Harry said:
What is the votage of the battery in a car? 12 V right?
Then why is Cigar-Lighter Adaptor 13.8V ?
Also, how the 13.8 (not 13.9 or 13.7) V is derived?
The '12v' battery, has a lot of different voltages associated with it.
For normal 'float' charging (in your car, with the engine running, and the
battery already reasonably charged, assuming it has a modern alternator
that doesn't keep trying to push too much voltage into the battery), the
cells need about 2.3v/cell (13.8v).
Fast charging, can use up to nearly 2.5v/cell (14.8v). Cars commonly used
14.5v, and older electrics on a car often charged all the time at this
higher voltage - which is why the electrolyte needed topping up so
often...
Idle, without significant load, typically about 12.9v.
Then during starting, down to under 7v momentarily, when 'cold cranking'.
'12v', is just a figure used to distinguish this system (with 6 cells),
from systems using 12 cells (lorries - nominally 24v, but up to 29v in the
real world), and ones using 3 cells (some older motor bikes). It bears
little real resemblance to the real figure seen in the car...
Best Wishes