Beginners Guide Part 5 - Batteries And Chargers
LiPo (lithium polymer) batteriesPlease, read this topic carefully, LiPo batteries are highly flammable and can explode. They even burn in the water because they do not need oxygen for burning.
Nowadays, this type of battery suppressed NiCd (Nickel-Cadmium) and NiMH (nickel-metal hydride) batteries because they have lower weight and low voltage drop compared to Nickel type batteries. The NiCd is still used to power the RC components in the glider (receiver and servo) as well as in the radio transmitters, but it is very likely that your new electric model will be driven just with LiPo batteries.
While NiCd and NiMH batteries for RC models typically consist of 4 to 8 cells, LiPo comes in 1 and up to 12 cells, although batteries over 6 cells are rarely used for beginner models.
The nominal voltage of a LiPo cell is 3.7V.
When the single cell is full charged voltage is 4.2 Volts.
Battery labels tell us how many cells a battery has, and the letter S is used for this label.
So in the specifications we have that 2S battery voltage is 7.4V, 3S battery 11.1V ...and so on.
The second label on the battery that is important is its capacity, expressed in mAh (milliamper hours).
The higher the capacity of your battery, the longer flight time will be.
Of course, batteries with higher capacity are much heavier, so you have to be careful not to overweight the overall weight of the model because the over-weighted model does not fly at all.
The third label that is very important for the battery is the C rating label.
We will discuss this a bit longer because it is a pretty confusing for the beginner.
The C-rating tells us how much current battery can constantly deliver.
The power of the battery that can be constantly supplied also depends on the capacity of the battery itself.
So we come up with the formula capacity (in Ah) times C-rating = max. electric current that battery can provide.
Let's take an example of 2200mAh battery labeled 20C:
2200mAh is 2.2 Ah (amper-hours) times 20 = 44A
Therefore, this battery is designed for a maximum constant discharge of 44A.
The same kind of battery, but with 40C label, can be constantly discharged with electric current of 88A.
Some battery series have two C-rating labels, for example the Turnigy Nano-Tech series, which are labeled as at 25-50C.
What does this mean?
This means that the battery is nominally rated as 25C and supports a constant discharge of 25C, but it also supports the so-called Burst (short duration about 1-2 seconds) discharge, when the current can jump up to 50C in very short time, without damaging the battery.
It should be noted that the batteries with a higher C-rating are more expensive than those with smaller rating and a few grams are heavier.
Also, if you buy battery from well known manufacturers (Gensace ..), you can be sure that the C-rating is exactly what it is written on the battery. Such batteries are sometimes several times more expensive than Chinese ones with the same cell count,capacity and C-rating.
If you try to buy cheapest battery do not be surprised to come up with a low-cost, high-C battery, which, after 2 charhing cycles, becomes unusable. This is because they often write that the battery is 25C, and in fact, it is barely 10 or 15C.
From my personal experience, I recommend always getting a battery of at least 30C, as good enough and not so expensive I recommend the Zippy Flightmax battery series from HobbyKing.
Each battery has 2 wire extensions. The one with thick wires and various types of connectors (the connector depends on the capacity and current that the battery can deliver - small connectors for small currents, bigger for large currents), that is the main output of the battery and connects to the ESC.
The second wire extension (3 wires upwards - depends on the number of cells in the battery) is called the balancing output, and is essentially the output from each cell in the battery, and is used when charging the battery to monitor the state of each cell in battery.
The disadvantages of LiPo cells are their high sensitivity, both on physical damage and on voltage. If the voltage of a particular cell falls down below a 3V per cell, battery can be permanently destroyed.
Also a damaged (deformed) battery can be a great risk of using or charging, as it may cause a fire or explosion.
Generally, a few basic rules should be taken when using the LiPo battery:
- Use only special chargers for LiPo
- Always monitor the battery while charging . Periodically check that it does not overheat.
- Do not charge the battery in the vehicle.
- Do not overload the battery with an inadequate motor / propeller (to pull more current than it can)
- Use only those ESCs designed for LiPo batteries.
- Make sure there is no short circuit.
- Never puncture, drill, distort or break the battery.
- Do not leave the LiPo battery in the model connected to the ESC
Storage of the batteriesBatteries, when not used for a long time, should be placed in the so-called Storage Mode. Every quality charger for the LiPo battery has this option, and it is about putting every cell in a battery to cell voltage of 3.8V.
This voltage allows the battery not to lose its properties when they are not in use for longer period.
Let's sum up everything we need to pay attention to:
- voltage below 3.2V per cell in use
- voltage below 3.6V per cell in standstill or storage
- charging over 4.2V per cell
- temperature above 60 degrees Celsius
- fast >0.5C charge at a temperature below 10 degrees Celsius
- draining more than 80% of battery capacity
- storage of fully charged battery (4,2V per cell)