# What can cause melted shunts?



## EVfun (Mar 14, 2010)

I vote for number 2. A shunt is a measured resistance that is designed to remain stable over a wide temperature range. A standard 50amp shunt (50mv) has 1 milliohm of resistance. That bar in the center will be kicking out 250 watts of heat at 500 amps!


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## Tesseract (Sep 27, 2008)

It sure looks like either extreme and persistent overcurrent OR a poor connection. A 50A shunt on the battery side, even with a high voltage, modest power (under 40kW peak) AC setup is a bit small - the shunt will routinely see 100A+, but it took more than just an occasional 2x overloading to melt the solder on that baby.


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## ereth (Jul 11, 2009)

I'm voting over current as well. You're up against a square-law with current here.

P = V*I = (I*R)*I = R*I^2

So run a 50A shunt at 100A (double) and instead of 2.5W it puts out 10W (four times). Let's say the shunt was designed for a 35 degree Celcius rise above ambient at 50A. At 100A, with four times the power you see four times the 35 degrees rise which is 130 degrees Celcius rise above ambient. Let's say it's 20 degrees inside your car so run it at this for a while and that little shunt will shoot up to 150 degrees Celcius.

Where does the solder melt? Depends on the solder but maybe 170 degrees Celcius. So, you would be right on the line at 100A--might hold but then again, might not. I've been pulling some pretty ball-park numbers out here and it's way too close to call.

Don't try running it at 150A any length of time though. Those same numbers and you get a final temperature of 335 degrees Celcius. That solder is gone.

You could try running two shunts in parallel. If you use 50mV @ 50A shunts you will get 50mV @ 100A.


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## favguy (May 2, 2008)

Surely it's No2. Should any shunt fitted not be rated at least as high as the maximum current that will flow through it? Why would you have a 50A shunt fitted in a circuit that pulls much more current than 50A, surely any such shunt will fail sooner or later under these conditions. Sounds like the kit supplier needs a talking to...

Or am I missing something? 

Paul


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## Tesseract (Sep 27, 2008)

favguy said:


> ...Should any shunt fitted not be rated at least as high as the maximum current that will flow through it?...l



If you have a shunt like the OP that drops 50mV at 50A - ie, 1mV/A - but you want to read up to 100A then you simply pair it with a meter that requires 100mV to display 100A (note how the sensitivity of 1mV/A is the same in both cases?).

This is acceptable IF you expect only brief excursions above the shunt's full scale rated current (e.g. - 50A). However, even a very lightweight EV is going to be pulling more than 50A from its traction battery on a routine basis unless it happens to be a very high voltage pack (e.g. - >288V).


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## Guest (Apr 29, 2010)

Why on earth would you have used a 50 amp shunt in the first place? That's asking for serious trouble or a melted dead shunt. 

Pete 

The shunt was way under rated for it's intended purpose. I won't even use a shunt that tiny in my Electric Cushman.


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## Guest (Apr 29, 2010)

Nice pics on why not to use an under rated shunt or parts. Thanks for the pics. 

Pete


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## racunniff (Jan 14, 2009)

gottdi said:


> Why on earth would you have used a 50 amp shunt in the first place? That's asking for serious trouble or a melted dead shunt.
> 
> Pete
> 
> The shunt was way under rated for it's intended purpose. I won't even use a shunt that tiny in my Electric Cushman.


Because it came with the kit. When I assembled the kit, I was ignorant of shunt ratings and had no strong idea of the amperages I'd be pulling.

I have since upgraded it to 150A / 50mv (which was why I pulled the shunt in the first place). My current setup (ahem) peaks at 150A occasionally, normally it runs at 100A or lower. I will be posting a warning to others who have used the original kit (I believe the manufacturers have since upgraded the kit to a 200A / 50mv shunt).


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## Guest (Apr 29, 2010)

Glad nothing serious happened. I can understand if it came in a kit. What were they thinking. 

I have an 800 amp shunt. Might consider at least 500 amp. 

Thanks again for the pics. I think I will use these for instruction and give others knowledge they need. Goes to show that even kits don't guarantee success.


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## halestorm (Apr 28, 2009)

racunniff said:


> I have since upgraded it to 150A / 50mv (which was why I pulled the shunt in the first place). My current setup (ahem) peaks at 150A occasionally, normally it runs at 100A or lower. I will be posting a warning to others who have used the original kit (I believe the manufacturers have since upgraded the kit to a 200A / 50mv shunt).



Maybe you're at 150A peaks because you're running a higher voltage pack now(?), but the stock pack is 144V and routinely pulls ~ 260-280A.

I'm running this same setup, though presumably with their 200A shunt since I've been running for a year now. I've not noticed any adverse effects (IE. failures) but my shunt is enclosed in a plastic box all by itself so I haven't looked at it since it was installed. Should I be concerned that I'm routinely running at more than its rating? Recall that I've been doing so for a year.


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## racunniff (Jan 14, 2009)

gottdi said:


> Glad nothing serious happened. I can understand if it came in a kit. What were they thinking.
> 
> I have an 800 amp shunt. Might consider at least 500 amp.
> 
> Thanks again for the pics. I think I will use these for instruction and give others knowledge they need. Goes to show that even kits don't guarantee success.


I hope they help - I certainly learned a lot from the experience (without suffering any negative effects other than mild embarrassment )


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## racunniff (Jan 14, 2009)

halestorm said:


> Maybe you're at 150A peaks because you're running a higher voltage pack now(?), but the stock pack is 144V and routinely pulls ~ 260-280A.
> 
> I'm running this same setup, though presumably with their 200A shunt since I've been running for a year now. I've not noticed any adverse effects (IE. failures) but my shunt is enclosed in a plastic box all by itself so I haven't looked at it since it was installed. Should I be concerned that I'm routinely running at more than its rating? Recall that I've been doing so for a year.


Yes, I'm now running 216V nominal and I've programmed the "performance" mode to pull about 150 amps peak (roughly 2C on my batteries, AGM wheelchair batteries). Normal usage is in the 50-100 amp range. The rule of thumb that I have run across for shunts is that they should be rated about 33% bigger than the typical sustained amp pull.

IMO - you should take a little time and inspect your shunt for evidence of overheating - discolored metal, melted plastic, etc. I ran for 2.5 years without thinking there was a problem. I got lucky.


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## EVfun (Mar 14, 2010)

Every time I see this thread the first thought through my head is...

*What can cause melted shunts?*

John Wayland
Roderick Wilde
Rich Rudman


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## TX_Dj (Jul 25, 2008)

evfun said:


> *what can cause melted shunts?*
> 
> john wayland
> roderick wilde
> rich rudman


truth!!! :d


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## halestorm (Apr 28, 2009)

So I did a cursory inspection of my shunt. I didn't look at the *back* of it, like you did, but from the front it seems OK...


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## racunniff (Jan 14, 2009)

halestorm said:


> So I did a cursory inspection of my shunt. I didn't look at the *back* of it, like you did, but from the front it seems OK...


That looks like a bigger than 50A shunt to me - maybe a 100A? What does your ammeter read?


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## halestorm (Apr 28, 2009)

racunniff said:


> That looks like a bigger than 50A shunt to me - maybe a 100A? What does your ammeter read?


Oh, yes, absolutely! EA claims it is a 200A shunt. They supplied a +/- 200A ammeter, and it is pegged (off-scale) while accelerating or driving on the freeway. A fellow EV-er with the exact same system measured his draw between 260-280A under acceleration.

So that was my concern; I have every reason to believe that I have a 200A shunt, however I am routinely (and for several minutes at a time) drawing on the order of ~250A through it.


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## TX_Dj (Jul 25, 2008)

Certainly sounds better to me than a 50A shunt taking 100-150A (2-3x rating) routinely.


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