Why car battery voltage varies so much?

Can be 11V when low up to 14-15 when engine running. What gives? How do all the doodads respond over such a big range?

by SJohnsson
November 21, 2016

The terminal voltage of a fully charged lead-acid battery with the engine not running, is roughly 12V. The lead-acid battery has an internal resistance that increases as the battery discharges, due to battery chemistry. Under a constant current load, the battery terminal voltage gradually decreases as the battery discharges. This is due to the gradually increasing voltage drop across that gradually increasing internal resistance.

With the engine running, the alternator generates a higher voltage (14V-15V) to force current back into the battery to reverse the discharge chemistry.


by raysbowen
December 04, 2016

2 Answers

Answer by febb

1) Because of users negligence 2) Because of engineers care :)

+2 votes
by febb
November 22, 2016

Definitely agree on #2 :)

by mrobbins
November 22, 2016

Answer by mikerogerswsm

When the engine is running the alternator is applying a greater voltage to force current into the battery and charge it up. All the doodads have to be designed to cope with this voltage change and with spikes and transients. Car electrics are a really rough world, not at all like regular smooth dc. Incandescent lamps manage gracefully all on their own. LED lights have their own current control circuitry. Electric motors may have intrinsic electromagnetic compensation so they run at constant speed or torque regardless of voltage. Electronic units usually have built-in filters and regulators to drop the voltage to a carefully controlled level. In the early days of electronic ignition the lessons were learned the hard way and solutions are well known now. Every piece of electronics has to meet SAE specifications and includes suitable protective circuitry to guard against being over stressed by the poor quality of the vehicle battery supply. End of rant.

+2 votes
by mikerogerswsm
December 03, 2016

Mike, thank you for sharing "car electrics" world insights! :)

By the way, I have been using Chevy Caprice for 15 years. Only inginition coil was blown one time. I was under impression, that there are no such thing like "gentle electronics" inside this beast. :)

I feel that America was truly great, engineering such wonderful dependable cars! :)

by febb
December 03, 2016

.....while we gave you the MG sports car with electrics by Joseph Lucas, Prince of Darkness!

by mikerogerswsm
December 05, 2016

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