# Why 13.8 V not 13.7 or 13.9?

Discussion in 'Electronic Design' started by Harry, Jun 14, 2007.

1. ### HarryGuest

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?

-- Harry

2. ### James BeckGuest

Nominal voltage of the lead acid battery.

3. ### GibboGuest

No it's the nominal *charge* voltage. The "at rest" voltage of a lead
acid battery is nowhere near that high.

4. ### James BeckGuest

OK, it is the nominal "float" voltage of a lead acid battery.
Feel better?

5. ### GibboGuest

Not especially so. Just that your first answer was wrong. Dress it up
with sarcasm however you like. Doesn't alter anything.

6. ### John PopelishGuest

To hold a 6 cell lead acid battery at full charge, you have
to apply 13.8 volts to it. The alternator is set to hold
about that voltage, to charge and maintain the battery at
full charge.

When it is not being charged but supplying a load (like when
then the output voltage stays near 12 volts for a
significant part of the discharge time.

7. ### Meat PlowGuest

Charge voltage in an auto should be 14.4.

8. ### GibboGuest

If only it was that simple.

9. ### EeyoreGuest

13.8V / 6 cells = 2.3V / cell, the nominal 'on charge' voltage.

Graham

10. ### Roger HamlettGuest

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
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...
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

11. ### Stanislaw FlattoGuest

Meat Plow wrote:

At what temp? (K, C, R, F) chose one, then define concentration of acid
in ppm and purity of solvent, then give us a number(s).

Do I hear anything???

Have fun

Stanislaw.

12. ### ian fieldGuest

One battery I had showed a label "charge voltage not to exceed 14.4V", IIRC
the Optimate battery conditioner float charges at 13.6V to minimise
electrolyte loss due to gassing.

13. ### Rich GriseGuest

But it _is_ the typical float voltage. Neither of them is "nominal" - that
just means "by name", i.e., the nominal voltage of a 12V battery is 12V,
even though the actual voltage is usually different from that.

As to why it's 13.8 instead of some other value is because that's the way
the chemistry works out.

Hope This Helps!
Rich

15. ### Rich GriseGuest

That's a non-answer. Where does the 2.3 come from? ;-)

Spoiler: It's the chemistry. ;-)

Cheers!
Rich

16. ### JamieGuest

your vehicles battery is actually less than that,
aprox 12 volts. The 13.8 is needed to put charge in the
battery.. The alternator (Charging system) has it's own
voltage regulator to maintain the 13.8 volts
The voltage has been selected as the standard to use with
common lead acid type batteries in this voltage range to put in a
charge, but not so that it'll damage the battery.

17. ### mcGuest

12.6 fully charged, 12.0 about half discharged...
When the car is running (and charging the battery), that is the normal
voltage of the electrical system.

18. ### JackShephardGuest

If you tried to charge a car battery with 12V, you would get NO charge.
In order to charge a battery, one must overcome the internal resistance
of the battery and actually FEED electrons into it.

The 13.8 volt figure does that, and does so without too much charging
current, as that must also be considered. One would not want one's
battery to go BOOM!

19. ### JackShephardGuest

The MeatTard strikes again.

Wrong, Slug-o!

When your IQ gets above that of a slug, come back and try again.

20. ### Brandon D CartwrightGuest

I don't know about that.I guess it depends who you are.

There are more than a few folk who would be pleasantly surprised if