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Bench Talk for Design Engineers

Bench Talk


Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

Batteries: Capacity, Cell Impedance, and Power Fading Lynnette Reese

Figure 1: Testing with a Fluke 500 Series Battery Analyzer. (Credit: FLUKE)

Demand for portable energy continues to grow. Rechargeable batteries are a focus of research and development because once energy is spent, the rechargeable battery can reverse the process; if energy is applied to the rechargeable battery, it becomes a self-contained chemical energy storage factory. At present, batteries are critical for electric cars, portable electronics devices that get daily use, and for storing energy obtained via renewable resources. A battery cell provides an electrochemical environment for the exchange of charge carriers at the electrodes of the battery; charge carriers are generated or discharged. The anode hosts an oxidation process where charge carriers (electrons) are let go and then flow through the electrical load, such as a portable headset. Recharging the battery forces electrons to flow in the opposite direction and create a chemical potential.

There’s a general expectation these days that a car, even an electric vehicle (EV) should operate for 10 to 30 years. Rechargeable batteries are critical for electric vehicles; therefore, it is ideal if the battery can experience numerous and repetitive charge and discharge cycles over years. However, long-term storage capacity is a challenging issue. Battery aging includes decay of total capacity, cell impedance, and capacity or power fading.

What is cell impedance?

Cell impedance, or battery impedance, gives you an idea of the available capacity of a battery. Battery impedance is internal resistance (IR) plus the reactance of the battery. The internal resistance, or total effective resistance, is a measurement of both the ionic resistance and the electrical resistance of the battery. The total effective resistance of a battery consists of ohmic resistance (a.k.a. electrical resistance) and ionic resistance. (It’s important to note here that the term “resistance” and “ohmic value” refer to pure resistance without reactance.) Ohmic resistance is the opposition to flow in an electrical circuit, and is due to the battery’s materials such as the battery jacket and terminals. Ionic resistance is due to the current flow inside the battery, and has to do with electrochemical factors such as the movement of ions, the conductivity of the electrolyte used, and the surface area of the electrode. The internal resistance of a battery is an indicator of the battery’s lifespan, not its capacity. Near the end of the battery’s life, the resistance will increase and the capacity will decrease, otherwise internal resistance stays flat. Measuring the battery’s internal resistance when it’s new will help when you think the battery might need to be replaced.

What is power fade?

As batteries go through repeated charge and discharge cycles, they start to deteriorate over time. Performance degradation begins to show in “capacity fade” as the battery can no longer store a charge. The battery also has increased internal resistance to transferring charge, and so battery voltage is reduced. Increased internal resistance is an indicator that end-of-life is near, however, it’s important to have an initial measurement of internal resistance to compare it with. In very cold temperatures. the electrochemical reactions in batteries slow down due to reduced ion movement in the electrolyte.

Battery Failure

Heat is the main cause of battery failure. To maintain rechargeable battery life, excess heat should be avoided. Charging a battery in hot conditions is less preferable. For example, charging a car’s batteries in a cool garage is preferable to charging a hot battery immediately after a long hot drive. Charge the battery in a cooler environment, if possible. Batteries live longer if a charge is maintained. Electric lawn mower batteries, for example, are going to last longer if they are periodically charged and topped off over the winter.  Overcharging and under-charging can also be a factor in shortening a battery’s life. Loose terminals will cause a battery to fail, but are easily remedied. Loose inter-cell connections can also cause a battery to fail, but are not as easy to fix. An old battery is another primary cause of failure.  

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Lynnette Reese holds a B.S.E.E from Louisiana State University in Baton Rouge. Lynnette has worked at Mouser Electronics, Texas Instruments, Freescale (now NXP), and Cypress Semiconductor. Lynnette has three kids and occasionally runs benign experiments on them. She is currently saving for the kids’ college and eventual therapy once they find out that cauliflower isn’t a rare albino broccoli (and other white lies.)

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