It’s time to get back to the basics. We’re going to talk about an important component, the battery also known as “LiPo”.
In this article, I will try to use accurate terms for all units of measure and other qualifiers so that everyone can get into good habits.
On FPV Drones, the battery is connected via the XT60 socket, and supplies it with electrical energy. There are other types of plugs like the XT30 (smaller) or the Dean format.
This energy is distributed via the PDB (Power Distribution Board) and supplies all the components of the FPV drone. The motors are the highest energy consumers on FPV drones The choice of a battery is therefore mainly based on the motors.
The characteristics of a Lipo battery
Of course, not all batteries are the same. It is essential for any pilot to know and master these notions because the quality of your flights depends on it, but also your safety.
Let’s see what are the parameters to take into account to choose and maintain your battery
Voltage expressed in Volts (V)
Voltage is a unit of electrical measurement expressed in Volts. On a battery, the voltage delivered is directly linked to the number of cells that make up this LiPo. To express this, we write the number of cells followed by an “S”. For example, a battery with 3 cells will be called “3S”.
For a LiPo battery, each cell has a nominal voltage of 3.7 volts. The elements (or cells) are connected in series. Their values are added and we obtain the following voltages for each battery:
- 3.7 volts for a 1S battery
- 7.4 volts for a 2S battery
- 11.1 volts for a 3S battery
- 14.8 volts for a 4S battery
- 18.5 volts for a 5S battery
- 22.2 volts for a 6S battery
I could have continued this list, but I think you will have understood the principle. These 6 types of batteries are the most common nowadays on FPV drones.
This voltage is said to be nominal, i.e. given as the one delivered in normal operating condition. This is the literal definition, but a cell’s voltage will change according to its state of charge. It will be equal to 4.2 volts when it is fully charged.
Never discharge a battery under 3.6 volts per cell if you want to keep your batteries healthy for a long time. If you discharge it under 3.6 volts per cell, the battery will not keep its power and will be unusable very fast.
Capacity of your battery in mAh
The capacity is a value expressed in milliAmpere per hour (mAh). This unit allows to quantify the quantity of energy present in a battery. A 1500mah battery will be able to deliver 1.5 amps for one hour (regardless of the voltage).
The greater the capacity is, the more current the battery will be able to deliver for a long period. One could deduce that it would be enough to take high capacity batteries to fly longer. In practice, this is not the case, and we will see why later in this article.
The “C” discharge value
The intensity of a current is a unit expressed in Amperes. The number of C’s written on the label is the multiplier coefficient of the amperage that the battery will be able to deliver during its discharge. That is to say that if I have a battery of 1500mah 100C, it will be theoretically able to discharge at a rate of 1,5 x 100 = 150 Amperes.
But what is the function of this coefficient and why is it theoretical?
This value allows you to choose a battery that can deliver the necessary current to your machine. To answer this question, it is important to know how much your device consumes. We will mainly take the consumption of the motors, the other components being negligible.
In the case where I have a FPV Drone with motors consuming each 40 amps maximum (with gas at full), I have a maximum consumption around 40 x 4 = 160A. For the good functioning of the FPV drone and the health of its battery, it will be necessary that the battery can deliver at least this current. Otherwise, not only will your FPV Drone be limited by the lack of available electrical power, but your battery may also deteriorate prematurely, or worse, catch fire…
Manufacturers recommend that you choose a LiPo that can deliver twice the current required by your quadcopter to preserve the health of the battery. In reality, it is very rare to be able to meet this requirement. The power consumption of our motors is far too high compared to the capacity of current batteries to deliver such currents. The pilot is often obliged to make do with a battery delivering an intensity equal to or barely superior to the consumption of his machine. This choice is therefore made at the expense of the life of LiPo batteries.
Why is this number theoretical?
Simply because it is the only parameter on which the manufacturers have no control during the manufacturing process. This characteristic is directly conditioned by the quality of the cells in the pack. The astronomical figures written on the labels are therefore purely marketing. At best, the brand will be honest, and will display a number consistent with the real quality of its products, at worst, this figure will be completely unrealistic compared to reality, and you will find yourself with an undersized battery.
The Weight of your battery
The weight of a LiPo is expressed like all weights in grams. It is customary to choose the lightest battery possible. Indeed, a heavier weight requires a stronger thrust from the motors, and therefore a higher consumption. This is the reason why choosing a battery with a higher capacity (and logically heavier) is not synonymous with a longer flight time. It is very important to choose a battery with an advantageous capacity/weight ratio in order to optimize the flight and its duration.
It should be remembered that a model aircraft such as a FPV drone must have its masses centered as much as possible around its center of gravity. This is generally located in the center of the X formed by the frame. If your battery is not in the center of the model, the heavier it is, the more the piloting will be affected and the motors will have to work hard to keep the FPV drone balanced!
The internal resistance of your battery
An electrical resistance is a value that we quantify in Ohms. It is a value that can vary considerably depending on the quality of the battery, its condition and the type of cell used. It is an important characteristic because following Ohm’s Law, it directly conditions the current delivered to the machine. The higher the resistance, the less your pack will be able to deliver the amperes that the quad’ needs to fly. It is considered that for conventional LiPo, the closer the resistance of a cell is to zero, the better the quality and health of the cell.
Be careful though, some LiPo cells will have an internal resistance at rest that might seem astronomical compared to other batteries. However, these will not limit the performance in flight! The type of cells used in these packs see their resistance drop drastically once hot and in use. This resistance rises during the rest phases to give them a better life span. This is the case of the batteries of the Tatuu brand for example.
However, it is a good practice to note the IR of your pack when it is new, and then to observe the evolution of this value during the aging of your battery. When the cells will have taken more than 15 or 20 mOhms more than their initial value, it will mean that the battery is at the end of its life.
After this overview, you should be able to see a bit more clearly what a LiPo battery is all about.
The discharge of your battery
The number one cause of LiPo degradation is improper use during discharge, and in particular too deep discharge.
A FPV LiPo battery must operate within a relatively small voltage range to remain healthy. The maximum voltage threshold is fixed at 4.20 volts per cell for a conventional LiPo, and 4.35 volts per cell for a LiPo LIHV. On the other hand, the minimum voltage threshold is left to the user. This threshold has been a source of debate for a long time, but I think it is useful to look at it from a different angle. The following reflection is the result of my research on the net and discussions with pilots and battery manufacturers.
A LiPo will give its power throughout its use, but will see its voltage drop sharply around 3.5 volts. It is known that it is dangerous to go below 3.3 volts per cell. This risk is all the greater as the consumption of your FPV Drone and/or the number of C delivered by the battery are large. The discharge will only take place more quickly. Your charger will simply refuse to charge a LiPo whose cells have a voltage lower than 3 volts.
The quality of cells
Our FPV drone batteries are composed of cells whose quality is qualified by a “grade”. The classic grades go from “A” for good quality cells, to “C” for those that do not meet the conditions required to be marketed. Some rarer cells have higher grades like “A+” or “A++”. I don’t think that the common man can have access to them in order to use them on a racing machine as we do, maybe one day …!
The different brands we use on our FPV drones have access, for the most part, to grade B cells. The resulting discharge curve is directly related to this manufacturing quality. The behavior of the battery will logically be different during the flight.
A conventional battery has a discharge curve similar to the one shown in the graph above, its voltage will drop more or less linearly until it drops almost vertically in the last part of the curve. A battery with better quality cells will have a higher curve, keeping a higher voltage throughout the discharge. Note that a quality battery will have less “SAG”, i.e. the voltage will drop less sharply during the current calls.
Proof by example
To illustrate this, I present a comparison of 2 LiPo batteries taken from the lipobench.com website. On this graph, we compare batteries whose capacity and C number are supposed to be similar, one from Infinity, and another from ThunderPower. Their performances are actually quite different, and we can see that the ThunderPower manages to maintain a higher voltage, while having less “SAG”. We can also see that at a deep discharge level, our better quality battery will have a hard time resuming a higher voltage, cell damage is inevitable at this voltage.
It is however very difficult to know what kind of cells each brand uses. But if one thing is certain, it is that you can very easily destroy a battery by flying too long while not using the full capacity will not affect it. There is no memory effect on this type of battery as there would be on a lead-acid battery.
What can we learn from this?
We can draw a good practice from this reflection. It is to follow the recommendations of some companies specializing in this type of battery, preferring to land when your LiPo has reached 3.7 volts per cell. The choice of quality batteries will guarantee you power and autonomy throughout the flight without having to go too low in voltage. You must be careful not to completely discharge your LiPo in order to optimize its life. Wanting to discharge too deeply means necessarily entering the zone where the voltage will drop drastically and damage the cells. Trying to get a longer flight time will have only one effect: damage your packs irreversibly. It is expensive for a few seconds of flight, isn’t it?
Charging your Lipo battery
For the charge, I reassure you, it will be much easier than for the discharge!
In order to charge your LiPo, you will of course need a … Charger compatible with these batteries. I won’t go into detail about this equipment, which will be the subject of another article.
Your battery, as we have seen, has a number of Cs dedicated to discharge. You can see that it also has a number of C dedicated to the charge! Often much more discreet on the label, it allows you to know at what intensity you can charge your battery.
Despite the presence of this multiplier, it is customary to charge your batteries at 1C. This is equivalent to one time the capacity of your battery. If you have a 1500mah battery, you would select a charge current of 1.5 amps on your charger. Similarly, if you are charging 450mah batteries, select 0.45 amps on the charger. You don’t even need to be good at math, just divide the capacity of your battery by 1000, easy!
The counter example
New sites are appearing on the web such as lipobench.com and allow us to know more precisely what our batteries are worth. An article on this site shows that charging your LiPo at a value higher than 1C does not harm the health of the battery (tested at 5C), so feel free to do your own tests, but know that the “traditional” charge at 1C remains a safe value in case of doubt.
Remember that to keep your batteries in top shape, ALWAYS charge in “balance” mode to keep your cells balanced.
Storage or How to store your Lipo batteries
Flammable and chemical products require appropriate storage conditions.
LiPo batteries are flammable, sensitive to heat (to a certain extent) and to perforation. It is therefore forbidden to let your little brother play with them, or to keep them in front of the window in full sun. The best practice is, in my opinion, at least :
- Avoid direct and prolonged contact with the sun (risk of heating)
- Store in an environment that is not very sensitive to fire (a fireproof bag or a clay pot for example)
- Store in an environment at room temperature and ventilated
For this aspect, I can only advise you to use a suitable envelope. Lipo safe” flexible bags are available everywhere on the net at affordable prices. They are a first step towards your safety. A safer way is to store your batteries in a box designed to hold ammunition. This is more expensive and bulky, but also more durable and adequate.
Your charger also has a mode called “storage” which is to be used without moderation! This mode is very useful to let your batteries rest between each session. It will charge or discharge your battery to a voltage of 3.8 volts per cell so that it does not age prematurely. I personally use this mode to reset all my batteries to this voltage after a flight session. It’s a bit time consuming, but it optimizes the life of your favorite batteries. Having a good cut-off value at the end of the flight also allows to be close to the storage voltage once on the ground. However, a passage to the charger allows to reach a controlled voltage and at the same time, an optimal balance between the cells.
I hope this article on our dear LiPo’s has been useful.
Fly safe !!!