13. Making Battery Quick-Test Feasible
When Sanyo, one of the largest battery manufacturers in the world, was asked, “Is it feasible to quick test batteries?” the engineer replied decisively, “No”. He based his conclusion on the difficulty of using a universal test formula that applies to all battery applications, — from wireless communications to mobile computing, and from power tools to forklifts and electric vehicles.
Several universities, research organizations and private companies, including Cadex, are striving to find a workable solution to battery quick testing. Many methods have been tried, and an equal number have failed because they were inaccurate, inconsistent and impractical.
When studying the characteristics relating to battery state-of-health and state-of-charge (SoH and SoC, respectively) some interesting effects can be observed. Unfortunately, these properties are cumbersome and non-linear, and worst of all, the parameters are unique for every battery type. This inherent complexity makes it difficult, if not impossible, to create a formula that works for all batteries.
In spite of these seemingly insurmountable odds, battery quick testing is possible. But the question is asked, “how accurate will it be, and how well will it adapt to continuously changing battery chemistries?” The cost of a commercial quick tester and the ease-of-use are other issues of concern.
13.1 Battery Specific Quick Testing
The secret of battery quick testing lies, to a large extent, in understanding how the battery is being loaded. Battery loads vary from short current bursts for a mobile phone using the GSM protocol, to long and fluctuating loads on laptops, and to intermittent heavy loads for power tools.
Because of these differences in loads, a battery for a digital mobile phone should be tested primarily for low impedance to assure a clean delivery of the current bursts, whereas a battery for a notebook should be examined mainly for the bulk in energy reserve. Ultra-low impedance is of less importance here. A battery for a power tool, on the other hand, needs both — low impedance and good power reserve.
Some quick testers simulate the equipment load and observe the voltage signature of the battery under these conditions. The readings are compared with the reference settings, which are stored in the tester. The resulting discrepancies are calculated against the anticipated or ideal settings and displayed as the SoH readings.
The first step in obtaining quick test readings is measuring the battery’s internal resistance, often referred to as impedance. Internal resistance measurements take only a few seconds to complete and provide a reasonably accurate indication of the battery’s condition, especially if a reference reading from a good battery is available for comparison.
Unfortunately, the impedance measurement alone provides only a rough sketch of the battery’s performance. The readings are affected by various battery conditions, which cannot always be controlled. For example, a fully charged battery that has just been removed from the charger shows a higher impedance reading than one that has rested for a few hours after charge. The elevated impedance is due to the increased interfacial resistance present after charging. Allowing the battery to rest for an hour or two will normalize the battery. Temperature also affects the readings. In addition, the chemistry, the number of cells connected in series and the rating of a battery influence the results. Many batteries also contain a protection circuit that further distorts the readings.