What is the IR Standard for Polymer Lithium Cells?

Polymer lithium cells are actually rechargeable batteries we use for everything from consumer electronics to electric motors. One of the most critical parameters of these cells is known as the IR standard. What is the IR standard for polymer lithium cells?
Looking Inside – Internal Resistance (IR) of Lithium cells
“Internal resistance” is basically what is discovered on the inside of the battery, which is a resistance to the flow of current through the battery. Constituents such as the electrolyte contribute to this; as does the phenomenon of weight of all things material. The less material there is, the better, and the more solid the internal material remaining, the better the current flows.
With regard to polymer lithium cells, this means their efficiency is comparatively lower, mainly present in their loss of ‘translate’ and potentially ‘translate’. Therefore a good IR is essential.
Benefits of a good IR standard?
– Efficiency – applies in many ways but mainly just that the less ‘lost’ in heat, the better
– Translation benefit – A less ‘full’ cell is able to handle lesser of a charge discharged, translating to real benefits someday on the roadIn any event, ensuring that it can be charged quickly is necessary for this sort of product, and is accordingly made easier by keeping its IR low. This is particularly important in industries that benefit from fast charging times, such as manufacturers of electric cars.
Longer Battery Life: High internal resistance (IR) indicates increased wear, since more heat will be given off during both charging and discharging which will damage the battery’s internal bearings over time. Lower IR indicates less strain on the battery, and thus extends its lifespan and maintenance intervals.
What is the normal IR standard for Polymer Lithium Cells?
The normal IR standard for polymer lithium cells is dependant upon a number of other factors, including the exact chemistry of the battery, the manufacturers design as well as the applications themselves. In general Polymer Lithium Cells should have an internal resistance of less than 50 mΩ (milli-ohm) for most standard applications. For the more high end cells which we have most recently seen introduced, such as for Electric Vehicles, this IR can be much lower and often needs to be under 20 mΩ.
Common Cell Types and IR Dependence on Application:
Standard Polymer Lithium Cells: Anywhere else – for comfortable portable electronics, polymer lithium cells typically have internal resistance from 20mΩ to 50 mΩ.
High Performance Cells: In cases such as Electric Cars or battery packs for grid energy, polymer lithium cells must meet very much stricter requirements with IR values often needing to be 10 mΩ or less.
Special Purpose Multiple-Use Cells: Some special and common designs, often used in aerospace, and high power applications may have IR standards of 1 mΩ or even lower.The actual values depend on the producer and the materials used for the cell. EIS and similar tests are performed to guarantee that polymer lithium cells fulfil the IR figures required.
How is IR Measured in Polymer Lithium Cells?
The IR for lithium cells is generally measured in the course of the discharge or charge cycle. A small AC current is applied to the battery and the voltage observed, allowing the manufacturer to derive the impedance (which is internal resistance).
For polymer lithium cells, the manufacturer will generally measure the IR at the following states:
SOC – The internal resistance may vary with state of charge, so the manufacturer will often assess the IR rating at a number of state of charge figures (such as 50% or 100%).
Temperature – The internal resistance of the lithium cell is prone to significant variation owing to temperature. The IR generally increases as the battery heats, hence the importance of compensating for temperature during IR testing – particularly in the high performance (extreme) battery sector
Factors Affecting the IR of Polymer Lithium Cells
Several issues tend to affect the internal resistance of polymer lithium cells. They are;
Electrolyte composition – The sort of electrolyte used in the cell will tend to have a significant effect on internal resistance. Solid state electrolytes, for example, usually have a higher IR than conventional liquid electrolyte cells.
Age / use – As the battery ages or is cycled through charge-discharge recharge, the internal resistance will tend to increase, that being a result of wear and tear on contacts, etc.
Temperature – both extremes of temperature can elevate IR in polymer cells. At extremes, the electrolyte may break down or its viscosity altered, either of which increases the IR.
How to Minimise the IR in Polymer Lithium Cells
Manufacturers will wish to minimise the internal resistance of the polymer lithium cells to improve performance. Some methods of accomplishing this are;
Component mounting – success of the anode and cathode materials will affect internal resistance. Good conductors will help to keep the IR low.
Manufacturing Method – Alteration of the actual method of manufacturing the battery, such that material is consistently aligned and uniformly coated, may lead to a more consistent and low IR.
Battery Layout – Design of the layout of the interior of the battery; thickness of electrodes, even ‘distribution’ of electrolyte.
Summary
IR is an important standard with regard to the Polymer Lithium cell. The better the short circuiting of terminals via internal connections, then the better the battery will tend to be in most parameters, including length of charge time and, in an optimum scenario, endurance of use.
Though specific IR is relative to the application and cell itself, the trend of optimisation remains. One would want the lowest Internal Resistance for the Lithium affordable and attainable.
While designing your own product, it may help to remember that as this is a factor of construction, practically it can be modified (within depending margins). The way in which Internal Resistance varies pendant to design with other characteristics may affect those other facets, while overall, determine its suitability to task.
Understanding of IR is therefore critical to arriving at the best battery, whether for fuel economy for a new type of performance car, or simply further down the field, for a top Bluetooth speaker.