Current Shunt

[Chris W]

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.


--
Chris W
KE5GIX

"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm"

Ham Radio Repeater Database.
http://hrrdb.com

 

[Sjouke Burry]

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 have done this without a shunt, just tap to points 10-20 cm apart
on the main wire, that gives you enough millivolts drop to
(inaccurately) measure the current.

 

[John Fields]

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.

---
Since your starter's only going to be running for a few seconds you
may very well be able to replace the 200A shunt with a 100A shunt
and let the starter current run through it.

Check this out:

http://www.emproshunts.com/eng.aspx

 

[Tim Wescott]

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.

Normal automotive practice is to run a big heavy cable straight to the
starter (or to a relay on an inner fender), with a smaller wire going
off of that to all of the rest of the electronics. If an ammeter is
installed at all the shunt is in that 'little' wire.

Do you have an overriding need to monitor the starter current?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[gearhead]

Normal automotive practice is to run a big heavy cable straight to the
starter (or to a relay on an inner fender), with a smaller wire going
off of that to all of the rest of the electronics.  If an ammeter is
installed at all the shunt is in that 'little' wire.

Do you have an overriding need to monitor the starter current?

Right, just measure the current after the starter.
That's how they did it when they were making cars with ammeters in
them, like my old IH Scout.

 

[z]

Right, just measure the current after the starter.
That's how they did it when they were making cars with ammeters in
them, like my old IH Scout.- Hide quoted text -

- Show quoted text -

yeah, i never saw anybody try to measure starter current, other than
maybe diagnostic purposes. it would be so out of scale with normal
current draw as to either pin the needle, or make the normal readings
infinitesimal; plus, you don't want to put a resistor in series with
the starter, even a small one, thereby reducing the current.

 

[Ross Herbert]

: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.

 

[Tim Wescott]

: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.

That's a good point. Many folks feel that if you put a meter on your
electrical system at all you should put in a volt meter. You can get a
lot more information about the _state_ of the health of your electrical
system, rather than the _trend_ (which is what an ammeter may or may not
give you).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[z]

: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_SHU...
:
: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 measurethe
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 thoughyou
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 probablywon'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 ofthe
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.

good point. i had a car with a factory ammeter once, and most of the
time the deflection was barely visible. it occurs to me that something
with a logarithmic scale would probably be more useful