optimal testing strategy

Question

Here I have a problem in understanding the optimal testing strategy.

In a scenario, You have been given two laptop batteries and a testing device. The testing device will make the battery connected to it (one battery can be connected at a time) deliver x mA, where x is an integer. The testing device is constructed in such a way that the battery will always explode if there’s any risk of explosion at that current.

Now we want a program that receives inputs. Input consists of multiple lines, one line per case (at most 200 test cases). Each line contains a single positive integer n between 1 and 4711 inclusive, giving the current at which the battery is known to explode. Input is terminated by a line containing 0, which should not produce any output.

And produce Output, where for each case, print one line containing the worst-case number of tests to perform when using an optimal testing strategy.

|-----------------|------------------|
|  Sample Input   |  Sample Output   |
|--------- -------|------------------|
    1                0
    2                1
    10               4
    23               7
    0    

This was a programming contest question marked as an easy question. but I do not get the definition of the optimal testing strategy. What can be the boundaries? Generally what this question means?