Lithium ion polymer batteries, or more commonly lithium polymer batteries (abbreviated "Li-Poly" or "Li-Po") are rechargeable batteries which have technologically evolved from lithium ion batteries. Their light weight, high power output and slow rate of self-discharge have made them extremely popular in R/C, especially electric aircraft.
It should be noted that in R/C applications, special care must be taken in their use. While much more volatile than nickel cadmium or nickel metal hydride batteries, lithium polymer batteries can be safely used many hundreds or thousands of times over if one follows the guidelines below:
- Chargers designed for use with Li-Po batteries must be used. Failure to do so runs the risk of explosion and/or fire.
- The leads must never be allowed to short together. Immediate damage to the Li-Po will result.
- Unlike their nickel-cadmium brethren, Li-Po batteries must never be allowed to discharge below a certain point. Therefore, low voltage cutouts (LVC) on ESCs or ESC/receiver combinations must be set so as not to over-discharge (see below).
- A model involved in a severe crash should never be immediately loaded into a vehicle due to risk of fire.
The majority of lithium battery fires happen during charging, so it makes sense to charge where a fire will not spread.
- Do not charge inside a vehicle, especially a moving vehicle.
- Charging in a battery safety bag or a heat-resistant ceramic container with a loose fitting lid is recommended. Flames, smoke and gas are released if a battery "vents." Safety bags help contain fires, but will not prevent the surface underneath from igniting by conduction.
- Metal containers can be used, but ensure the charging wires cannot be cut or shorted.
- Keep batteries separated so that a fire cannot damage other batteries.
- The charging container should be a short distance away from anything flammable.
Additional safety checks will prevent the vast majority of charging fires:
- If you are using a charger that can charge more than one chemistry type, double check that the correct type is selected.
- If you are using a charger capable of charging more than one capacity of lipo pack double check that the charger current setting does not exceed the charge C rating of the battery.
- If you are using a charger capable of charging more than one voltage of lipo pack, double check that you have selected the right voltage or number cells on the charger.
There have been very few cases of batteries suddenly exploding when they are not being used, abused or charged (i.e. during transport and storage). Lithium batteries are commonly air-freighted protected by a few layers of bubble wrap and small versions are carried around in mobile phones. Should shipping an R/C pack be necessary, pack it so it cannot be physically damaged.
Some fires have been caused because a dog was attracted to the smell of a lithium battery and bit it.
If a battery's outer case is punctured, the lithium inside is highly volatile and will react violently with water. Such a pack may be disposed of by first discharging it with an automotive lamp, carefully slitting the outer skin of each cell with a razor blade or X-Acto knife and neutralizing it for several hours in saltwater. Since the chemicals are considered environmentally friendly, the pack may then be disposed of in the regular trash.
Explanation of the alphanumeric designation
The alphanumeric designation used in R/C applications refers to the number of cells in the battery and the way they are interconnected.
- "3S" denotes a three-cell battery wired in series, thus multiplying the voltage to three times that of a single cell.
- "2P" denotes two batteries connected in parallel, to give twice the current capacity.
Thus, a 3S2P battery contains 6 cells, and has three times the voltage and twice the capacity of any individual cell. All cells in a pack should be identical.
Most lipo packs currently on sale do not use parallel arrangements, and not all manufacturers now state the number of cells directly such as "3S". They may simply indicate the voltage, ie "11.1V".
An extra designation such as 25C indicates the C rating of the battery pack.
2200mAh or 2.2Ah indicates the capacity of the battery.
What's the difference between Lithium Ion and Lithium Polymer?
Lithium ion cells are cylindrical and have a rigid metal case. Lithium polymer cells have a flexible, foil-type (polymer laminate) case, but they still contain organic solvent. Since no metal battery cell casing is needed, the battery can be lighter and it can be specifically shaped to fit the device it will power. Because of the denser packaging without the holes between cylindrical cells and the lack of metal casing, the energy density of Li-Poly batteries is over 20% higher than that of a classical Li-Ion battery and approximately three times better than NiCd and NiMH batteries.
The voltage of a Li-Poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-Poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination. During discharge on load, the load has to be removed as soon as the voltage drops below approximately 3.0 V per cell (used in a series combination), or else the battery will loose capacitcy and may no longer accept a charge.
Recent design improvements have increased maximum discharge currents from two times the cell capacity to 15 or even 20 times it. In March 2005, Toshiba announced a new design offering a much faster (about 1-3 minutes) rate of charge. These cells have yet to reach the market but should have a dramatic effect on the power tool and electric vehicle industries, and a major effect on consumer electronics; especially electrically-powered model airplanes.
Some chargers can be set to charge Lithium and other types of batteries. They must be set to the li-po setting, and failure to set the correct battery type is one of the most common causes of LiPo fires. The next most common causes are incorrectly setting the number of cells in a pack, and attempting to charge a physically damaged or over-discharged pack.
As stated earlier, the lithium in a Li-Poly cell is hazardous. However, with good common sense, lithium polymer batteries can be safely handled for many cycles and just as safely disposed of. Lithium batteries used for regular E-flight typically last a couple of years and a few hundred cycles.
Many batteries, especially the larger packs now come with a second, smaller, multiwire plug for balancing. A lithium balancer can be plugged into this either during charging or afterward to ensure that all cells are at the same voltage. If a battery is not balanced, some cells may be overcharged, others may be over-discharged and the life of the pack can be reduced.
Balancing is required because lithium batteries are not automatically balanced by a small overcharge in the way nickel-cadmium batteries are.
There are many different lithium battery balancers on the market, no standard plug and no clear agreement about the "best" balancer or method of balancer. Not all balancers are compatible with all chargers, so some research may be necessary.
- Great Planes/Electrifly Li-Po page
- Kokam R/C technical specs
- Battery University is an on-line resource that provides practical battery knowledge for engineers, educators, students and battery users alike. The papers address battery chemistry, best battery choices and ways to make your battery last longer.
- LiPo Fires videos in the Utah Fliers video collection. Well worth watching to see how large a fire you have to deal with.