Lithium Polymer Batteries
This information is provided by Mike Redmond of Arourra (www.aurorra.co.uk) who would be happy to advise on Electric Power for F3A aircraft, equipment and accessories
The Facts About Lithium Polymer Batteries
Over the last two years a revolution has started in the use of lithium polymer batteries and brushless motors to an extent that power systems from the smallest to the largest flying models can now be produced that will meet or exceed the capability of any IC system.
Successful use of lithium polymer batteries in the powering of large models is achievable as long as these batteries are used within their real specification.
Any successful use of lithium is really only going to be truly a success if the power delivery meets or exceeds that of any IC equivalent and also the lifetime of the batteries is such that the cost per flight is at least half that of using an IC engine.
As with all power systems what really sorts out the “men from the boys” is the high power delivery applications.
In this article I am going to try to explain the real facts about Lithium Polymer and the importance of selecting the correct cells for an application. An understanding of these facts is very important for successful, winning use.
When designing an electric power system one must first decide upon the power that is going to be required to fly the model and more importantly fly the model in a way that is fit for purpose. Within the aerobatic competition scene there are a few basic requirements however completing a schedule flight in all weather conditions whereby the pilot experiences no power fade in any manoeuvre is the immediate aim.
We have used the first term which in my opinion is the most important term in electrics, POWER. Once in the design of a system the power requirement is determined then the Lithium Polymer battery system must be designed to be able to deliver this power safely and not too stressed as to have only a short lifetime of use.
The real cost of Lithium is related to the lifetime of use
I want to now define various parameters as they are defined and understood in the commercial world of batteries. Unfortunately marketing of Lithium specifications within the R/C area has got so excessive that the true performance specs of the batteries has been lost.
AH or Ampere Hours
If a battery is discharged at a constant current of 1 amp for 1 hour then 1Ampere Hour of energy is taken from the battery. Many times you will see the capacity of a battery defined in terms of Ampere hours and this value is the amount of energy the battery holds. For example a 2 ampere hour battery if it delivers 1 amp for 1 hour will be half drained while a delivery of 2 amps for 1 hour will fully drain the battery.
The true capacity of a battery is measured by discharging the battery at a low current rate and measure the amount of time it takes to fully discharge the battery. For example if you discharge a battery at 1amps discharge current and it takes 120minutes to completely discharge it then it s true capacity is 2AH or 2000MAH (milliamp hours). The commercial accepted way to test the rating of a battery is to discharge it at a current of 1/2C where C is the rated capacity of the battery.
Thus a 3000mah ( 3AH) battery would be discharged at 3/2 (1/2C) amps i.e. 1.5Amps . If it has been correctly rated @3AH then it will take over 2 hours to fully discharge.
Whenever a battery is put under load i.e. current is drawn from it then its voltage will pull down to a lower value. How much the battery pulls down under load is dependent upon how capable it is of supplying the current.
This is becoming a much abused parameter; in my opinion the use of burst specifications etc and over marketing using specifications with no definition is a real cause for concern.
Any high energy source is dangerous if abused, Lithium last year was undergoing a lot of safety misinformation but it is like any other high energy source and that is if it is misused it is dangerous. My concern at the moment is that the over marketing is leading people into a false sense of security, you must be sure of the specification of the product that you select especially with the big power models. The very least consequence of abuse will be short lifetime of the product and the very worse consequence is the short lifetime of the user!.
Lithium cells in general are composed of 20 various chemicals, the formulation of which will vary between each manufacturer. This formulation is of importance for the performance on capacity and the ability to deliver current. I mentioned previously the ability of the cell to deliver current but with any high energy source there has to be a maximum current that it can deliver safely on a continuous basis. Commercially this is a safety rating and it is related to the fact that the cell will not get so hot that the lowest boiling point chemical used in the formulation starts to boil and give off gas ( puffing of the cell pouch). Commercially the maximum current rating of the cell is related to the capacity of the cell and recently it appears that everybody is marketing 20C rated cells .What this rating should mean is that the cell can be discharged at this maximum current from fully charged to fully discharged state without ever getting hotter than 140F and pulling below 3Volts cell voltage.
For example a 2000mah 20C rated pack should be able to be fully discharged at 40amps continuously without ever getting hotter than 140F and pulling below 3V per cell .A 3000mah pack can be discharged at 60amps on the same basis.
If you were to charge a pack and discharge it at its continuous rating for the whole of its lifetime you will get about 25 to 50 cycles of use from it but it will be safe to do so. This temperature of 140F is derived from the lowest boiling point chemical used in the cell in that we assume that if the outside temperature of the pack is 140F then the inside temperature of the pack will be at least 150F, some more margin of 10 to 15 F is allowed to ensure we are well away from that boiling point.
I think you can also see another piece of misinformation creep in here, cooling the cells in use has no effect whatsoever on over run packs as the heat is being generated inside the cells. To have a true indication of exactly what the temperature is inside that cell you must not cool the pack. There is a very good analogy here in that how many of us have been badly sun burned on a very hot sunny and windy day, the surface of the skin where the nerves are did not warn of burning because the surface layer was being cooled by the wind but sure enough under the first skin layer we were being literally roasted.
Another parameter that truly relates the performance of a cell is its internal resistance.
The reason that one cell gets hotter than another for the same power delivery is that the internal resistances differ. The current being supplied by a cell has to go through the internal resistance of its structure and in so doing generates heat ( like current through the thin element of a light bulb). It is this internal resistance that causes the cell voltage to pull down under increasing current, remember you want the cell to deliver POWER.
If a cell has a high internal resistance then some of the power it is capable of delivering is lost in heating up the cell and does not get to the motor.
Motors do not just need current they need voltage and current
Power =Volts x Amps
This is why some lithium packs deliver more power to the motor than others and the pilot will notice this for the same current delivery their pack voltages are higher. I usually call the cell that can deliver more power for the same current as being stiffer”.
Stiffer packs can deliver more power, this is why defining the safe continuous current rating is important and must be on the on the basis of temperature developed under load as well as the pull down of voltage.
Burst ratings are often quoted now and what does it mean? Well it means absolutely nothing in real terms. If you were to short the terminals of a battery ( please do not try this with even the smallest of lithium cells) then the ultimate parameter that will decide the value of the short circuit current that will flow would be the internal resistance of the pack, for how long that current will flow will be determined by the capacity of the pack. Most lithium cells can deliver greater than 50C or 60C when short circuited come to that so can Nickel Metal Hydride, but it will only be for a short period of time. Consider a 2000mah cell with a 60C burst rating. A 60C burst on a 2000mah cell is a current of 120 amps, this sounds good but the cell will be fully drained in 60 seconds at this rate.
To help increase the lifetime of lithium cells there is now available a true cell balancing charger. This charger cell balances all the way through the charge cycle of the packs each cell in the pack is effectively being charged individually and at the same time and to a very high accuracy. It is the only method available to charge and cell balance at high current levels.
During the charge cycle of a lithium cell there is high current delivery while the cell is being charged up to about 80% of capacity. In the last 20% of top up the charge current falls away linearly to a low level. Any damage to a cell in an unbalanced pack will occur at this 80% point.
“Cell balancing” circuits have been available for a year or more but act more like charge levelers, they have a current limitation of 300ma to 1 amp and can only have some effect at very low charge rates. Quite often people are using these devices at charge rates which essentially mean that they cannot fully cell balance particularly at this crucial 80% of capacity point when the maximum charge current is flowing. At the very best they may cell balance to a level of 50 to 100 mill volts and only then on very slow charging of up to 6 hours.
A true cell balancing charger will maintain cell balance during the 80% danger point of the charge cycle and at maximum charge current and hold the cells to a better than 10 mill volt balance and this occurs at every charge.
Please do not be afraid to have a go with the lithium but beware and buy and select wisely I hope the above brief discussion has helped to put some fact to these often used terms that most people do not really understand.
Lithium and brushless is the future, performance is available now and the cost per flight can be achieved that is much better than existing high performance IC. Vibration damage of the airframe and expensive digital servos is non existent moreover brushless motors do not wear other than easily replaced bearings.