored motor at high speed. At high speed most ESC’s will alter the timing for the sequence of power pulses that are sent to the motors windings. In turn improving efficiency and performance.

Sensorless motors also are lighter in weight, less complicated and less prone to failure. It is quite easy to damage the thin wire loom that sends all the data from the sensored motor to the ESC.

Applications of Sensored and Sensorless Brushless Motors

The number one reason why you may require a sensored motor for your application is due to requiring torque at extremely low speeds. We are talking around zero RPM. Applications where there is a requirement for torque at very low speeds are RC Car based. They include Rock Crawling, or any type of very slow speed RC Car action.

Also, cars with high gearing that require immediate torque can be quite problematic at low speeds. They may consist of high speed on road cars or high speed RC drag racing cars. The low gearing in these RC vehicles requires the motor to develop a lot of torque from a start. A request for the motor to accelerate quickly from a stop can cause a great amount of power to be drawn. When the brushless motor is not in sync with the ESC, the reliability of quick, smooth acceleration is compromised. A sensored motor in these applications would help.

When it comes to other applications, the requirement for a sensored motor is low. For example, RC helicopter, boats and airplanes of all types work very well with a sensorless motor. Most of the operation of these motors in these applications are done at higher speed. One of the most important parts is even at 0 RPM, there is a very low torque requirement in getting the system going. A propeller at slow speed does not load the motor significantly enough to warrant a sensored motor. Dropping the requirement of added sensors and wire looms to a motor in these applications saves precious weight and complication. Ultimately, this may lead to increased reliability.

Sensored / Sensorless Brushless Hybrid ESC’s

A perfect example of this system is in my 1/8 scale brushless buggy. I am using a castle system that is capable of both sensored operation and sensorless operation. What ends up happening is during acceleration from a stop, the motor/ESC combination is in sync utilizing the sensors inside of the sensored brushless motor. Once a small amount of speed is gained, the ESC switches from using the sensors to using sensorless synchronization. Here you gain all of the advantages as described above. It is an excellent solution if you are looking for smooth operation from zero RPM and excellent high speed performance.

Can a Sensored Motor work with a Sensorless ESC

Taking a sensored motor and using it with a sensorless ESC will work with no problem. The motor will simply act as a sensorless motor since the ESC will figure out rotor synchronization based on back EMF generated. In other words, a sensorless ESC will operate a sensored motor as a sensorless motor.

For obvious reasons you can not take a sensored ESC and use it with a sensorless motor. If the sensored ESC has the ability to operate as a sensorless ESC, the motor will also be treated as above – sensorless synchronization.

Brushless DC motors used in radio control applications contain windings just like every other electric motor. However, what is more unique about them verse an AC induction motor, is that they contain a permanent magnet. The permanent magnet in the brushless motor has the opportunity to interact with the make up of the winding section of the motor. This is where the cogging begins.

Cogging in Brushless Motors used in RC

What is Cogging in a Brushless Motor

As mentioned above, we know cogging of a brushless motor starts with the interaction of the permanent magnet on a brushless motor and the windings. Looking more specifically at the winding section of the motor, cogging occurs in a slotted stator. A slotted stator is made up of thin steel lamination’s that are stacked up and the motor windings are placed in the created slots. It is these iron slots that create the cogging torque on brushless motors.

The permanent magnet as it is rotated by the motor shaft, passes by these slots. The magnet then interacts with the steel lamination’s creating that bump that you know of.  If you don’t know what I am referring to here, you can grab any slotted brushless motor and try it out. Grab a brushless motor and slowly rotate the shaft. Depending on the motor, you will feel the strength of the cogging torque applied by the interaction of the iron core and the permanent magnets. I have noticed that 6 Pole inrunners tend to have the strongest cogging torque.

Does Cogging Exist in a Sensored Motor?

Sensored motors have been designed as a result of sensorless motors not being able to determine rotor position. It is critical for the ESC to know exactly where the rotor is relative to the windings to accelerate the motor from a stop. As a result of a sensorless motor, the motor stutters on takeoff. It does seem that most would contribute this phenomenon to cogging. In fact this is not the definition of cogging. We described cogging as the result of the interaction between the slotted stator and permanent magnets in a motor. A sensored motor only adds a sensor to aid in determining the rotors position. Therefore cogging does in fact exist in a brushless sensored motor.

How Does Cogging Torque Effect Performance?

The only area in our RC applications where cogging torque can effect performance is in the startup from zero RPM. This is more than likely where confusion of the term cogging all began. In a sensorless application the ESC must overcome the additional torque caused from the magnetic field interacting with the stator slots.  The ESC must do this as it is figuring out the position of the rotor. As the ESC sends a pulse to the windings of the motor and moves the rotor as a result, the cogging torque applied changes the position of the rotor slightly. This slight positional change is something that the ESC must overcome and figure out. Most ESC’s have no problem overcoming the effect of cogging torque applied. Once the motor is rotating, cogging does not effect the performance any longer.

Can I see Cogging Torque acting on my RC

Yes, you can. If you have an RC car that is geared very high, it will be easier to pick out. (On Road Application) The easiest way that you would be able to see cogging in action is to get your RC car rolling under its own power and then let off the throttle completely. As the car begins to slow to a crawl you can hear cogging in action. You will only be able to hear it just before the car comes to a stop.

Another way to hear cogging is to push your RC car very slowly along the ground. The car should sound very smooth as you push it across the ground. However, where there is cogging present, you will hear rattling and backlash of the gears as positive and negative torque values are applied to the motor shaft as a result of cogging.

LiPo batteries is one area of the RC hobby that can be fairly expensive. Especially if all the RC vehicles that you prefer to run, uses a different pack. The typical lifespan that we can expect from a LiPo battery pack can on average about 2 to 3 years.

For certain RC applications, the LiPo battery lifespan may very well be outside of this range. For example RC EDF jets or RC boats may have a shorter lifespan as these vehicles draw a lot of power from the pack. On the other hand trainer style RC airplanes, or scale RC boats pull significantly less power. These packs may last many years. For these reasons specifically, it is very important for us to maximize the battery lifespan so that we don’t need to be purchasing new batteries every year or two. That can be very expensive! Prolonging the life of your LiPo battery pack is not hard, follow these 5 ways:

Maximum Discharge Rate – LiPo Battery Lifespan

LiPo battery packs have a rating that allows one to understand the maximum amount of current the pack can discharge continuously. This is one of the most important specifications on a battery pack that you must consider for your application. The specification that we are referring to is known as the C rating. The C rating of a battery pack is explained further in this article.

In order to maximize the life of your battery, it is  important to be certain that you have a lot of head room in the C rating of your pack. My recommendation is to only draw 50% of the maximum continuous power output that your battery specifications specify. If you have the ability to reduce the percentage, indeed do so! The smaller the load factor of the battery pack, the more that you are able to prolong the life of the battery pack.

Maximum Discharge or Run Time – LiPo Battery Lifespan

Maximum discharge refers to the maximum amount of capacity that you should pull from your LiPo battery pack. The magic rule that helps prolong the lifespan of a LiPo is 80/20. The meaning behind this rule is that the maximum discharged capacity should be no more than 80%. This leaves a minimum capacity in the pack of 20%. Following this rule will help increase the lifespan of your battery pack.

Be certain that you know the maximum run time that you have available in your RC to not discharge more than 80% of the capacity. The resting voltage of a battery pack that doesn’t exceed 80% discharge is approximately 3.7v.

Storage of a LiPo Battery – LiPo Battery Lifespan

Storing a LiPo battery pack is defined as not using the battery pack for more than 10 days. If you don’t plan to use the battery pack in the next 10 days, it is highly recommended to place the battery pack in to storage mode. What does this mean? Well let’s go through it. To maximize the lifespan of the battery pack, it is critically important to store the pack correctly.

To store a LiPo battery pack correctly, many chargers on the market can help with this. However, if your charger does not have a charging mode specific to storing the pack, you can do this manually.  The voltage of the battery pack is a good indicator for an optimal storage specification. The recommended storage voltage for a LiPo battery pack is 3.80 to 3.85 volts. Battery capacity will be approximately 40-45% at the correct storage voltage.

Leaving your battery pack at maximum charge 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.

Maximum LiPo Battery Temperature – LiPo Battery Lifespan

This factor that effects LiPo longevity is fairly straight forward. It doesn’t matter if you are or are not following any of the above tips if your LiPo is getting too hot. Like all electronic components, LiPo battery packs are no exemption. The amount of heat that your LiPo generates while being discharged is directly proportional to the lifespan of the pack. A pack that constantly gets hot will not last as long as a pack that stays cooler.

The maximum recommended temperature for a LiPo battery pack while in use is 60C or 140F. This is an absolute maximum. You can be certain that if your LiPo is achieving this temperature every run that you are degrading the lifespan. To maximize lifespan, decrease the amount of heat that your battery pack builds. You can do this by reducing the load of the battery pack. To reduce load, increase the amount of LiPo battery packs being used by wiring them in parallel. Or decrease the amount of load on your battery by reducing prop size or gearing.

Battery Charge Rate / Peak Voltage – LiPo Battery Lifespan

Battery packs being sold on the market today are advertising 5C and upwards of even 10+C charge rates! These are crazy! I believe anything more than 3C is crazy. 3C charging is very fast. What I have found is the most optimal battery charge rate when considering both quick charge and lifespan is 2C. Within about 30 minutes you can have a pack fully charged and you are not pushing the pack. You can be more conservative towards battery life by charging at a 1C charge rate.

The maximum specified fully charged voltage of a LiPo battery pack is 4.20 volts. Charging to this voltage actually degrades the lifespan of the LiPo battery. If you are to charge the battery pack up to a maximum below the 4.20v mark, you can increase the lifespan of the pack. Tesla actually uses this method to increase the lifespan of their lithium based batteries. What I do is take the battery pack off the charger as I see the pack starting to ramp down in current closer to end of charge. If your charger allows, you can also set the maximum peak voltage of a LiPo. Setting the maximum charge voltage to something less than 4.20 volts will prolong the lifespan. Setting anywhere from 4.10 to 4.18v will help, and obviously the lower you go, the longer the lifespan potential you will see.

Unfortunately everything in life is a trade off. Trading run time for battery life is one thing that we can do to increase the lifespan of our batteries. Ultimately, you get to decide if this is worth it.