【正文】 ethod we choose. Furthermore, improving the reliability and flexibility of a PV system, adapting to different requirements of voltage and power easily are mainly related to the structure of a control system. However, the conventional method we used can not effectively slove these problems as we expected.2 Conventional Control SystemGenerally, leafacid batteries are used to form the energy storage part of a standalone PV system. Each cell of the batteries is built up in series to obtain different voltage and power for various requirements. Adopting Single Controller shows the structure of PV system in which only one controller is used.PVarraysPVcontroller++ Single controller systemIn this system, only one controller is adopted to manage the whole battery group. It is very simple apparently. However, there are several disadvantages in this kind of system.①Low reliability. Once this controller fails, it would not charge any of the batteries. The whole system voltage will drop down to zero if no maintenance is done for a certain period, and lead to the paralysis of the whole system. In this condition, the plete batteries will be severely damage and should be replaced.②Low flexibility. Any changes of system voltage and capacity expansion of the system will cause changing of the controller. And it is hard to achieve a large capacity system in a real project with a single controller because the system current will be very high.③Bad performance to prevent voltage drifting of battery cells. As we know, voltage drifting problem is always exist in storage battery cells. The more the cells in one series, the bigger the problem is. All we know in this kind of solution is only the voltage of the whole battery group. We have no idea about what voltage that one cell or several cells are. The fact is, the voltage among the batteries are different from each other to a certain extent as time goes by,. Some will be higher and while some will be lower than others. The controller judges whether the batteries are in normal condition or not just according to the whole system voltage of the batteries, though some of the batteries may have been already damaged because of deepdischarge or overcharge. More and More batteries will be damaged if no maintenance is taken for a long period of time.To solve some of those problems, an improved control system will be shown as follows: Adopting Multiple Modularized Controllers shows the structure of PV system with multiple modularized controllers.PVarraysPV controllersPV controllersPV controllers++ Multiple controller systemIn this system, several modularized controllers are adopted to control the whole system. Each control module has the same function as the controller in single controller system showed above except the power capacity. It is more reliable than the single controller system because this system can still be operated even if more than one controller failed, for batteries can be charged by the other controllers. The modularition control method makes it easy to expand the power capacity of a PV system. But it is still hard to change the system voltage. If you want to design different voltage PV systems you have to choose different controller module with different voltage. It mendably resolves the problem① and part of problem② that exist in single controller system. However, it can not solve problem③ as well.To effectively resolve the above problems, we attempted to develop a new design for PV control system.3 Proposed Control SystemFrom both users and designers39。 point of view, the two conventional methods are not that ideal. To resolve problem①, ②and③mentioned above, here this paper puts forward a brandnew idea of design, in which the control system has a better structure and control method than the other two, that is one control unit of the system only controls one part of the batteries instead of the entire batteries.The structure of this new design is shown in . We only draw a small portion of the entire system here, and several of it constitute the system. This structure mainly resolves the voltage drifting problem and have many advantages at the same time. The proposed system differs from those used in the past in that the batteries in this system are divided into many units, each of which is controlled by a group of controllers. That is to say, each unit can be controlled by one to several controllers in parallel, depending on the capacity of batteries and PV modules. You can use 6 or 12 cells built up as one unit in general, or you can use any number of cells even one cell as one unit if you like. It mainly depends on what kind of controllers you have and what precision you want for your control system to prevent voltage drifting.The advantages of this control system are illuminated as follows:First of all, it resolves the voltage drifting problem to a great extent. The possibility that one cell is drifting is proportional to the number of cells in series. So in this control system the voltage drifting problem is many times less than in the other two conventional solutions and that is very important and beneficial to the lifetime of the batteries.Secondly, this system has high reliability which is guaranteed by the redundant design and high flexibility. You can use just one kind of PV controller you have to build up a PV system of any voltage and power you want. PVarrayscontrollerPVarrayscontrollerPVarrayscontrollerPVarrayscontrollerPVarrayscontrollerPVarrayscontroller++++ Proposed control system4 ConclusionsAccording to what we have discussed above, we can apparently conclude that the proposed control system has higher reliability, higher flexibility and better performance to the batteries of standalone PV system pared to the conventional ones. Owing to these advantages, we can not only extend the