Discharging process can approximated by using a Constant Voltage Source Circuit Model as illustrated on Fig 19 below. The reason we use Constant Voltage Source circuit model, because there is a fact that at condition of near to empty, the battery (without load) still has voltage that close to peak voltage when battery at the full condition. Based on this fact, it is difficult to use capacitance model for discharging process. At Constant Voltage Source Circuit Model, VDC models Constant Voltage Source of the battery, while Resistance Rs models and represents the battery energy content. Gp represents the battery leak, which is quitely equal to G(V, T,i) function on battery charging model.
By assumption that VDC is constant, then the battery energy Ebatt can be represented by the value of RS, which represents internal series resistance of the battery at discharging process. RS can be calculated by the following simple formula:
It is requires an algorithm to calculate RS as representation of battery stored energy at real time condition. It is for reader exercise to develop thus Rs as a function of battery stored energy. The illustration of stored energy vs Rs is shown on Fig 20, below:
Figure 20. Illustration of stored energy vs Rs 
By understanding the information in this Chapter, the available information can be used to develop energy management concept for solar cell system. The main point is monitoring flow-in current to and flow-out current from batteries, which then combined with the information according to available energy in batteries, will give accurate information regarding to the energy in the system. The accurate information is required by the system when deciding integration process to keep maintain sustainability and also to conduct the business of selling and buying electric energy in the grid system.