September 28

Lithium Polymer Batteries (LIPOs)

Essentially there are three types of battery suitable for powering your plane. These are:

Nickel Metal Hydride (NiMH)

Lithium-Ion (A123)

Lithium- Polymer (Lipo)

Let me say right away that we will not be considering NiMH batteries for our trainer. NiMHs have fallen out of fashion with model flyers due to their greater weight and volume than Lipos for a given battery capacity.

A123 Batteries

The label for just about every Lipo battery tells you to never leave your battery charging while unattended.  While this is definitely a good practice to follow, watching any battery charge is almost as excited as watching grass grow…

A123 batteries may be a perfect alternative if you don’t feel comfortable charging a typical lipo pack. However, there are some things to consider before making the change. First things first.  What is an A123 battery?

A123 Batteries use Lithium Iron Phosphate (LiFePO4) as the cathode. Conventional Lipo Packs have Lithium Cobalt Oxide (LiCoO2) cathodes. Without some electrical knowledge these terms will mean very little but don’t worry about it, just accept it!

With A123 Systems’ patented “nanophosphate”technology, the particles of lithium that gain or loose electrons during charging and discharging are extremely tiny. This makes it possible for the cells to be charged and discharged at very high rates without overheating or exploding as is possible with LiPo packs. This technology also allows for the cells to be charged and discharged many more times than LiPo’s without loosing capacity.

LiFePO4 batteries are considered a “Lithium-ion” battery because they use a liquid electrolyte solution as opposed to the solid “gel-type” polymer used in “Li-poly or “Lipo”packs. This is why most A123 cells are cylindrical.

The chemistry of LiFePO4 battery packs is much more stable than the LiCoO2 chemistry found in conventional LiPo’s making them much less likely to overheat or catch fire when overcharged, put into deep discharge or crashed.

 Disadvantages of A123 Batteries

So far so good right?  So why isn’t everyone using A123 packs instead of Lipos?

The big trade off is the fact that LiFePO4 battery packs are heavier and larger than Lipo packs of the same capacity. This means shorter flight times or heavier batteries. This is why A123’s are generally used more with RC trucks and cars than with airplanes.

There are some RC pilots, however, that feel the shorter flight times of LiFePO4 batteries is a tradeoff worth making for a safer more stable battery that can be charged very quickly without worrying about burning their place down!

Lithium Polymer Batteries

lithium polymer battery, or more correctly lithium-ion polymer battery (abbreviated as LiPo or Li-poly), is a rechargeable battery of lithium-ion technology in a pouch format. Unlike cylindrical cells, LiPos come in a soft package or pouch, which makes them lighter but also lack rigidity.

The name “lithium polymer” (LiPo) is widespread among users of radio-controlled models, where it may indicate a single cell or a battery pack with a number of cells connected together.

Each individual lithium polymer cell has a nominal voltage of 3.7 volts and when fully charged this voltage rises to 4.2 volts. It is extremely important that a LiPo battery cell is not  discharged below 3.0 volts or charged beyond 4.2 volts. Doing so will most certainly damage the battery!

An RC LiPo battery pack is designed to deliver or discharge a maximum amount of current. This value is determined by the “C” rating of the battery. To determine the maximum allowed current, simply multiply the “C rating” by the capacity of the battery pack. For example, a 2200mah LiPo battery pack with a 30C rating can safely provide 30 x 2200mah = 66,000 mA or 66 amps.

Discharging a LiPo battery pack at a higher current than it’s rated for is dangerous and it will drastically shorten the life of your battery. On the other hand, having a battery that is too heavy for your airplane will make it fly like a brick. Striking that delicate balance between weight and capacity is crucial.

LiPo’s are different to any other type of battery and must be charged with a charger specifically designed for LiPo batteries. Even with the correct type of charger, you could very easily wind up seeing unwanted fireworks if you charge lipo batteries too quickly!

LiPo batteries should most definitely be respected. But there’s no reason to be afraid of them. If you follow the advice on LiPo battery safety you will reduce the chances of an incident to near zero.

Following the LiPo battery safety guidelines below will greatly reduce the chances of having a fire.

 Safe Battery Charging Techniques

  • Always let your LiPo batteries cool to ambient temperature after a flight before charging. Make sure your charger is designed specifically for LiPo battery packs.
  • Double check that your charger has counted the cells correctly and is charging at or below 1C charge rate.
  • Charge your batteries on a solid non-flammable surface in a well ventilated area.
  • Have a fire extinguisher nearby while charging your LiPo batteries.
  • Always check your LiPo batteries for any damage after a crash. If the LiPo back is punctured or damaged in any way, don’t use it. 
  • Never, under any circumstances, leave your batteries unattended whilst charging . 

On very rare occasions a LiPo pack may catch fire even if you’ve done everything correctly. That’s why it’s good practice to use a fireproof LiPo charging bag. I always charge my batteries in a charging bag. It’s also very convenient for transporting batteries when going to the flying field. Charging bags are inexpensive and could potentially protect your house, workshop or car from a fire. Consider them a very cheap form of home or car insurance!

I have gone into quite a bit of detail here regarding Lipos and there use so you’ve probably realised that I am recommending them for your trainer. I have been using lipos now for 8 or 9 years and never a fire. Treat them with the respect they deserve and they will give you good service over a long period.

Choosing the right Lipo

Ok, now we are ready to do some simple sums to determine the correct size of lipo for your trainer. If we go back to the page on Electric Motors we calculated the current consumption of our motor using either a 3 cell or 4 cell lipo. We worked out that with a 3 cell lipo we would need to draw a maximum 54 amps to produce 600watts of power. Alternatively, a 4 cell lipo would need to supply 40.5 amps.

3S, 4000mah, 20C lipo
3S, 4000mah, 20C lipo

 This picture shows a 3 cell lipo rated at 4000mah capacity and a discharge rate of 20C. The current rating of 4000mah indicates that under ideal conditions this battery can deliver 4000 milliamps or 4 amps for one hour or 40amps for 1 tenth of an hour (6 minutes). We know that it is capapble of handling the demand as the “C” rating of 20 tells us that it can safely deliver 4 amps x 20 = 80 amps. For maximum output we need 54 amps from our 3 cell lipo so flight duration will be approximately 40/54 x 6 = 4.44 minutes.

Now I know that this sounds like a short flight but we must bear in mind that this is maximum consumption and most of our flying will be undertaken at around 50% throttle setting. Current consumption is exponentially proportional to the throttle setting and so we can anticipate a current draw of Less than 25 amps for most of the flight. Doing our calculation again the result is 40/25 x 6 = 9.6 minutes.

In light of this we can expect our flight duration to average around 6 to 8 minutes. This is very much on a par with flight durations from an IC powered plane.

Let us now take a look at what we can expect from a 4 cell lipo with the same current rating.

4S, 4000mah, 35C lipo
4S, 4000mah, 35C lipo

We can see that apart from the “C” rating, the only other difference is that we have 4 cells. From our calculations on the Electric Motor page, because we have a higher voltage, we only need 40.5 amps to give us the maximum 600watts of power we require.

Let us round this down to 40 amps for ease of calculation. Once again, our lipo is capable of delivering 4000 milliamps or 4 amps for one hour. We know that it is capable of delivering 40 amps because the “C” rating of 35 means that our lipo can deliver 35 x 4 amps = 140 amps. by a similar calculation as above we can expect 40/40 x 6 = 6 minutes of flight duration at full throttle. Once again, knowing that we will be flying most of the time at around 50% throttle and that this setting will demand less than half the full throttle current, we can recalculate 40/20 x 6 = 12 minutes. Allowing for a few blasts of higher throttle, we can expect flights of around 10 minutes.

So what is the penalty of using 4 cells as against 3 cells? In essence only additional weight and a proportionally more expensive purchase. The extra weight should not be a problem for our trainer as the high lift wing section of our plane is more than capable of lifting a few extra grams of battery weight. The depth of your pocket is relevant to the additional expense.

My recommendation would definitely be the 4 cell version even if you go for a lower “C” rating of around 20C to save money.

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