There is a need for rapid and accurate ways to test the state of health (“SoH”) of electrochemical storage batteries. Consider, for example, the situation experienced by a company who uses a large number of two-way radios, cellular telephones, or other devices powered by rechargeable electrochemical batteries. The batteries in such devices degrade as they age. It is generally not viable to wait to replace each battery until it fails completely. This would interfere with the overall reliability of the equipment powered by the batteries. On the other hand, replacing batteries prematurely represents a significant unwarranted cost. It is therefore desirable to monitor the SoH of batteries so that the batteries can be replaced at an appropriate point well before complete failure. According to the IEEE 450 standard, which applies to vented lead-acid batteries used for standby operation, a battery should be replaced when it has degraded to the point that its capacity is 80% of the capacity that the battery had when new. Other standards and guidelines recommend replacement schedules for batteries of other types.
Electrochemical impedance spectrum analysis methods may be used to test electrochemical batteries. Such methods excite a battery under test with waveforms having various frequencies and monitor a response of the battery to the excitation waveforms. Information about the condition of the battery can be derived from relationships between the exciting waveform(s) and the response(s).
Huet F., A review of impedance measurements for determination of the state of charge or state of health of secondary batteries, Journal of Power Sources 70(1998), 56–69 discloses that the response of a battery to a single exciting waveform having a frequency in the range of 83–90 Hz can be used to assess the state of health (“SoH”) of a battery. Various investigators have determined that such methods do provide reasonably reliable estimates of battery capacity for batteries which have capacities in the range of 1% to 75% of their nominal values. Such methods have been found to be unreliable for measuring capacities of 80% or more (Noworolski, Z. et al., Can a battery ohmic tester distinguish a good cell from a pool of better ones?, Telecommunications Energy Conference 2002; and, Feder, D. O., Performance measurements and reliability of VRLA batteries, Proceedings of the 1995 INTELEC Conference).
It can be seen that current testing methods have serious problems because such methods are least accurate in the range above 80% capacity, which is the same range in which the greatest accuracy is desired. Ideally a test would permit accurate identification of those batteries which have degraded to a replacement threshold (typically about 80% of their nominal capacities) in a group of batteries which have capacities in excess of the replacement threshold.
Some investigators have suggested that improved accuracy in SoH measurements might be achieved by performing spectroscopy at low frequencies. However, the use of lower frequencies increases the time required to perform each test. The time taken for each test is at least several periods of the excitation waveform.
There is a need for rapid and accurate methods for determining the capacities of electrochemical storage batteries.