Current 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.

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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

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?

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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

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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.

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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.

: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 .... : :

: :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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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

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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