Car battery peak current?
L. West
What is the peak current demand on a 12V auto battery?
Does this occur at the moment the starter motor is first
engaged?
Will measuring the resistance across a starter give a
reasonable value from I = V/R? (I don't know R yet.)
........................................................
Alan R. Winstanley
In article <33223A...@engr.uark.edu>, "L. West" <L...@engr.uark.edu>
writes
last time this came up, we reckoned something like 200-300 amps -ish,
peak.
--
Alan Winstanley email: al...@epemag.demon.co.uk Views are my own.
- Everyday Practical Electronics Magazine - In our 26th year!
Wimborne, Dorset, BH21 1PF, United Kingdom
http://www.epemag.wimborne.co.uk
Terry King
<< What is the peak current demand on a 12V auto battery?
Does this occur at the moment the starter motor is first
engaged?
Will measuring the resistance across a starter give a
reasonable value from I = V/R? (I don't know R yet.)
In a 12V Automotive system, the inrush current is from 200A to 400A,
as I understand. (There is a 2:1 or more ratio between the heavy-duty
starter on a big V8 engine, and the starter on a small 4 Cylinder
engine). There is some Inductance to the starter motor but I don't
think it is enough to slow the rise of current appreciably.
You can measure the resistance of the starter motor by using a known
low-value resistor in series with it, passing a known current thru it,
and measuring the voltage drop across the motor. You are making a
big low-value '4-wire' ohmmeter when you do this.
I have measured the internal resistance of big circuit breakers,
100A fuses, power wiring, terminal strips, lugs etc. this way.
What You Need:
A. A small value, fairly high-power RESISTOR to supply current thru,
OR a variable power supply that will supply a short-circuit current
of 10A or so without damage.
A resistor value of .5 (that's 1/2) ohm with a power rating of, uh,
100 watts would work. (Hard to find, but available).
OR: Use one or two 12V headlights in parallel. They will draw 8 to
20 amps, and be OK with the power dissipation!.
B. A modern Digital MultiMETER that has a 200Mv or so scale, so that
you can make meaningful measurements of 20 to 50 Millivolts.
C. A DC AMMETER (Reads Amps!) of 10 to 50 amps full scale. You can find
these in many auto accessory stores and Marine supply stores.
Connect the Multimeter across the Starter Motor (Or circuit breaker, or
switch contacts, or piece of suspect wire etc: whatever you want to
measure).
Connect the resistor (or headlamp) from the + side of the 12V battery
or power supply, thru the Ammeter, to the load (Motor etc).
You have a series circuit that will put a fairly high
(10 Amps or more) current thru the device you are testing.
Don't leave it connected longer than you need to measure the values!
Note the CURRENT thru the device, and the VOLTAGE across it. Now
Just Do Ohms Law:
Example: 10 AMPs, 50 Millivolts (.05 V) so: R=E/I, R=.05/10 = .005 OHMS
Which is usually written as 5 Milliohms.
OK?? MOST of the voltage drops across the 'intentional resistor'
which sets some reasonable current. The digital multimeters we have
today can measure a few millivolts well.
Another example: the current is some odd value:
17 amps. and .12 volts: R=E/I R=.12/17 R= .007 Ohms.
NOW: You can figure the INITIAL INRUSH CURRENT of your starter motor!
Say you measured .05 Ohms (50 Milliohms) Then:
I=E/R (if your battery is good): I= 12/.05 I= 240 Amps
What Really Happens when you start the car with the usual circuit:
In the first few 1/10's of a second,
the starter motor starts to spin, the battery voltage drops under load
to 10V or so, and then the current starts to drop rapidly as the motor
speeds up and generates a lot of "Back EMF'.
IF you REALLY needed to know just what was happening in
detail OVER TIME, like Ford or GM Engineers, you'd need some computer
based voltage and current measurements that could take a lot of samples
over a couple of seconds and print and graph it all.
(Hey, are you sorry you asked??)
Regards,
Terry King
..In The Woods In Vermont
Richard Torrens
>
> What is the peak current demand on a 12V auto battery?
> Does this occur at the moment the starter motor is first
> engaged?
> Will measuring the resistance across a starter give a
> reasonable value from I = V/R? (I don't know R yet.)
We actualy use cat starter motors for testing some of our controllers.
It takes 35 amps or so before the motor even starts to move.
The motor will easily take an inrush curent of 200 amps or more.
The initial current surge is limited by circuit inductance: as it starts to
build up the motor will start to turn and the current will quickly peak.
--
/| Richard Torrens
/ |
/ | 4 Q D - 4...@argonet.co.uk
/ | |
/ /| | We manufacture
/ / | | MOSFET controllers for battery operated motors
/_/__| |____
/_____ ____\ See us on http://www.argonet.co.uk/users/4qd
/ _ \| | _ \ our www site also contains FAQ sheet on motors & controllers
| | | | | | | | and a selection of interesting circuit diagrams
| |_| | | |_| |
\__\_\ |____/ We use an Acorn StrongARM RISC-PC 32 bit computer.
JB
L. West wrote:
>
> What is the peak current demand on a 12V auto battery?
> Does this occur at the moment the starter motor is first
> engaged?
> Will measuring the resistance across a starter give a
> reasonable value from I = V/R? (I don't know R yet.)
>
300-800A...?
Yep.
Yep.
<<JB
Chip McCoy
compression ratio, viscosity of oil, etc. On the average domestic
automobile the "average" current draw at start up is approximately 150
to 250 Amps.
Chip McCoy
Supervisor of Advanced Service Engineering
Delco Electronics
JB
JB wrote:
>
> L. West wrote:
> >
> > What is the peak current demand on a 12V auto battery?
> > Does this occur at the moment the starter motor is first
> > engaged?
> > Will measuring the resistance across a starter give a
> > reasonable value from I = V/R? (I don't know R yet.)
> >
> 300-800A...?
> Yep.
> Yep.
>
> <<JB
The "startup current" may be calculated from coil resistance.
<<JB