Study on battery charging methodStudy on battery charging method
Abstract: according to the characteristics of the battery, the polarization phenomenon of Yuasa battery charge and discharge process, proposed and analyzed several charging mode, and its development prospects.
Battery due to its low manufacturing cost, large capacity, low cost and widely used. However, if used improperly, its life will be greatly shortened. Many factors affect the life of the battery, and the charging right, can effectively extend the service life of the battery.
The results show that the battery charging process has the greatest impact on the battery life, and the discharge process is less affected. That is to say, most of the battery is not bad, but "bad charge". Thus, a good charger plays an important role in the service life of the battery.
1 theoretical basis of battery charging
In mid 60s, the charging process of American scientist Maas to open the battery for a large number of experimental studies, and put forward with the lowest gassing rate as the premise, the battery charging curve is acceptable, as shown in figure 1. Experiments show that if the charging current varies according to this curve, the charging time can be greatly reduced, and the capacity and lifetime of the battery are not affected. In principle, this curve is called the best charging curve, which laid the research direction of fast charging method, [1, 2].
Fig. 1 optimum charging curve
As you can see from Figure 1: the initial charging current is very large, but it decays very fast. The main reason is that polarization occurs during charging. In the sealed battery charging process, generated oxygen and hydrogen, when oxygen can not be absorbed, will accumulate in the positive plate (positive plate to produce oxygen, to increase the internal pressure of the battery), the battery temperature rise, while the positive plate area is reduced, the internal resistance increased, the polarization phenomena appear in the so-called.
Battery is reversible. The chemical reactions of discharge and charge are as follows:
It is obvious that the charging process and the discharge process are reciprocal reactions. The reversible process is the equilibrium process of thermodynamics. In order to ensure that the battery can always be charged under equilibrium, the current must be reduced as much as possible through the battery. The ideal condition is that the applied voltage is equal to the electromotive force of the battery itself. However, practice shows that, when charging the battery, the applied voltage must be increased to a certain value for the numerical and as electrode materials, various factors concentration difference in different extent than the equilibrium value of the battery electromotive force. In chemical reactions, the phenomenon that the electromotive force exceeds the thermodynamic equilibrium is polarization.
Generally speaking, there are 3 reasons for polarization.
1) in the process of ohmic polarization, positive and negative ions move towards the poles. In the course of ion migration, a certain resistance is inevitably called ohmic internal resistance. In order to overcome this internal resistance, additional voltage must be applied to overcome the resistance and promote ion migration. The voltage is converted to the environment in a thermal manner, giving rise to so-called ohmic polarization. As the charging current increases dramatically, ohmic polarization will cause high temperatures during charging.
2) when the concentration polarization current flows through the battery, in order to maintain the normal reaction, the most ideal condition is that the reactant on the electrode surface can be replenished in time, and the product can leave in time. In fact, the diffusion rate of the reactants and reactants is much slower than that of the chemical reaction, resulting in a change in the concentration of the electrolyte solution near the plate. That is to say, from the electrode surface to the middle solution, the electrolyte concentration distribution is uneven. This phenomenon is called concentration polarization.
3) electrochemical polarization is caused by the rate of electrochemical reaction on the electrode, which lags behind the speed of the electrons on the electrode. For example: before the negative discharge of the battery, the electrode surface has a negative charge, the solution near it has a positive charge, and the two are in equilibrium. When discharged, an electron is released immediately to an external circuit. The surface of the electrode negative charge is reduced, and the oxidation of metal dissolution was slow to Mee Me, not to add reducing electrode surface, surface charge change of the electrode. This decrease in negative surface charge state promoting electrons leaving the electrode metal, metal ions into the solution Me, Mee, Me accelerated reaction. There is always a time to reach a new dynamic balance. However, compared with the pre discharge, the number of negative charges on the electrode surface decreases, and the corresponding electrode potential becomes positive. That is, the electrochemical polarization voltage increases, thus seriously hindering the normal charging current. Similarly, when the positive discharge of the battery, the number of positive charges on the electrode surface decreases, and the electrode potential becomes negative.
These 3 polarization phenomena are serious with the increase of charging current.
2 Research on charging method
2.1 routine charging method
The conventional charging system is designed according to the internationally accepted rules of thumb before 1940. The most famous is "ampere hour rule": the charging current in amperes, should not exceed the battery to be charged ah. In fact, the speed of conventional charging is limited by the temperature rise of the battery during charging and the generation of gas. This phenomenon is of great importance to the minimum time required for battery charging.
Generally speaking, there are 3 conventional charging methods.
2.1.1 constant current charging method
The constant current charging method is a method of charging by adjusting the output voltage of the charging device or changing the resistance in series with the battery to keep the charging current constant