Car 12v plug voltage during engine ignition - Page 1
Forum section: electronics / general

author: Cihan
author_rank: Newbie
timestamp: December 04, 2022, 09:17:54 pm
content: Did anyone measure 12v plug voltage during engine ignition? I wonder if there's dangerously high voltage fluctuation in voltage since I don't unplug phone from charger during engine start. Unfortunately I don't own an oscilloscope.

author: John B
author_rank: Frequent Contributor
timestamp: December 04, 2022, 09:38:33 pm
content: I've measured using a 100kHz bandwidth multimeter, so it may not catch high frequency spikes. Depending on the battery health it can easily drop down to 6V

author: tom66
author_rank: Super Contributor
timestamp: December 04, 2022, 10:04:21 pm
content: On my old car, it actually dropped to zero, because the BCM isolated all non-critical loads during engine start (possibly to improve start performance at low temperatures). On my vehicle (small European city car) the crank voltage dropped to about 7V at the battery as the starter solenoid engaged, then hovered around 11V while the engine was started. I didn't see any high spikes, I suspect those are more likely the closer to the battery or alternator you go, or only occur under certain circumstances like high engine revs when load is removed (so-called "load dump").

author: Circlotron
author_rank: Super Contributor
timestamp: December 04, 2022, 10:20:25 pm
content: I expect the most likely time for a spike to occur is when the starter solenoid releases and several hundred amps is interrupted. The 1 metre or so of cable from the battery to the starter solenoid would have a certain amount of inductance and I’ve seen situations where a feed connection to the rest of the vehicle is made from the solenoid terminal. There couldn’t be a worse place, I’m sure. Similar in principle to the busy end of a boost converter inductor, but with no diode feeding the following capacitor!

author: jwet
author_rank: Frequent Contributor
timestamp: December 04, 2022, 10:27:39 pm
content: Starting is really not a very tough action for loads. As others have said, either actuating the starter turns off the lighter socket or you get get the cold cranking droop which can go pretty low on a very cold morning- 6V isn't unusual.The tougher actions are what are called "load dumps" where a large load is turned off and the alternator is cranking out a bunch of current, this is where you can the big spikes. The bad loads are the AC compressor magnetic clutch, power windows and seats or a loose connection to something like headlights or a loose connection in the charging system. Each connect and disconnect causes a good spike with a lot of energy. Since about 2015, a lot of the better cars have a central suppression system, a monster tranzorb like device that is up stream of everything usually in the charging system or before the main fuse block.My best advice is to use good quality accessory adapters. Don't use the ones from the dollar store or a gas station. The good one's have some decent surge protection. Don't ruin your $1000 phone trying to save $20 on an adapter.If you're curious, there is an ISO spec numbered ISO7637 that lays out all the horrors of vehicle power distribution. The real spec costs money but there is a pretty good wiki and some other resources on line. ISO 7637 is the parent spec of some other specs that it references for charging system and trucks. Trucks are really bad.

author: Ian.M
author_rank: Super Contributor
timestamp: December 04, 2022, 10:41:19 pm
content: The LTspice circuit simulator supports simulating various ISO7637 automotive transients. Seehttps://www.analog.com/en/technical-articles/ltspice-models-of-iso-7637-2-iso-16750-2-transients.htmlEven if you don't use LTspice, the linked article serves as a quick reference to the typical transient waveforms and likely voltages your device may experience. They are *nasty*, and the only remaining issue is how frequently they may occur, which is dependent on the vehicle's usage, and design and condition of its electrical system, which can be hard to assess, but is loosely correlated with the vehicle's age and maintenance history.