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RC Lipo Batteries Info


A lithium polymer battery, or more correctly lithium-ion polymer battery (abbreviated variously as LiPo, LIP, Li-poly and others), is a rechargeable battery of lithium-ion technology in a pouch format. Unlike cylindrical and prismatic cells, LiPos come in a soft package or pouch, which makes them lighter but also less rigid.

The denomination "lithium polymer" has caused confusion among battery users. It may be interpreted in two ways. Originally, "lithium polymer" stood for a developing technology using a polymer electrolyte instead of the more common liquid electrolyte. The result is a "plastic" cell, which theoretically could be thin, flexible, and manufactured in different shapes, without risk of electrolyte leakage. These batteries are available although the technology has not been fully developed and commercialized and research is ongoing.

The second meaning appeared when some manufacturers started applying the "polymer" denomination to lithium-ion cells in pouch format. This is the most extended use nowadays, where "polymer" went from indicating a "polymer electrolyte" to mean a "polymer casing", that is, the soft, external pouch. While the design is usually flat, and lightweight, it is not a true polymer cell, as the electrolyte is still in liquid form, albeit it may be "plasticized" or "gelled" through a polymer additive.  These cells are sometimes known as "LiPo", however, from the technological point of view, they are the same as the ones marketed simply as "Li-ion", as the underlying electro chemistry is the same.

This article concerns the second, more extended meaning among the general public.

The name "lithium polymer" (LiPo) is more widespread among users of radio-controlled models, where it may indicate a single cell or a battery pack with cells connected in series or parallel. The more general term "lithium-ion" (Li-ion) is used almost everywhere else, including consumer electronics such as mobile phones and notebook computers, and battery electric vehicles.


History


LiPo cells follow the history of lithium-ion and lithium-metal cells which underwent significant research during the 1980s, reaching a significant milestone with Sony's first commercial cylindrical Li-ion cell in 1991. After that, other packaging techniques evolved, including the pouch format now also called "LiPo". 


Working principle


Just as with other lithium-ion cells, LiPos work on the principle of intercalation and de-intercalation of lithium ions from a positive electrode material and a negative electrode material, with the liquid electrolyte providing a conductive medium. To prevent the electrodes from touching each other directly, a micro porous separator is in between which allows only the ions and not the electrode particles to migrate from one side to the other.


Charging


Just as with other kinds of lithium-ion cells, the voltage of a LiPo cell depends on its chemistry and varies from about 2.7-3.0 V (discharged) to about 4.20-4.35 V (fully charged), for cells based on lithium-metal-oxides (such as LiCoO2), and around 1.8-2.0 V (discharged) to 3.6-3.8 V (charged) for those based on lithium-iron-phosphate (LiFePO4).

The exact voltage ratings should be specified in product data sheets, with the understanding that the cells should be protected by an electronic circuit that won't allow them to overcharge nor over-discharge under use.

For LiPo battery packs with cells connected in series, a specialised charger may monitor the charge on a per-cell basis so that all cells are brought to the same state of charge (SOC).


Applying pressure on LiPo cells


Unlike lithium-ion cylindrical and prismatic cells, which have a rigid metal case, LiPo cells have a flexible, foil-type (polymer laminate) case, so they are relatively unconstrained. By themselves the cells are over 20% lighter than equivalent cylindrical cells of the same capacity.

Being lightweight is an advantage when the application requires minimum weight, such as in the case of radio controlled models. However, it has been investigated that moderate pressure on the stack of layers that compose the cell results in increased capacity retention, because the contact between the components is maximized and de lamination and deformation is prevented, which is associated with increase of cell impedance and degradation.


Applications


LiPo cells provide manufacturers with compelling advantages. They can easily produce batteries of almost any desired shape. For example, the space and weight requirements of mobile phones and notebook computers can be completely satisfied.


Radio controlled equipment and Airsoft


LiPo batteries have just about taken over in the world of radio-controlled aircraft, radio-controlled cars and large scale model trains, where the advantages of lower weight and increased capacity and power delivery justify the price.

LiPo packs also see widespread use in airsoft, where their higher discharge currents and better energy density compared to more traditional NiMH batteries has very noticeable performance gain (higher rate of fire). The high discharge currents do damage the switch contacts due to arcing (causing the contacts to oxidise and often deposit carbon), so it is advised to either use a solid-state MOSFET switch or clean the trigger contacts regularly.


Personal electronics


LiPo batteries are pervasive in mobile phones, tablet computers, very thin laptop computers, portable media players, wireless controllers for video game consoles, electronic cigarettes, and other applications where small form factors are sought and the high energy density outweighs cost considerations.


Electric vehicles


Lithium-ion cells in pouch format are being investigated to power battery electric vehicles. While it is possible to use a large number of cells of small capacity to obtain required levels of power and energy to drive a vehicle, some manufacturers and research centres are looking into large-format lithium-ion cells of capacities exceeding 50 Ah for this purpose. With higher energy content per cell, the number of cells and electrical connections in a battery pack would certainly decrease but the danger associated with individual cells of such high capacity might be greater.


Safety


LiPo cells are affected by the same problems as other lithium-ion cells. This means that overcharge, over-discharge, over-temperature, short circuit, crush and nail penetration may all result in a catastrophic failure, including the pouch rupturing, the electrolyte leaking, and fire.

All Li-ion cells expand at high levels of state of charge (SOC) or over-charge, due to slight vaporization of the electrolyte. This may result in de lamination, and thus bad contact of the internal layers of the cell, which in turn brings diminished reliability and overall cycle life of the cell. This is very noticeable for LiPos, which can visibly inflate due to lack of a hard case to contain their expansion.

Compared to cylindrical Li-ion cells, LiPos lack integrated safety devices such as a current interrupting device (CID) or a positive temperature coefficient (PTC) material that is able to protect against an over-current or an over-temperature.


Lithium cells with true polymer electrolyte


Although the name "lithium polymer" (LiPo) is mostly applied to lithium-ion cells in pouch format, which still contain a liquid electrolyte, there are electrochemical cells with actual polymer electrolytes, which however have not reached full commercialization and are still a topic of research. 


This article was adapted from wikipedia to suit our page content. For the original wikipedia page please follow the link: wikipedia article