Radio Control Electric Flight
Information and Guidelines
for entering the exciting world of
Electric Powered R/C Flying
This document is broken into 5 parts
Part 1 - Introduction
Part 2 - Understanding power requirements & motors
Part 3 - Understanding batteries
Part 4 - Construction considerations
Part 5 - Conclusions
3. Batteries and how many cells do I need?
The selection of battery type and cell count is a bit tricky, but it often comes down to “What battery packs do I have “ and what current will they allow the motor to draw, when the selected propeller is fitted. And, does this give me enough power to achieve the required flight characteristics?
Whilst the public euphoria almost demands the use of Li Poly battery packs, there are some good reasons why the use of tried and tested Nickel cadmium and Nickel metal hydride battery packs are a good starting point. Why, yes, of course they are heavier and not much cheaper than Li Poly packs, but they are very much more robust in every sense. They take a lot more abuse than Li Po’s and anyway, you have a lot of other technology to get to know without adding the vaugeries of new batteries too! So this is my suggestion for a good starting point.
I’ve found that 8 to 10 cell packs of Ni Cad or Ni MmH of the various types and capacities are quite a workable starting point. You see, if we need more power (Watts.) we can add an extra cells, because extra cells means extra volts and probably extra amps both will give increased power.
Obviously, there is a limit to the number of cells your speed controller is rated for and just as importantly, your chosen model will carry and still have an acceptable wing loading. However, that’s the subject of a later paragraph!
But here are a few ideas.
For a High Wing Trainer – one cell for each 50 square inches of wing area.
For a low wing sport aerobatic Model – one cell for every 35 square inches of wing area.
Another rule of thumb for cell count relates to the IC engine capacity we are used to and is as follows:-
15 Size 7 – 8 cells
25 Size 10 – 14 cells
40 Size 16 – 28 cells
60 Size 29 – 40 cells
Above this, your on your own. Get help!!!
4.Which Batteries are right for me?
Unfortunately, not all cells are OK for electric flying. This is due to the simple fact that we need fast Charging AND Discharging performance, without ruining the cells. The cells I have chosen give reliable performance and will take fast charging over and over again. Reliable performance to me is:-
1. A discharge rate of 15 to 40amps dependant upon the requirements of your model.
2. A recharge in about 30 to 40 minutes.
3. Hundreds of cycles before I have to buy new cells.
Some say these cells will cycle up to one thousand times but I personally doubt this claim when we treat them so aggressively.
When I use Ni Cad or Ni Mi cells I mostly used Sanyo & Gold Peak and I have listed my choices below.
Lithium Iron & Li Poly Cells are also now well established on the market and they do have excellent energy density potential; but again, the reader is cautioned to get lots of good advice before buying a large number of these packs.
So, what cells DO work for us? I suggest the following:-
Nickel Cadmium & Nickel metal Hydride types:-
Gold Peak GP1100
Gold Peak GP2200
Gold Peak GP3300 to GP4700
(Intellect Brand are comparable to GP)
The price of cells whether NiCd or NiMh multiplied by the number you need for a given model can be quite expensive. Doubled, of course if you decide to have a second pack for more frequent flying. So, get advice BEFORE you make the purchase. These packs will be with you for a long time some experts say up to 500 cycles. Therefore, it is obviously no good buying cells that don’t suit your particular needs, even though in another application they may be perfectly suitable.
Lithium Polymer and other new light weight battery Packs.
Like cylinder battery cells there are many Lithium Polymer Li Po’s they are usually referred to) on the market. These cells carry dire warnings that you need to become very familiar with, however, with intelligent use and correct selection they are fantastic!
The biggest advantage of Li Poly’s is that they weigh around one third, yes 1/3 rd of the weight of a similar capacity pack of cylinder batteries. This means you can build light, fly longer and your fuel cell will probably be smaller than the older types of batteries.
BUT, there are some very important things to know about Li Po cells, and here are some of the most significant pointers you need to know all about:-
1. Cell Voltage is usually 3.7 Volts not the 1.2 volts you may be used to.
2. Li Po’s are very susceptible to over charging and over discharging.
3. You must always use a dedicated Li Poly charging device operated from either a suitable car battery or good quality regulated power supply.
4. Always charge the Li Poly pack with it resting on a non combustible surface, like the garage concrete floor.
5. Install a smoke detector above the location where you normally charge your packs.
6. Never, cause uncontrolled discharging by shorting out the battery pack leads. An instant short circuit can render your brand new pack useless.
7. Never store your packs in a deep discharge state. Store at at least 60% fully charged state.
8. Do not connect packs of dissimilar capacity together because it may lead to over charging or discharging of individual cells and subsequent ruin of the pack.
9. Occasionally use a cell balancer in conjuction with your charger. Follow the instructions carefully.
Lithium Manganese and other new light weight battery Packs.
Two new technologies have been introduced into our market recently. Both are showing promise as a development of light weight battery packs, but neither has the capacity or discharge characteristics that Li poly packs now provide. Here is some information about them:-
Lithium Manganese: These cells are a bit heavier than the Li Po’s we are now using, are physically larger and have less capacity. I haven’t seen any of them at our local hobby shops, so don’t rush into purchasing these on the internet as there will be no local after sales service.
Lithium Nanophosphate called A123’s: These cells are now available through one local internet based Electric flight specialist shops. These cells are 3.3 volts nominal and are supplied in iron cans like other technologies. Whilst there is a substantial weight saving over NiCd and Ni Mi cells, these too are only currently sold in 1300 to 2300mAhr cells. Further they are larger that the sub-C cells we had gotten used to. The good news is that the manufacturer is supplying this technology to domestic car hybrid and government road, air and military applications. The USA is gearing up to be far less dependant on fossil fuels in the years ahead which can only help us with our need for high energy dense light weight fuel cells.
There is just one other thing to be aware of, if you go for A123 cells, your charging system will need to accommodate the different charging requirements of Lithium Nanophosphate. Some existing chargers like Schulze can be upgraded with a OEM supplied upgrade chip. The abbreviation for A123’s is Li Fe.
How would you know this without asking someone more experienced than yourself? Check with other experienced electric flyers before committing to buy a certain type of Battery pack or cells.
|Disclaimer: To the reader... please feel free to download, use and copy this Treatise on the understanding that it is for 'private' use only and must not be used in any way for commercial purposes without the writers permission.
Note also, that the information is a work in process and may be amended at any time should the writer or the Club decide so to do. No responsibility or liability is expressed or implied with respect to the information contained in the Introduction to Electric Flight material.
A copy of this article is also available in pdf format below.