Each sensor draws a robust test current for a very short time from each cell, this results in a small momentary change in the terminal voltage. The waveshape and parameters of the change in the terminal voltage are analyzed, and from this the Vigilant can determine the cell and strap resistance. The sensors analyze the cell energy and automatically optimize the test current to maintain a safe level of test. The actual energy drawn from the cell during the test is insignificant to the battery and is not detrimental to the cell in any way.