What Lipo Battery to Buy and Why

Lithium-polymer batteries are the fourth most like type of battery to be commercialized for EV applications. The discovery of nonmetallic solids capable of conducting ions has allowed has allowed for the development of these batteries. Lithium-polymer batteries have anodes made of either lithium or carbon intercalated with lithium. One candidate cathode under evalution contains vandadium oxide. This particular battery chemistry has one of greastest potentials for the highest specific energy and power. Unfortunately, design challenges assoicated with kinetics of the battery electrodes, the ability of the cathode and anode to absorb and release lithium ions, has resulted in lower specific power and limited cycle life for lithium-polymer batteries.

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.

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 RC car lipo is usually comprised of lipo battery 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 11.1v lipo battery 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.

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 3×3.7v=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)

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 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 lipo 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 8.5×6 propeller that needs about 4.0A to fly normally at around 50% throttle we will do the next calculation:

We need 4000mah lipo battery 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 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.

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.

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 Lipo 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.