LiPo Batteries for RC Aircraft
There are a few battery options available for RC aircraft, but most flyers choose LiPo. These batteries use lithium-ion technology with a polymer (as opposed to a liquid) electrolyte. LiPo batteries are lightweight but with a large energy capacity. They are also rechargeable and come in a number of configurations which can be tailored to suit specific aircraft design specifications. They range upwards from the tiny, ultra-lightweight batteries used in Ultra Micro planes, such as the Sport Cub models. This makes them the most popular choice for RC aircraft enthusiasts.
As with all the other components in a RC aircraft design, the battery must be chosen to meet the overall design requirements of the aircraft. It is also essential that all the component parts are compatible with each other in terms of performance and capacity. So, battery choice is determined by:
- The size of the aircraft (weight, wingspan, propellor size);
- The desired flying characteristics of the aircraft (basic trainer, sport flying, 3D aerobatics etc.);
- Flight time requirements;
- Aircraft motors and speed controllers.
So, how do you make the right battery choice? Let’s begin with the aircraft itself. The weight of the aircraft and the desired flying style are the major factors to consider here. These can then be used to determine the power requirements of the system. The general wisdom amongst RC enthusiasts is that for a basic trainer style, about 188 Watts (W) per kg of aircraft is suitable (or 85W per lb). These requirements change for different styles to 243W per kg (110W per lb) for sport flying and up to about 331W per kg (150W per lb) for 3D aerobatics.
For an example, let’s use the Joytrainer Mini model from Joyplanes (joyplanes.com). This is a common model for both novice and experienced flyers. It has the following specifications:
- Wingspan – 1050mm (3.4 ft)
- Length – 720mm (2.4 ft)
- Max allowable weight (including battery) – 860g (1.9 lb).
If we use the maximum weight, this gives us an upper limit for the required power of 188W x 0.860kg = 85.2W when used as a trainer aircraft. We will return to this value later when considering the current requirements of the motor, speed controller and battery.
The next consideration is the battery cell count. LiPo batteries contain a number of individual cell units, each one providing a working voltage of 3.7V. The number of cells in the battery pack determines the total voltage provided. In general, more voltage equates to more power, but comes at the expense of extra battery weight. For the Joytrainer Mini, there are two recommended cell configurations. Two cell batteries are suitable for beginner flights in light wind conditions, and three cells can supply more power which will provide greater manoeuvrability and cope with more wind.
The number of cells in a battery pack is denoted by 2S (two cells), 3S (three cells) etc on the battery packaging. For our Joytrainer Mini aircraft, the two commonly recommended battery packs are 2S for beginners or stable flights and 3S for those occasions when extra power is required. This means that the available voltage is either 7.4V (2 x 3.7V) for a 2S battery) or 11.1V (3 x 3.7V for a 3S battery). The voltage size is an important factor in determining how much power is available for the aircraft motors.
So, how much power do you generally need?
If you remember, this depends on the aircraft size (in particular, its weight) and the flying style. If we fly our Joytrainer Mini in steady flight, this requires the motors to produce a maximum power of about 85.2W. A simple bit of physics now allows us to calculate the current required to produce this power. Electrical power is calculated using the relationship, power = current x voltage. Assuming we are using a 3S battery pack (voltage = 11.1V) at a power of 85.2W, this means that 85.2/11.1 = 7.7Amps of current will be flowing through the aircraft’s circuitry. If we use the lower voltage 2S battery, this will increase the current requirements to 11.5 Amps (lower voltage means greater current to produce the same power).
Why is it important to know the value of the current in the circuit?
The main reason is that the other components in the circuit need to be chosen so that they can carry this current without overheating. The two most important components in question are the motor and the electronic speed controller (ESC). These two components carry the same current (connected in series). For safety, it is usual to build in a little headroom when choosing the ESC and motor rating (say 20% above the maximum expected current). So for our Joytrainer Mini using a 3S battery, this means that the motor and ESC must be rated for at least 1.2 x 7.7 = 9.24 Amps. For the same aircraft using a 2S battery pack, the rating needs to be at least 1.2 x 11.5 = 13.8A for both components.
How long will the battery be able to deliver this current?
Battery packs store energy in the form of electrical charge which is then lost as the battery delivers current during use. (Current is basically the amount of charge flowing each second). The capacity of the battery is a measure of how much overall charge the battery can store. It is measured in units called mAh (milliamp hours). This tells you how many mA can be delivered in one hour before a fully charged battery becomes completely discharged. So, a 2200mAh capacity battery can deliver 2200mA for an hour before it becomes totally flat. Batteries can deliver charge at a faster rate for a correspondingly shorter time. Doubling the current means that the same amount of charge will be delivered in half the time. The 2200mAh battery can deliver 4400 mA for 30 minutes, or 6600mA for 20 minutes etc. The extra current can be useful for some flight manoeuvres, but two things need to be borne in mind. First, the current drawn shouldn’t be greater than the current rating for the other components, or they could be damaged. Secondly, greater current comes with a subsequent faster loss of battery charge and so a shorter time between charging sessions (and shorter flight times).
A final point about drawing current from a LiPo battery is worth noting. LiPo batteries do not like being completely discharged and you should try and avoid pulling more than 80% of the total capacity of the battery before you recharge it. So, for a 2200 mAh battery, this works out at 0.8 x 2200 = 1760 mAh worth of charge. This will obviously restrict your flying time by 80% too. [SHOULD THIS NOT BE 20%?]
A final property of a LiPo battery is its C-rating. This is an important characteristic as it indicates how much current can safely be drawn from the battery at any time. It also indicates how much current can be put into the battery during the recharging process.
Understanding the current requirements of your flight can be complicated and will probably need to be gleaned from flight experience. In short however, the current pulled from the battery goes up and down depending on how you are flying. A greater number of propellor revolutions uses more current and vice versa.
It is useful to be able to estimate the average current during a flight, and also to know how much current you are drawing during your more demanding manoeuvres. A typical motor current for the MC2206 2000KV model used in the Joytrainer Mini can vary from 2A (motor idling) to 10A (level flight) to 25A (maximum short burst current). The values depend on the propellor size used and how fast the prop is turning.
The C rating allows its users to calculate the maximum current that can be drawn from the LiPo battery without damaging or overheating the battery itself. The formula is as follows:
Maximum current drawn = Capacity x C Rating.
Let’s take an example to understand the formula, for a battery with 3S 22000 mAh 25C LiPo, the maximum current drawn is going to be 2200mA x 25C = 55,000mA (or 55A). This means we can discharge 55A continuously without hurting or damaging the LiPo battery.
A battery with a higher C rating can deliver more current safely, and that means higher performance. The big difference between batteries with different C ratings is the fact that the higher-rated batteries suffer less from voltage drops under load. This ability to maintain a higher and more consistent voltage is what typically generates the high performance we enjoy. This is particularly important when you have multiple motors turning multiple rotors.
C-rating is also key to note when recharging the battery. As calculated above, a 2200mAh battery with a C-rating of 25C will safely allow 55A of current to flow into it during charging. This will speed up charging periods considerably. It is important to set the charger correctly for the battery being charged. This means setting the maximum voltage and also the charging current so that neither of the battery’s limits are exceeded.
We have all heard the scary stories of LiPo batteries bursting into flames, but such incidents are few and far between and LiPos are only as dangerous as you allow them to be!
Anyway here’s a few precautions you can take:
- Use a lithium safe fireproof charge bag.
- Never leave a charging battery unattended.
- Always charge on an inflammable surface with a metal cookie sheet under the battery.
- Unplug the battery as soon as its finished charging.
If your LiPo is swelling, getting hot during charging or giving off a sweet, pungent smell, these are signs it is going bad and it is time to dispose of it.
Also worth noting is that LiPo batteries which come in RC cars etc. have a hard plastic shell, while those used in planes come shrink-wrapped, helping to reduce weight.
If your LiPo is swelling, getting hot during charging or giving off a sweet, pungent smell, these are signs it is going bad and it is time to dispose of it.
Also worth noting is that LiPo batteries which come in RC cars etc. have a hard plastic shell, while those used in planes come shrink-wrapped, helping to reduce weight.