Car Battery Startup and Shutdown Voltage Curves

Written 2020-09-23

Tags:Battery Voltage 


About two months after I wrote this, both my battery and alternator died within a few weeks of each other. This test should be conducted again with a young battery and alternator, but I've already returned the logging meter back to my friend.

Test Setup

My car is a 2002 Toyota Highlander with either a 100 or 130 Amp alternator. The car battery was last replaced about five years ago with an auto parts store brand. I used a Mooshimeter for data logging at about roughly 1.5Hz logging rate. Of note, my car currently sometimes has a low idle, I believe due to a sticky idle air control valve.



Around 2 seconds I opened the car door and sat down. Around 7 seconds I turned the key to the on-position. Around 15 seconds I started the car, sagging the voltage significantly, then the alternator began and the voltage quickly approached 14v. I then left for the grocery store.



After unpacking the groceries, I reattached the meter, waited a few seconds, then shut off the engine completely. The voltage dropped quickly initially then recovered a little and relaxed slower. It appears that around one minute after shutdown another load was removed.

Interestingly, the voltage lowered between leaving the house and returning. This could be alternator regulation changing to float-charge, or it could be that my engine was idling lower when I returned then when I left.


This is, of course, a single data-point taken with a low sample-rate. I was simply looking to see if I could pick out the alternator easily. If you are looking to design your own vehicle-powered device, please refer to ISO 7637.