In most applications batteries are subjected to a complex current profile which is determined by the charging unit and the loads. Microcycles are here defined as rapid changes of the battery current direction between charging and discharging. Such microcycles are common in battery applications when both charging and supply occur over the same interval, or when reactive power is supplied through an inverter. That is the case in many different applications, from autonomous wind and/or solar systems to vehicles with generators and electric vehicles with feed back of braking power. The effect is particularly strong in systems incorporating loads with varying power such as single phase AC loads or loads with simple on/off control algorithms. Microcycle current oscillations or pulses are usually in the frequency range of about 0.05 to a few hundred Hertz. The impact on different types of batteries caused by this has not yet been thoroughly investigated. To investigate the impact of microcycles it was first necessary to build a test unit. The test equipment had to fulfil as a main requirement the generation of rectangular pulses for charging and discharging, frequencies in the range from 0.01 to 15.000 Hz and currents from 2 to 200A. In addition a special mode called 'zero Ah control' was produced. The results of laboratory tests on flooded lead acid batteries show that batteries lose capacity and require longer time and more power for charging if they have been subjected to microcycles. These effects were reversible in our laboratory environment but are expected not to be reversible in field environments, in particular with autonomous PV or electric vehicle systems. Models to explain these results are still under investigation. However, it is clear that the unequal current distribution in the electrodes caused by the ohmic resistance of the electrodes and the polarisation of the electrochemical reaction play a major rote. Therefore the observed effects are probably not limited to lead acid batteries. Concluding the work so far these is a need to take a much closer look at the fuctuation of battery current for future designs of electrical systems which use a battery as power source or sink.
The impact of microcycles on batteries in different applications
Der Einfluß von Mikrozyklen auf Batterien bei verschiedenen Anwendungen
2001
12 Seiten, 8 Bilder, 3 Tabellen, 5 Quellen
Conference paper
Storage medium
English
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