Re: Current Shunt
- From: Ross Herbert <rherber1@xxxxxxxxxxxxxx>
- Date: Thu, 28 Feb 2008 02:13:15 GMT
On Tue, 26 Feb 2008 13:41:05 -0600, Chris W <1qazse4@xxxxxxx> wrote:
:I was considering putting a current shunt in my car to monitor current
:draw from the battery. I found a 200 A current shunt and was wondering
:if that would be enough. Obviously during normal operation I'm not
:going to be pulling near that. But during start I could draw
:significantly more than 200 A. The shunt I am looking at is ....
:
:http://www.allelectronics.com/cgi-bin/item/SNT-200/search/200_AMP_SHUNT,_50MV=200_AMPS_.html
:
:I'm guessing since it is rated at 200A, it can handle 200A continuously.
: If it can, it doesn't seem like short bursts of high current during
:start would cause it to heat up too much to cause any problems. If the
:200A shunt isn't going to handle the start current, I guess I could find
:a way to have it measure current for everything but the starter motor.
:Then I could get away with a 100 A shunt.
I don't see that any useful information can be gained by trying to measure the
starter current. The shunt would have to be installed in series with the main
cable to the distribution fuse box.
When you think about it what useful information can be had by installing an
ammeter in a vehicle used for domestic journeys anyway? It's not as though you
are going to be continually glancing at the dancing pointer to keep yourself
ammused while driving, because it doesn't really tell you a lot.
If your electrics develop a fault which drains the battery an ammeter is
probably not going to help in finding the cause. At best it will tell you if
your alternator is charging the battery or not, or if there is a short circuit
to chassis which drains the battery - and you will probaly only get an
indication of this if you take the time to look at the ammeter when switching
off the ignition. If an alternator fault occurs while driving you probably won't
be in any situation where it can be fixed instantly so knowing how many amps
your battery is discharging is not really helpful. The possibility of either of
these situations occurring during the lifetime of a vehicle is next to zero so
vehicle manufacturers figured correctly that all the driver really needs is an
alternator warning indicator.
In all the cars I've owned since the mid 50's (only 5) only one has had an
electrical problem or developed a situation where an ammeter may have helped,
and that was caused by a body repairer. Somebody had pranged my rear end and the
trunk lid needed repairing. The repairer had to slightly straighten one of the
trunk lid support struts, which he apparently did in situ using an oxy-acetylene
torch. He didn't wait for the strut to cool before closing the trunk lid to
check the alignment before repainting the strut and the heat caused the trunk
light switch plunger to melt thus causing the trunk light to remain permanently
on. It took about a week for the battery to go flat but an ammeter (if
installed) would probably not have indicated such a small current drain anyway,
even if I had bothered to look at it when switching off the ignition. And even
if it did, it wouldn't have told me where the fault was. I had to do that by
analysis using a standard DMM.
.
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