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Li-Ion & Li-Po & Lipo Hybrids & LiFe Batteries

2014/7/18      view:

Li-Ion & Li-Po & Lipo Hybrids & LiFe Batteries

In the RC world today, most battery packs are of the LiPo type. Here 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 and are cared for in the same way; the differences are in how the cells are packaged and the type of electrolyte that is used.


Li-Ion batteries use an organic liquid solvent as the electrolyte. This electrolyte is responsible for the ion exchange between the electrodes (anode and cathode) just like any type of battery. This organic solvent based electrolyte is highly flammable and the reason why Li-Ion batteries are more volatile and can catch fire or explode if mistreated. Li-Ion batteries are usually encased in a hard metal can (again like a more conventional battery) adding weight and not allowing many different options as far as shape and size.

Max charge voltage: 4.1V/cell

Nominal voltage: 3.6V/cell


A true LiPo battery doesn’t use a liquid electrolyte but instead uses a dry electrolyte polymer that resembles a thin plastic film. This film is sandwiched (actually laminated) between the anode and cathode of the battery allowing for 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 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 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.

Max charge voltage: 4.2V

Nominal voltage: 3.7V/cell

LiPo Hybrids

All RC LiPo batteries out there at the time of this write up (May/09) 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 electrolyte into the polymer, the ion exchange rate is improved immensely. Since the electrolyte is gelled, there is less chance of leakage, but it is still flammable. LiPo hybrids are not as dangerous as Li-Ion’s but they can still catch fire or explode if over charged, shorted, or punctured.

When first introduced, LiPo batteries were more expensive than Li-Ion because they are more difficult 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 thin carbon coated anode and cathodes in an alternating pattern on the front and back side of the polymer film.

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 sealed into the foil pouch along with the "greasy" gel/liquid electrolyte which incidentally has a very sweet solvent smell much like nail polish remover/acetone - no wonder it's flammable.

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 great deal 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 you 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!

One interesting characteristic hybrid LiPo batteries share to an extent with their dry counterparts is they do get more efficient at ion exchange once warmed up. If you have ever noticed your RC model seem to gain a little more power a minute or so after working the battery; what you are experiencing is the increase in ion exchange efficiency once the battery chemistry warms up.

This should have you thinking that if you fly your electric RC helicopter or plane in the winter time, you might want to keep your RC LiPo battery packs in a warm place prior to the flight.

Max charge voltage: 4.2V

Nominal voltage: 3.7V/cell


Max charge voltage: 3.6V

Nominal voltage: 3.3/cell

Batteries Standard charge and discharge Table

Type 1S norminal voltage(V) 1S max voltage(V) 1S minimum voltage(V) Standard charge(C) Recommend charge(C)    Max charge(C) Standard Discharge(C) Max discharge(C)
LiPo 3.7 4.2 3 0.2 0.71 12 × ×
Li-Ion 3.6 4.1 2.8 0.2 0.2~0.5 1 x x
LiFe 3.3 3.6 1.6 1 3 x x
3.2 3.65 2 2 15
NiHM 1.2 1.45~1.5 0.9 0.1 0.3 0.4 0.5 1
NiCd 1.25 1.5~1.6 1 0.1 0.3 0.4 0.5 1
PB 2 2.32.42 1.7 0.15 0.22

Customized Battery charge & Cut-off Voltage
Battery Charge Volatage LiPo: 4.18-4.3V/cell 
LiIon: 4.08-4.2V/cell
LiFe: 3.58-3.7V/Cell
NiCd: Delta peak detection 3-15mV/cell
NiMH: Delta peak detection 3-15mV/cell
Discharge Cut-off Voltage LiPo: 3.0-3.3V/cell
LiIon: 2.9-3.2V/cell
LiFe: 2.6-2.9V/cell
NiCd/NiMH: 0.1-1.1V/cell