Batteries power electric
LiPo batteries (short for Lithium Polymer) are a type of rechargeable battery that has taken the electric RC world by storm, especially for planes and helicopters. They are the main reason electric flight is now a very viable option over fuel powered models.
RC LiPo batteries have three main things going for them that make them the perfect battery choice for RC planes and even more so for RC helicopters over conventional rechargeable battery types such as NiCad, or NiMH.
• RC LiPo batteries are light weight and can be made in almost any shape and size.
• RC LiPo batteries have large capacities, meaning they hold lots of power in a small package.
• RC LiPo batteries have high discharge rates to power the most demanding electric motors.
In short, LiPo’s provide high energy storage to weight ratios in an endless variety of shapes and sizes.
These benefits are important in any RC model, but for airplanes and helicopters they are the reason electric flight has become so popular. Face it, electric cars and boats have been around for decades, it wasn’t until LiPo battery technology arrived on the scene that electric planes and helicopters started showing up and are now surpassing nitro power and even turbine in terms of power to weight ratios.
There are a few down sides with RC LiPo batteries however; once again proving there is no perfect solution.
• RC LiPo batteries are still expensive compared to NiCad and NiMH, but coming down in price all the time.
• Although getting better, RC LiPo’s don’t last that long, perhaps only 300-400 charge cycles (much less if not cared for properly). That said, I have heard some people getting over 1000 cycles if all the rules are followed.
• Safety issues - because of the volatile electrolyte used in LiPo’s, they can catch fire or explode.
• RC LiPo batteries require unique and proper care if they are going to last for any length of time more so than any other battery technology. Charging, discharging, and storage all affect the lifespan – get it wrong and a LiPo is garbage in as little as one mistake.
Before I start talking about the actual care & ratings of RC LiPo batteries, I thought I should go over the basics first. Feel free to skip down the page if you don’t care about the actual make up of a lithium battery and just “want to know what the heli to do with them and what to look for when buying them”.
Differences in Lithium Ion (Li-Ion) Lithium Polymer (LiPo) batteries.
In the RC world today, most battery packs are of the LiPo type. I thought I should include a short discussion on the Li-Ion type of pack just in case you come across one as they are used in some higher end radios. Li-Ion and LiPo batteries have essentially the same chemical make-up, they both rely on lithium ion exchange between the lithium carbon cathode & anode, and are cared for in the same way; the primary differences are in how the cells are packaged and the type of electrolyte that is used.
Li-Ion
Li-Ion batteries use a flammable solvent based organic liquid as the electrolyte. This electrolyte is responsible for the lithium ion exchange between the electrodes (anode and cathode) just like any type of battery. Li-Ion batteries are usually encased in a hard metal can (again like a more conventional battery) to keep the electrodes wound up tight against the separator sheet adding weight and not allowing many different options as far as shape and size.
LiPo
A true LiPo battery doesn’t use a liquid electrolyte but instead uses a dry electrolyte polymer separator sheet that resembles a thin plastic film. This separator is sandwiched (actually laminated) between the anode and cathode of the battery (lithium carbon coated aluminum & copper plates) allowing for the lithium ion exchange – thus the name lithium polymer. This method allows for a very thin and wide range of shapes and sizes of cells.
The problem with true LiPo cell construction is the lithium ion exchange through the dry electrolyte polymer is slow and thus greatly reduces the discharge and charging rates. This problem can be somewhat overcome by heating up the battery to allow for a faster lithium ion exchange through the polymer between anode and cathode, but is not practical for most applications.
If they could crack this problem, the safety risk of lithium batteries would be greatly reduced. With the big push towards electric cars and energy storage, there is no doubt some pretty huge developments will be made in ultra light weight dry and safe LiPo’s in the coming years. Seeing that theoretically this type of battery could be made flexible, almost like a fabric, just think of the possibilities.
LiPo Hybrids
All RC LiPo batteries out there at the time of this write up (January 2013) are actually a hybrid lithium polymer battery. The correct name for this type of battery is lithium-ion polymer, but the battery world of today simply calls them lithium polymer even though they are not a true dry type LiPo battery.
By introducing a gelled organic/solvent based electrolyte to saturate the polymer separator, the lithium ion exchange rate between anode and cathode is improved immensely. LiPo hybrids like Li-Ion can still burst and catch on fire if over charged, shorted, punctured, or incinerated.
When first introduced, LiPo batteries were more expensive than Li-Ion because they are more labor intensive to manufacture. Fortunately prices have dropped substantially since they have become as, if not more popular than Li-Ion battery technology. This holds especially true for electric powered RC aircraft and the real driver behind LiPo battery research – portable communication/entertainment devices.
LiPo hybrids use the same flat cell structure as their dry counter parts meaning they have the same flexibility with sizes and shapes allowing for very specialized shaped battery packs perfect for use in our RC models.
Almost every RC LiPo battery cell is packaged in a foil pouch coincidentally called a pouch cell. The picture to the right shows a typical 2 cell LiPo RC battery pack.
Pouch cells are the perfect solution for building multi celled battery packs since the flat pouch cell can be stacked with no wasted air spaces like found within round celled battery packs. Of course since LiPo’s use this light weight pouch instead of a metal can, less weight is the result making LiPo’s the best choice over Li-Ion in a weight conscious application such as RC aircraft.
If you ever open up a LiPo foil pouch cell, this is what you will find. A long piece of very thin plastic film (the polymer) with the thin lithium carbon coated aluminum & copper anode & cathode electrodes laminated in an alternating pattern on the front and back side of the polymer separator film. The works will be saturated with the greasy solvent based organic electrolyte.
This long film (over 7 feet long in the case of this 5000 mAh cell), is then folded accordion style back and forth upon itself. The entire folded cell matrix is then heat sealed into the foil pouch along with the gelled electrolyte which incidentally has a very sweet solvent smell much like nail polish remover/acetone.
If you're wondering what the burnt hole is in the center of all the cell folds, I purposely drove a nail through this cell to discharge it rapidly & watch the fireworks. The cell rapidly ballooned out, burst, and vented a fair amount of flammable electrolyte but never caught on fire. On the positive side, if it would have burst into flame, I wouldn't have this picture to show the "guts". I only did this because I dropped this heavy 6S 5000mAh LiPo pack on the hard concrete floor (yes - very dumb & costly butter finger moment) and one cell was damaged in the process. Lesson learned, don't carry more LiPo's than you can safely hold!
Here's a good video of the processes involved in manufacturing LiPo cells.
