Lithium polymer batteries
The first choice as energy storage for mobile devices.
|Nominal voltage||3.7 V nominal|
|End-of-charge voltage||4.2 V|
|Energy density||100 – 160 Wh/Kg|
|Charging rate (C-rate)||Typically 0.5 – 1C|
|Discharge rate (C-rate)||0.2 C – 3 C|
|No. of cycles||300 - 1000|
|Temperature range||-20°C ~ +60°C|
|Self-discharge||~ 3% / per month|
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It works the same way as lithium ion batteries. Therefore anything stated about lithium polymer batteries applies to lithium ion batteries as well.
One advantage is the almost infinite number of sizes since steel housings are not required. A plastic-coated aluminum foil is used instead of such housing. Therefore, very thin cells can be manufactured.
Lithium polymer cells are also a bit lighter. Moreover, the variety of different sizes presents greater freedom in the design of the final product. Custom sizes are available at relatively low production volumes so that the existing space for the battery can be utilized optimally.
The disadvantage is a higher sensitivity of the battery due to the lack of a solid housing. This must be strictly observed in the use of the battery, because mechanical damage must be avoided at all costs.
Lithium polymer cells are also negatively affected by overcharging. Therefore, lithium polymer batteries are provided with a protection circuit that protects them from overcharging. These batteries should only be charged with chargers specified for that battery’s chemistry.
Lithium polymer cells are also called soft or pouch cells.
Just like lithium ion cells, lithium polymer cells consist of a graphite electrode (negative) and a lithium metal oxide electrode (positive). The lithium metal oxide can be manganese, nickel or cobalt. Its composition influences the properties of lithium rechargeable batteries and is different depending on the manufacturer and grade. The nominal voltage of lithium ion cells depends on the electrode material and is 3.6 or 3.7 volts. The charge voltage is usually 4.2 volts.
The electrodes are separated by a separator to prevent short-circuiting between the electrodes. The separator is permeable for lithium ions. The cathode acts as a sponge. It can thus absorb a large number of ions.
There is a wide range available now, but mostly prismatic cells.
The capacity of a lithium battery is reduced over time even without use, mainly because of the reaction of lithium with the electrolyte. The rate of decomposition increases with cell voltage and temperature. A deep discharge below 2.0V can damage the battery permanently. It is therefore advisable to store such batteries at room temperature and with a charge of 60%, a compromise between accelerated aging and self-discharge.
When it is cold, the chemical processes take place at a slower pace (including the decomposition of the battery during aging) and the viscosity of the electrolytes used in lithium cells increases greatly, so the internal resistance in lithium ion batteries increases in cold conditions too, thus decreasing the usable power. Moreover, the electrolytes may freeze at temperatures of around -25 ° C. Some manufacturers specify a working range of 0-40 ° C. 18-25 ° C is ideal.
Because of the soft housing, there is an increase in the thickness of the cycles (swelling), which must be planned as part of housing design.