the voltage level of vehicle diagnostic system 28 39 s internal battery 45 internal battery 45 is used to power vehicle diagnostic system 28 when the system is disconnected from an external power source cpu 30 uses a signal on internal battery lead 44 to determine the condition of internal battery 45 if the battery 39 s voltage is low cpu 30 sends a message to display 36 requesting that the user connect the diagnostic system to an external power source cpu 30 also receives regulator harness input 46 which is received from regulator harness 48 via analog to digital converter 50 regulator harness 48 is used during testing of a vehicle 39 s electrical system it is connected to the vehicle 39 s alternator to test the performance of the alternator and is controlled by cpu 30 through control line 52 during electrical testing cpu 30 also receives positive battery terminal lead 54 and negative battery terminal lead 56 positive battery terminal lead 54 and negative battery terminal lead 56 are connected through analog to digital converter 58 to the respective positive and negative terminals of battery 60 positive battery terminal lead 54 and negative battery terminal lead 56 are used by cpu 30 to determine the voltage at the two terminals of battery 60 the two terminals of battery 60 are also connected to filter 62 which isolates a ripple found in the voltage difference between the two terminals filter 62 produces filtered signal 64 which is input to counter timer 66 counter timer 66 calculates the time between peaks in the ripple of filtered signal 64 the time between peaks is provided to cpu 30 along counter bus 68 based on the value.