# car battery recharging question

Discussion in 'Electronic Design' started by Kooner, Nov 1, 2003.

1. ### KoonerGuest

My car battery measures 12.4V when the engine is off. When the engine
is running the battery measures 14.4V. Why is there a 2V difference?
Even if the battery internal resistance is 0.01ohms and the trickle
current is 0.1A that would give a voltage drop of 0.001V. So why don't
I measure 12.401V when the car is running? What is the schematic
representation of a battery? Can anyone recommend a good book on
rechargeable batteries? Thanks,
Kooner

2. ### Jim ThompsonGuest

Lead acid cells have a tremendous TC.

Cold cells can require 15.1V to charge, nominal temperatures require
typically 14.6V, hot is 13.3V.

(Numbers give on my website.)

...Jim Thompson

3. ### John WoodgateGuest

I read in sci.electronics.design that Jim Thompson
Yes, but the overvoltage when charging is not primarily a thermal
effect. The battery doesn't only have its linear resistance and off-
charge e.m.f; it also has 'partial' or 'contact' voltages (AKA 'redox
potentials') at each plate when being charged. These are the
consequences of the chemical reactions that are taking place at the
plates, even at trickle charge rates.
The equation is a negative exponential (Nernst equation, IIRC), like the
Vbe equation of a bipolar transistor.

4. ### Kevin McMurtrieGuest

No battery is 100% efficient. More power goes in than comes out.

14.4V is a typical charge voltage for cycle-use lead-acid.

5. ### Tim ShoppaGuest

The representation you are using - a "perfect battery" + internal resistance -
is certainly the most common. But it's not perfect, and nobody will
pretend it's perfect. I think your model for the internal resistance
is
too low for anything but the larger truck/marine batteries - a 0.01
ohm internal resistance would imply that the battery would deliver 1200 Amps
to a dead short, and that's too high by a factor of several for a small
car battery. That's not to say that dropping a wrench on the battery
terminal is a safe activity !

At 14.4V the battery is not being trickle-charged. The alternator
is probably putting more than 10 Amps into it at that point. To further
complicate things the alternator doesn't produce flat DC but instead a
very highly pulsing DC.

Some modern alternator voltage regulators will fall down to a
"trickle charge" after a start, but the classic alternator VR designs
know no such concept. As the electrolyte boils away, that's why you have
to add water... It's not the kindest thing to do to a storage battery
but they're built to take this kind of abuse.

Tim.

6. ### Jim ThompsonGuest

On 1 Nov 2003 14:53:44 -0800, (Tim Shoppa)
wrote:

[snip]
The biggest problem is that the auto manufacturers, particularly
Detroit are CHEAP. Not a single alternator regulator actually senses
the battery voltage directly. So wiring harness drops determine
whether you have an over- or under-charged battery.

...Jim Thompson

7. ### GPGGuest

Alternators are generally wired direct to the battery with a (non adjustable)
internal regulator. Old systems had an external mechanical reg, similar to dc
generator but no cutout relay (not needed)and could be adjusted.

8. ### JeffGuest

My car battery is rated for 1000CCA (garanteed amperage after 10 seconds and
not dropping below 7.2V at something like 0 deg C) and 1250 CA (garanteed
amperage after 10 seconds and not dropping below 7.2V at about room temp) ,
granted it's slightly bigger then some of those motor cycle sized batteries
used on some imports. Short circuit current would be much higher. With my
battery at room temp, 0.0038 ohms of resistamce is calculated, however this
would be somewhat higher then the intial resistance. 3000 - 5000 A would be
the intial SC current.