Selecting the correct LiPo battery pack for your RC vehicle is not as easy as just selecting the cell count. One of the most important parts of the correct battery pack selection is also considering the C rating. What is the C rating? Well let’s take a look!

What is the C Rating?

All LiPo battery packs have a C rating associated with them. Actually, they have not just one C rating, but a few C ratings that we will discuss shortly. A C rating is a value given to the LiPo battery pack that refers to the maximum discharge rate of the LiPo.  Examples of C ratings could be anything between 30C to 65C. Numbers outside of this range are possible as well. LiPo’s typically have the C rating marked right on the front of the LiPo pack. Can you spot the rating on these packs?

2X 4s 4000mAh with a C Rating of 45C

How to calculate Max Discharge using the  C Rating?

The maximum discharge rate of the LiPo is highly dependent on the capacity of the battery pack. In general the larger the battery pack in terms of capacity measured in mAh, the higher theoretical maximum continuous discharge rate you should expect. With this being said, the capacity of the battery pack is part of the calculation. We first take the capacity of the battery pack in mAh and convert this to Ah. Next we use the maximum continuous discharge rating in order to perform the calculation. Take these two values and multiply them together.

If we look at the image above consisting of a 4000mAh 4s pack, it has a C rating of 45C. Firstly, 4000mAh is converted to 4Ah. We then take 4Ah and multiply it by 45C. The resulting value is 180 Amps.

4Ah x 45C = 180Amps

Our Turnigy Graphene pack is capable of providing 180A continuously according to the rating provided by the manufacture.

Check out this page to help calculate battery pack specifications.

Other C Ratings of a LiPo Battery Pack

There are other C ratings that can be found on LiPo battery packs that aren’t typically spoken about. The second C rating that is also quite important, is the rating provided for charging a LiPo battery. This C rating is specifically for the rate at which you can charge the battery pack at.

Charge Rate C rating

There are many chargers on the market these days that are able to charge at a high charge rate, significantly decreasing the amount of charge time. In order to be certain you can charge your pack quickly, you must verify the charge rate using the charge rate C rating.  When LiPo’s first came out, typical charge rates were known as 1C. Typical C ratings for charging in today’s day is anywhere from 2C to 15C where 15C charge rates are crazy!

The Turnigy Graphene pack in the above example has a charge rate of 10C. To determine the maximum charge rate, we first convert the pack capacity to Ah from mAh. 4000mAh is equal to 4Ah. Next we multiply this value by the charging C rating. 10C x 4Ah = 40 Amps maximum charge rate.  Keep in mind that this is an extremely high charge rate, in fact I typically do not charge any faster than 3C. An absolute personal maximum is 5C.

Peak Discharge C rating

The last and final C rating that you may come across when looking at LiPo battery packs, is the C rating for maximum peak discharge rate. The peak rating may or may not appear on the battery advertised. Typical peak ratings could be anywhere between 50 to 100% more than the rating for maximum continuous discharge.

What C rating is required for my Application?

The correct answer here really boils down to budgets, battery pack weights/sizing and required discharge rate. Having the battery pack with the highest C rating is always best for the power system and battery pack health.

It is best to have a discharge rate overhead of 30%. If you work out a maximum power system discharge of 100 amps. Your battery pack should should deliver at least 30% more or 130 total amps. Never match system draw to maximum continuous discharge rates of the battery pack.

2s 860mAh with a C Rating of 35C

For example, I would select this 860mAh at 35C for a load that will discharge at a maximum continuous current of 23 Amps.

35C x 0.86Ah = 30 Amps
30 Amps / ( 1 + 30%) = 30 A / 1.3
23 Amps

Having a battery pack that can deliver 1000 Amps for a motor that will only use 10 Amps may seem like overkill, and it probably is. However, there is no reason for concern that the battery pack could over power the motor. Keep in mind that a load only pulls the amount of current that it requires.

In conclusion, the best C rating for your pack is a value that will allow 30% overhead in discharge rate, fits your budget, and is the correct size and weight for your application.

It’s known that like most things in life, LiPo batteries do not last forever. Eventually, LiPo batteries degrade to a point which makes them become unusable for your application.  The real question is, how do you know when an RC LiPo Battery is DEAD? Did you know that using a dead battery can actually be dangerous and could lead to catastrophic failure of the battery pack? A dead battery pack can also place extra strain on your ESC due to higher ripple voltages. If you are looking for more information on LiPo batteries it is recommended to check our LiPo batteries page.

Visual Inspection – When a LiPo Battery is DEAD

The most obvious way to determine if a LiPo battery is dead is to visually inspect it. What you are looking for when you are inspecting your LiPo battery pack is any sort of ballooning of the pack itself. A ballooning pack is caused when gases are released inside of the LiPo battery. Gases being released from a cell in a LiPo battery can happen for a number of reasons.

Generally, excessive heat is a contributor to packs that enlarge or balloon. If this happens to you during charging you must dispose of the pack immediately. You should dispose of the pack if this does happen at any time during use or no use. Ballooned LiPo battery packs can be very dangerous. 

How do you know a LiPo Battery is Dead?

Performance Test – When a LiPo Battery is DEAD

One of the easiest ways to determine if a LiPo battery pack is dead, is to review it’s performance characteristics.  This can be done by  placing the LiPo battery in the application that you intent to run the battery pack in. There are a few things to watch out for that would suggest your battery is around or past is expiry date. Let’s take a look.

Aggressive Run – Dead LiPo Test

The performance test  is quite simple and does not require much preparation other than a fully charged battery pack. What you will want to do is fully charge the battery and then place this in to your most power hungry vehicle that would of course use that battery pack.  You must be experienced running this vehicle to ensure that there are no power system concerns and everything has ran well before.

Run the battery pack more aggressively than you typically would. If your flying an airplane or driving a boat, maintain a higher average throttle setting than you typically would. If you are running an RC car accelerate more aggressively and frequently than you typically would. Keep in mind that if you are basing your run off a timer and you are driving more aggressively, you will burn more power. Don’t forget to consider this when setting your run time “timer.”

Only discharge the pack down to a maximum of 20%. Do not over discharge the pack as this won’t help or make your test any more conservative.

Once you have run your RC vehicle, these three items is what you should look for.

1. Are you getting the expected performance out of your battery for the entire duration of your run?
2. During the duration of the run, were there no low voltage cutoffs during the run?
3. Is your battery operating at less than 60 degrees celcius or 140 degrees farenheit? (Dangerous if not)

If you have answered yes to these questions above, your pack is in fair condition. If you have answered no to any of the above questions, this could be a sign that your battery pack is in poor condition.

Looking at the Numbers – When a LiPo Battery is DEAD

If you have a more advanced charger, you may be able to use the information it offers to help you out. Take a look at your LiPo charger and see if it is possible for it to measure internal resistance often abbreviated IR. The unit of measurement is commonly in milli-ohms. (m Ω)

To use the resistance value determined by our charger, we first need to start with a discharged battery and use the charge function. Or you may start with a fully charged battery and use the discharge function. Discharging a pack to get a resistance reading is only recommended if your charger can discharge a significant amount of power.

It’s highly recommended to use the resistance determined by the charger during a charge cycle as you will be charging the battery regularly presumably. It’s also best to know the internal resistance of your packs cells when you purchased them new.  Determine the internal resistance  by charging your LiPo batteries and recording the internal resistance computed by your charger.

Measuring RC LiPo cell internal resistance (IR)

Once you have charged your pack and record the internal resistance you can determine the health of the cells in your LiPo battery pack. Generally speaking, if the resistance goes up by 2-3 times the original amount, your LiPo cells are suffering in performance. Depending on the application these packs are used in, 2-3 times the internal resistance may make them completely useless.

If you don’t know the internal resistance per cell of your LiPo pack when purchased new, here is a quick reference chart. Keep in mind these values are general values.

Conclusion – When a LiPo Battery is DEAD

The method I typically use is actually a combination of the above.  If I’m happy using a half dead LiPo in an application that allows it, even with the weaker performance / capacity, then so be it.  As long as the LiPo is not in a dangerous state.When I can no longer use the LiPo battery pack, it is disposed of accordingly and a replacement is purchased.

There are many options that we have for RC brushless motors regardless of the application. Motor come in many sizes to fit certain power ranges. One of the biggest factors that could effect your brushless motor purchase, is a brushless inrunner option or a brushless outrunner. Which motor option would you choose, inrunner or outrunner?

Key Differences – Brushless Inrunner vs Outrunner Motor

Take a look at the image below. You can see that the brushless outrunner motor has the output shaft, connected to a propeller in this case attached to the case of the motor. This would suggest that the motor shaft when spun would also spin the outer motor case.   This is exactly what happens. The permanent magnets on the outrunner are placed on the rotor and the rotor spins on the outside case. On the inside of the motor are the stator windings which do not rotate, they are fixed in position.

Brushless Outrunner Motor vs Inrunner Motor. Outrunner on right side

On the inrunner motor, you essentially have the complete opposite true for how it is built. On the outer side of the motor is the case. The case in this situation does not rotate and is fixed. The stator windings are placed on the inside face of the case. When you spin the motor shaft of an inrunner, you are spinning the rotor which also contains the permanent magnets much like the outrunner. The difference of course being that they are now at the center of the motor.  For most, this would be the more conventional type of electric motor, especially if you are familiar with large AC motors or even old brushed DC motors.

Performance Differences – Brushless Inrunner vs Outrunner Motor

This can be easily debated as to which motor has the best performance when you dive deep in to the specifics. For simplicity let’s loosely consider motors of equal size and weight in order to compare the possible performance differences.

Physical Size differences

Generally speaking brushless outrunner motors will have a larger diameter and a smaller length vs a comparable inrunner motor of similar weights.  Conversely, Inrunners are smaller in diameter and typically larger in length.   Physical size is one area that your application may be limited in, however there are other trade offs that would have to be considered as we will get in to below.

RPM / Volt (Kv)

When you consider the RPM per volt of a brushless motor, (rotation speed per one volt applied) this is one of the biggest factors in choosing the correct motor for your application. Often times when one does not correctly select the appropriate Kv motor, risk of burning a power system component out is greatly increased. An inrunner motor of equal size to a brushless outrunner motor will have higher Kv. Although different motor wind selections (same size motor with Kv options) provides a decent range, outrunner motors will typically have a lower Kv value. This is key in your selection of a brushless motor to directly fit your application.

How does an outrunner produce lower Kv? Well, we already did speak about physical size difference. Physical size does represent a primary factor that effects kv. The larger can diameter of the outrunner allows a higher quantity of magnets to be used in the outer case. More magnets alternating magnetic poles forces the ESC to switch more rapidly slowing down the overall speed as there is more work to be done by the ESC. You could also more simply look at it as the larger diameter creates a larger circumference for the motor to travel in one rotation. The larger can diameter also represents a larger moment arm for an outrunner that is a good segue in to the next topic.

Torque Comparison of a Brushless Outrunner vs Inrunner motor

The larger moment arm that we have spoken about above converts directly in to more torque being created. Therefore the brushless motor will generate more torque as a general comparison against an inrunner motor. The relationship ties in with the fact that outrunners do have a lower RPM per volt. The relationship with Kv and torque are inversely proportional. As RPM per volt (Kv) increases, torque of the motor decreases.

Efficiency the same between inrunners and outrunners?

This can be a tough question to answer actually as the true answer has many dependencies. These dependencies can be anything from the effective size comparison to quality of the motor, manufacturer of the motor, power output of the motor and several more. In general for most applications in RC, a brushless inrunner motor has the potential to be more efficient than a brushless outrunner motor.

Waste Heat of Inrunner vs Outrunners

Let’s consider waste heat that a motor outputs. If we look at the brushless outrunner motor, heat must be dissipated through the center of the motor. The source of heat generation is of course the stator windings of the motor. The stator windings of the outrunner is located at the core of the motor. heat must transfer from the windings to the center of the motor being the motor shaft. Heat is then conducted outward through the motor shaft. An outrunners solution to heat is to  have large accessible cooling vents in the case to allow air to flow over the windings directly.

Brushless Outrunner Motor with large vents to the stator windings

An inrunner motor on the other hand has the stator windings directly on the inner face of the outer can. This direct point of contact provides an excellent surface with a very large area for heat to transfer. Air is a very poor conductor of heat. Increasing the amount of surface area for heat to gather in order to dissipate in to the air is how an inrunner can get rid of excess heat energy.

Being able to get rid of waste heat allows the inrunner motor to run cool being more efficient for similar power outputs on a motor of equal size/weight.

Common Brushless Outrunner vs Inrunner motor Applications

Motors that can be fit as direct drive offer simplicity which increases reliability, reduces weight that a transmission would add, and reduces amount of component wear items. The amount of load or torque required also plays in to the overall equations of an inrunner vs outrunner choice. Lastly required RPM is considered.

Here’s a chart to outline the most common approach for motor selection. Note that not in all cases this chart is followed. Deviating from it is entirely possible and really depends on other factors that we didn’t speak about. These can be availability, cost and other similar factors.

In general when I am selecting a motor, I am looking for simplicity and reliability. My first pick would be an inrunner motor and if this does not fit, then I select an outrunner.

Summary of Differences – Brushless Inrunner vs Outrunner Motor