Backwoods R/C's Since 2001

Now that I have bored you to death on RC LiPo battery basics, time to get into the main topics at hand. First are ratings, specifically voltage and capacity. These are the two main numbers you will need when going battery shopping.. There is a third number you will also need to be aware of which I will get to in just a bit.

VOLTAGE

Unlike conventional NiCad or NiMH battery cells that have a voltage of 1.2 volts per cell, LiPo battery cells are rated at 3.7 volts per cell. The benefit here is fewer cells can be used to make up a battery pack and in some cases like on the Blade mCX, one 3.7 volt cell is all that is needed to power the model.

Other than the smallest of electric RC models, RC LiPo battery packs will have at least two or more cells hooked up in series to provide higher voltages. For larger RC models that number can be as high as 6 cells (even more). Here is a list of LiPo RC battery pack voltages with cell counts. If you are wondering what the 2-6S in parenthesis means; it is a way the battery manufactures indicate how my cells hooked in series (S) the battery pack contains.

3.7 volt battery = 1 cell x 3.7 volts 7.4 volt battery = 2 cells x 3.7 volts (2S)11.1 volt battery = 3 cells x 3.7 volts (3S)14.8 volt battery = 4 cells x 3.7 volts (4S)18.5 volt battery = 5 cells x 3.7 volts (5S)22.2 volt battery = 6 cells x 3.7 volts (6S)I should point out you may run across packs or cells hooked up in parallel to increase the capacity. This is indicated by a number followed by a "P". Example: 3S2P would indicate 2, three celled series packs hooked up in parallel to double the capacity.

So, those are the voltages you need to know and each RC model or more specifically, the motor/speed controller combination will indicate what voltage is required for correct operation/RPM. This number has to be followed to the letter in most cases since a change in voltage equates to a change in RPM and will require changing the gearing - not something I want to get into. If a model calls for a 3 cell (3S) 11.1 volt battery – that is what must be used unless you want to open a whole new can of worms.

A quick word on motor ratings...

Many people new to electric flight get confused by brushless electric motor ratings, specifically the Kv rating thinking Kv = kilo-volts (1 kV = 1000 volts). This is not the case at all. The Kv rating of a brushless motor refers to how many RPM it turns per volt. An example might be something like a 1000 Kv motor with a voltage range of 10 - 25 volts. That would mean this motor will turn at about 10,000 RPM @ 10 volts up to around 25,000 RPM @ 25 volts.

I don't want to start into motor ratings; battery ratings are plenty to get through... I just thought I would make mention of it since I do get that "Kilo-Volt" question often.

CAPACITY

Capacity indicates how much power the battery pack can hold and is indicated in milliamp hours (mAh). This is just a fancy way of saying how much load or drain (measured in milliamps) can be put on the battery for 1 hour at which time the battery will be fully discharged.

For example a RC LiPo battery that is rated at 1000 mAh would be completely discharged in one hour with a 1000 milliamp load placed on it. If this same battery had a 500 milliamp load placed on it, it would take 2 hours to drain down. If the load was increased to around 15,000 milliamps (15 amps) a very common current drain in a 400-500 sized RC helicopter, the time to drain the battery would be only about 4 minutes.

As you can see, for a RC model with that kind of current draw, it would be very advantageous to use a larger capacity battery pack such as a 2000 mAh pack. This larger pack used with a 15 amp draw would double the time to about 8 minutes till the pack was discharged.

The main thing to get out of this is if you want more flight time; increase the capacity of your battery pack. Unlike voltage, capacity can be changed around to give you more or less flight time. Of course because of size restrictions and weight you have to stay within a certain battery capacity range seeing that the more capacity a battery pack has, the larger and heavier it will be.

DISCHARGE RATE

Remember that third number I was talking about when you go RC LiPo battery shopping? Yes, discharge rate is that number. This one is probably the single most over rated & miss understood of all battery ratings.

Discharge rate is simply how fast a battery can be discharged safely. Remember that ion exchange thing further up the page? Well the faster the ions can flow from anode to cathode in a battery will indicate the discharge rate. In the RC LiPo battery world it is called the “C” rating.

What does it mean?

Well Capacity begins with “C” so that should give you a pretty good idea. A battery with a discharge rating of 10C would mean you could discharge it at a rate 10 times more than the capacity of the pack, a 15C pack = 15 times more, a 20C pack = 20 times more, and so on.

Let's use our 1000 mAh battery as an example; if it was rated at 10C that would mean you could pull a maximum sustained load up to 10,000 milliamps or 10 amps off that battery (10 x 1000 milliamps = 10,000 milliamps or 10 amps). From a time stand point, this equals 166 mA of draw a minute so the 1000 mAh pack would be exhausted in about 6 minutes.

This is calculated by first determining the mA per minute of the pack. 1000 mAh divided by 60 minutes = 16.6 mA's per minute. You then multiply that number by the C rating (10 in this case) = 166 mA of draw per minute divided into the packs capacity (1000 mA) = 6.02 minutes.

How about a 20C rating on a 2000 mAh battery? 20 x 2000 = 40,000 milliamps or 40 amps. Time wise, a 40 amp draw on this pack would exhaust it in about 3 minutes (2000/60= 33.3 mA minutes multiplied by 20c = 666 mA per minute - divided into the packs capacity of 2000 mA = 3 minutes). As you can see, that is a pretty short flight and unless you are drawing the maximum power for the entire flight, it is unlikely you would ever come close to those numbers.

Most RC LiPo Battery packs will show the continuous C rating and some are now indicating a burst rating as well. A burst rating indicates the battery discharge rate for short bursts of extended power. An example might be something like “Discharge rate = 20C Continuous / 40C Bursts”

The higher the C rating, usually the more expensive the battery - this is where you can save some money. Getting a high discharge rated pack when there is no way you could possibly pull the full amount of power is not required. You can’t go too low on your C rating either or you will damage your battery and possibly your ESC (electronic speed control).

So how do you know what C rating to get when purchasing your LiPo RC Battery Pack? Time for a little more math - sorry...

You need to know your ESC’s maximum amp rating which is usually determined by the motor it will be used with. Since the ESC is what ultimately delivers the power to the motor, that is why the ESC’s amp rating is more important than the motor’s watt rating. All ESC’s will have an amp rating and from that it is easy to figure out what C rating your battery should be.

For example, an ESC rated at 30 amps used in conjunction with a 2000 mAh battery pack would need a battery with a minimum C rating of 15C (15 x 2000 milliamps = 30,000 milliamps = 30 amps).

Now for a RC boat or car where it is doubtful you would be using full power all the time and only be using full power intermittently, you might be able to save a few bucks by using a 12C battery with a 15C burst rate for example.

I feel getting a pack with a constant discharge rate equal to what the ESC is able to deliver is the economical way to go.

This calculation method won't push the pack past its maximum discharge rate, and it allows a nice buffer zone (unless you are using full power the entire flight (not very likely). A pack that is not pushed to the threshold of its discharge capacity will run cooler improving the overall life span of the battery pack and will end up saving some coin in the long run.

Time for another example to go through all this:

If we had a Trex 450 RC heli with 35 amp ESC and a 2100 mAh 20C battery, we can calculate if this battery is good enough for even the most demanding 3D pilot. The Battery is capible of producing 42 amps of maximum power (2100 mA X 20C = 42000 mA = 42 amps). More than enough for a 35 amp ESC.

Don't forget the time calculation either. At a full 31.5 amps of draw, our battery would be exhausted in about 4 minutes. I know a Trex 450 with a 2100 mAh battery pack will give at least 6 minutes of flight when pushing some hard sport type flying and only the most aggressive 3D flying could come close to exhausting that pack in 4 minutes.

As you can see "by the numbers", unless you are pushing things to the limit, spending serious cash on high discharge rate packs is somewhat of a waste.

That said, RC LiPo packs are coming down in price all the time. If you find a 30C pack for the same price as a 20C when that is all you need, go for the 30C pack - it will run cooler and most likely last a little longer. Like most things, pushing a Lipo pack hard close to its limits will wear it out in short order. Having a little wiggle room on the high side will ensure that won't happen.

Lastly feel your packs after running them. I'm afraid to say it, but just because a pack says it is rated at 20C doesn't necessary mean it is in real world applications since nothing runs at 100% efficiency. A LiPo battery after a flight or race will be warm. The warmer it gets, the more stress is being placed on it. If you are running a 20C and it is very warm or worse, hot to the touch after using it, your should definitely consider going up to a 25C or 30C pack.

The other thing that will heat a pack up fast is if you push it right down to 3.0 volts per cell under load. Even if you have a 40C pack and can only draw half that amount, if you push it hard right down to 3 volts per cell - it will become very warm/hot and will shorten its life.