The maximum power is the most direct index to evaluate the quality of battery. In order to find the maximum power, experiments can be tried again and again; as for the battery model, to establish it in advance, then to obtain the derivative. By using the relationship between function expression and function derivation under maximum power, the battery can be evaluated by the first derivative value of volt ampere function at the maximum power. Further investigation of the symbol state of the second derivative of the volt ampere function can evaluate the battery performance under the single rational function without paying attention to the maximum power condition. Based on the function of the first and second derivative of the volt ampere function in the battery evaluation, a method of battery evaluation, rooted in the battery parameters in the volt ampere function, is constructed. Via the method, the derivation and deduction of the maximum power in the model can be avoided, and the repeated search for the maximum power in the experiment can also be avoided.