By John Thompson, Westek Electronics Pty Ltd

Analytical testing of batteries is often dismissed as an arcane science. The facts are that with appropriate testing methods, it is possible to determine battery conditions for primary and secondary batteries. For applications such as UPS systems, testing of the battery bank can be critically important. However there are also production quality t, shelf life, and effective charging/discharging issues under real-life situations which call for testing apparatus. This short note deals with a series of battery testing apparatus providing comprehensive battery parameters information.
 
We distinguish between simple battery testers used in the service shop environment, which provide for some charge/discharge testing in addition to voltage output and those systems described here that provide programmed discharge and charging utilizing tailored protocols, impedance testing as well as specific application tests such as pulsed discharge (mobile phone services for example). Load tests obviously provide the ‘authoritative answer’ but they are often a cumbersome test—and require the taking out of service of the battery being tested. Battery impedance testing can be carried out without the necessity of taking batteries out of service.
 
The mechanisms responsible for battery deterioration include loss of active chemicals through corrosion processes and evaporation, changes in the morphology of active chemicals by way of dendritic growth and crystal formations at the electrodes, passivation of electrodes through the build-up of resistive layers, shorting of individual cells, and electrode cracking such as can occur with Lithium polymers. These various processes are often impossible to assess by design engineers who are incorporating primary or secondary batteries into their designs—yet the effective battery is often a critical parameter in customer satisfaction with the product..
 
Battery testing ideally ought to provide answers to questions including how much charge is left, whether deterioration has occurred, whether the battery meets manufacturers specifications, the latter being difficult to determine with the exception, of course, of intelligent batteries (which store original performance data on an on-board chip that also records recharge records, etc), and so forth. In terms of predictive behaviour, impedance measurements if taken consistently on a particular battery or battery cell will correlate with percentage remaining battery life. The equivalent circuit below indicates the elements making up the equivalent circuit of a cell.

Racid represents the resistance of the electrolyte, Rmetal is the resistance of grids, posts and leads, Relectr is the charge transfer resistance, C is cell capacitance, and L is aggregate inductance. The relative impedance change of this circuit can be related to remaining battery life. It might sound too good to be true but with consistent multi-cell measurements as a comparison base, predictions on the basis of impedance measurement become reliable. Thus an on-line impedance monitoring regime on a string of VSLA batteries in a large UPS installation will provide significant benefits in terms of ensuring reliable operation during power failure periods.
 
The Basytech HPS series is optimised for a power range of 100W to 30 kW. The modular concept of the power amplifier makes the system in a wide range configurable. The key features include parallel connection of channels with voltages up to 60V, impedance measurements in frequency ranges from 10 micro Hz to 100 kHz; high speed control and data acquisition to meet pulse profile requirements, from 1 to 400 independent channels per system operating under CC/CV mode, RS232 or Ethernet communication, optional environmental chamber temperature control and extensive software options.
 
All data that is used for operation of the system (i.e. battery parameters) and all data measured with the system is stored in a database system. The database system permits the use the well known SQL (SQL-92) database language. Usually the SQL language, commands are automatically generated by the BaSyTest Software.
 
The importance of a battery testing system such as the Basytech HPS system as briefly described above cannot be understated. In state of health (SOH) determinations, for example, it is generally accepted that this be fixed at the number of charge/discharge cycles required to reduce the battery energy storage capacity to 80% of its rated value. For mission-critical applications such as communication hub UPS systems, determination of SOH values for the individual batteries may well have to be part of regular internal maintenance procedures.
 
In manufacturing and distribution of secondary battery-operated power tools, quality assurance procedures may well necessitate testing regimes of batteries prior to distribution/sale of tools and such testing may also have to be done in elevated temperature environments. The Basytech HPS system lends itself to control of environmental chambers.

 

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