In most applications, batteries can be discharged only to a certain point without damaging them or significantly reducing their lifetime. These are often referred to as low-rate applications. Starved-electrolyte sealed-lead battery voltages typically remain relatively constant until they reach the knee of their discharge curve where they then drop off quite rapidly.
Capacity Measurement
A Battery discharger measures the capacity of a cell or battery by measuring how much energy it delivers to the load. This can be done in various ways. One method is to measure the discharge current and integrate it over time. However, this approach is not as accurate as a pulsed discharge method since the actual discharge current may vary from one measurement to the next.
A more precise technique involves using the voltage plateau of the Peukert curve to estimate capacity. This allows for a more accurate assessment of performance by ignoring discharges that occur before or after the peak of the voltage plateau.
Another way to measure capacity is to use a dual estimation method that utilizes two KFs, one for SoC and the other for battery capacity. This approach can be less demanding on computational resources than a joint estimation method, although it still requires the battery model to be adaptive. Moreover, the two KFs must have different weights to provide accurate results.
Battery Life Testing
Battery testing can be a complex task, ranging from the characterization of the smallest cell used in portable devices to the evaluation of Lithium-ion battery packs for electric vehicles (EVs). Automated test equipment, environmental chambers, and data acquisition systems are crucial to performing tests that meet stringent quality standards.
Cycle testing reveals how a battery performs under different conditions, and provides insight into the factors that influence its useful life. As shown in Figure 4-48, for example, changing the charge voltage for a 16-hour battery cycle can dramatically decrease life projections. Likewise, if the battery is designed to deliver power in a continuous discharge for an extended period of time, then the capacity degradation curve must be traced to ensure that the battery will meet the required performance level. This is done by charging and discharging the battery at varying rates, measuring the open circuit potential with a Keithley SMU with pulse capability, and analyzing the data with a digital multimeter.
Self-Discharging Current Rate Measurement
The rate at which lithium ions move across the electrodes in battery cells is a major factor influencing early capacity trajectory and subsequent decline. Distinguishing reversible from irreversible capacity loss is essential to battery life testing, but can be complicated by the effects of polarization and anode overhang.
The best way to measure a battery’s self-discharge is to directly measure its internal self-discharging current, called ISD. This method takes less time to evaluate the battery than analyzing the change in its open-circuit voltage (OCV) over time.
This is accomplished by connecting the battery to a low-noise, highly stable DC power supply. The power supply maintains the battery at a constant voltage and SOC, while measuring its ISD with a microammeter. The measured value is then displayed to the operator, allowing for fast characterization of the cell.
Temperature Range
Temperature is one of the major factors that influence battery performance and lifespan. High temperatures increase internal heat generation and reduce the capacity of batteries. While low temperatures slow down chemical reactions and make it difficult for the battery to perform its intended function.
Considering the importance of temperature in batteries, several methods have been developed to estimate the battery internal temperature. Those based on impedance acquisition and real-time measurement are more preferred due to their low calculation cost and high accuracy.
They can establish empirical correspondences between the imaginary part, real part or phase change of the battery’s impedance and its bulk temperature. Some of them also use simplified thermal models to estimate battery internal temperature distribution. However, they still have to work out a balance between accuracy and computational efficiency.
The Bottom Lines
Battery dischargers help protect batteries from over-discharge, maintain optimal storage voltage, balance battery cells, and test battery performance. Read our Battery discharger Buying Guide to find the best option for your needs. This HTRC LiPo battery discharger is one of the best picks for buyers on a budget since it offers a decent power rating, voltage range, and even includes a 1 year long warranty.
