RC Lipo Batteries Buying, Usage and Storage Basic Guide

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Intro

This guide is intended for the new user that wishes to start in the RC world and needs a basic knowledge about Lithium Polymer batteries.
Different types of batteries and connectors
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Different types of batteries and connectors

What Lipo Battery to buy and why

Lithium Polymer batteries are the most reliable when it comes to Remote Control Models due to their compact size and weight to power ratio.
Different types of connectors make the Lipo batteries easy to be used with a wide variety of Remote Control helicopters, airplanes, cars, boats, quadcopters and so on.
Lipo Batteries are classified by 3 main categories:
  1. Number of cells or voltage
  2. Capacity or mAh rating
  3. Discharge Rate
Turnigy A-Spec 3S 2200mah 65C - 130C markings
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Turnigy A-Spec 3S 2200mah 65C - 130C markings

What all the numbers mean

1. Number of cells or voltage.
Lipo Battery packs start from as low as 1 cell packs and go up to big 12 cells battery packs.
2. Capacity or mAh rating
Basically the higher the number the more the model will run.
3. Discharge rate.
Multiplying the mAh rating with the discharge rate will give you the maximum sustained load that the battery can handle without damage.
The discharge rate is expressed by two numbers, the first one states the normal load and the second one is for burst discharge (this will be used for no longer than 10 seconds and usually only for emergencies).
Turnigy Nanotech 750mAh
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Turnigy Nanotech 750mAh

Single cell battery packs

Each cell is rated at 3.7v nominal voltage. Why nominal? Because a fully charger cell has 4.2v and a fully discharged one will have about 3.5v.
A 1S battery is usually comprised of 1 cell that has the negative(black) and positive(red) wires hard soldered to its terminals and these wires are used for powering up the system.
Depending on the battery make and purpose this might also be used for charging the battery.
In some cases a different connector will be used for charging.
The battery in the picture shows the same cell made by the same manufacturer but with different cables used for different interfaces.
The cell is a Turnigy Nanotech  750mAh 35C constant discharge with 70C burst discharge. 
Weighing only 19 grams this battery will keep a Syma X5C with an all up weight of 100 grams flying for about 10 minutes.
Syma X5C Explorers with an upgraded battery Turnigy 3.7v 750mAh discharge rate 35-70C
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Syma X5C Explorers with an upgraded battery Turnigy 3.7v 750mAh discharge rate 35-70C
3S 2200mah battery with exposed connections
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3S 2200mah battery with exposed connections

Multiple cell battery packs

The image on the right shows what you will typically find in a multi cell battery.
All the cells are stacked together wrapped in plastic.
The cells are wired in series and this gives the name and the voltage of the battery pack.
In this configuration the voltage adds up and the capacity remains the same as the individual cell.
So a 3S battery will have 3x3.7v=11.1v.
This is the usual type of configuration and you might find it marked as 3S1P. Where the first 2 characters (3S) represent the number of cells and the last 2 characters (1P) represents how many packs are inside a battery.
If you connect 2 3S batteries in parallel you will have a 3S battery with 2 times the capacity. This is a 2 pack battery and the marking for it would be 3S2P.
More than one pack is not very common so this type of marking is not widely used but you might run across it when looking for dedicated receiver battery packs.
This is the principle that works with all the range of battery packs.
Usual configurations are as follows:

3.7 volt battery = 1 cell x 3.7 volts (1S)
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)
29.6 volt battery = 8 cells x 3.7 volts (8S)
37.0 volt battery = 10 cells x 3.7 volts (10S)
44.4 volt battery = 12 cells x 3.7 volts (12S)

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Capacity or mAh Rating

The best way to explain this is that the more milliamperes the battery stores after a full charge the more it will be able to keep your model running.
So the next thing would be to think "Hey i want my plane to fly for longer i'll just put a higher capacity battery inside!". Well, there is a catch. The higher the mAh rating is, the cells get bigger in size and weight also. So if your plane flies 8 minutes with a 2000mah battery it might fly for 10 minutes with a 2200mah battery but a 10000mah pack might be to heavy and the plane will not fly at all.
This is where weight plays an important role. You need to be able to maintain the centre of gravity and also the ability of the model to lift, carry or float with the added weight.
Discharge rate explained
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Discharge rate explained

Discharge rating

You can view the discharge rate as a container that has some mahs inside. 
This container has an opening on the bottom. The bigger the opening the faster the mahs flow out of it.

You should read the following if you do not want to destroy your batteries.

Following our example the 20C battery will be able to provide 44000 milliamperes per hour or 44 Amperes.
If you want to calculate how long your model will run on this battery you can do the next calculation:
multiply the capacity by the maximum load of the battery
2200mah * 20C = 44000mah (these are milliamperes per hour)
Divide the result by 60 minutes
44000mah / 60 minutes = 733ma (power draw per minute)
Divide the capacity of the battery by the power draw per minute
2200mah / 733ma = 3.01 minutes

So from a theoretical point of view this battery will hold on for 3 minutes in a system that needs 44 amps to work normally.

If we put this battery in a flying wing that is powered by a NTM prop drive 2826 with a 8.5x6 propeller that needs about 4.0A to fly normally at around 50% throttle we will do the next calculation:

We need 4000mah to operate the motor and we divide this by 60 minutes to find the power draw per minute
4000mah / 60 = 66.6ma per minute.
We then divide the capacity of the battery by the draw per minute result
2200mah / 66.6ma = 33.3 minutes (about 3 minutes and 20 seconds)

Here comes the 80% discharge rule:
This rule says that a lipo battery should never be discharged with more than 80% of its capacity.
So for a 2200mah battery this will be:
2200mah * 80 / 100 = 1760mah
At the end of the discharge cycle there should be 440mah still left in the battery.
This will maintain battery life and you will enjoy it for many flights.

This discharge limit will usually be about 3.7v per cell. Using a battery monitor will alert you when this limit is reached and you know when to bring your model back.

Remaking our calculation based on this 80% rule we have:
1760mah / 66.6ma = 26.4 minutes of flight (about 24 minutes and 25 seconds)
Because there are also other devices that need some power in order to keep the plane flying this time will be shorter. These devices are servos, receiver and the BEC(battery elimination circuit). So it will be somewhere around 20 minutes on a healthy freshly charged battery.
This can be seen in the following youtube video. It might not be full of stuns, loops, inverted verticals and so on but it does fly for 20 beautiful minutes.

The C rating is important when you buy a lipo battery because if your system uses under normal load more than the constant discharge rate the battery will puff or swell, its internal resistance will rapidly deteriorate, the cells will become more and more unbalanced and the battery does not like that.
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The bad things about Lipos

You might hear a lot of people saying that Lipos are dangerous, they explode, they swell, puff, make a lot of smoke not work for more than 10-15 times.
It is true. If you mistreat them, Lipos are a waste of money.
Main causes of Lipo failure:
  • Overcharge
  • Over discharge
  • Not enough discharge rate
  • Charging the battery with fast chargers for too many times
  • Not using balance chargers
  • Running the battery at its full burst C rating repeatedly for more than 10 seconds.
  • Physical damage to the battery during landings or crashes
  • Short circuits in the system
Basically any abuse on the battery will decrease its life time. Serious abuse or damage can lead some times to explosions, fire and aggressive swelling.

Obviously there are those remote cases when the batteries have a manufacturing fault but usually these faults can be detected before or on the first use.

Storage

After discharge we saw that the battery may drop to 20% of its capacity.
For storage it is recommended to have the battery at about 50% capacity.
This will be about 3.8v per cell.
Never store them in hot temperatures.
Never leave them in your car exposed to direct sunlight when out on the field.
Never store the batteries fully discharged for more than 2-3 days. Best practice should be to charge them the day before using them and then charge them back to storage value for when you will not use them for a long period of time.

Ending Note

This guide is based on personal experience gathered throughout many flying hours.
It is not a perfect guide for using lipo batteries but it should give you a very good idea on what to look for and what to expect from a Lithium Polymer battery.
I hope you will have great and memorable moments in the hobby.


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