High temperature is used in this type of test for 2 purposes. First, high temperature is recognized as a method of stressing the battery and accelerating failure. For this reason, life in test is used to predict life in the application. Second, it proves the ability of the battery to perform at the high temperature. The latter of these reasons is straight forward, but the former requires some discussion.
High temperature accelerates certain failure modes in the battery.
The most important of these failure modes is positive grid corrosion. For a given battery design, at the manufacturers recommended float voltage, it is possible to construct a graph of battery life as a function of battery temperature. The shape of this curve will vary somewhat from battery design to battery design and most battery manufacturers can provide specific data showing this relationship. In the absence of this information, a rough approximation is to assume that the life of the battery is reduced by half for each 8 to 10 C rise in temperature. Using this information, it is possible to conduct a float test at high temperature, in the range of 50 C to 80 C, and based on the life achieved in the test predict the life at reduced temperatures.