Does Your Battery Monitor Shunt Heat Up?

BrianZ

[Caution:  If you find math/equations boring or painful, you can jump directly from the end of the first paragraph to "My Conclusion" below]

I recently questioned whether a battery monitor shunt mounted inside a box (as suggested by Victron), needs vents for cooling, as some have suggested.  The BMV-712 shunt is a fairly massive device, and may be the largest conductor in a T@B; so, I wouldn't expect it to generate much heat with its low resistance and the relatively small DC currents we typically see.  After all, this shunt is rated at a whopping 500 Amps!  With our limited use of battery & only 100W of solar power, the largest current I've seen measured by our monitor was less than 12A, and that was during charging.  When dry camping, we run both the fridge & Alde heater on propane, and no air conditioning of course, so typically only use around 20AH per day of battery power.

In searching for more info on this shunt heating question, I came across the following info in a solar forum, showing how they derived the power, in watts, that goes into heating by using basic electronics equations..

The Watts of heating power comes from an alternate definition of Watts being equal to Amps squared multiplied by Resistance, where resistance is found using the more basic expression of Ohm's law:

V = A x R, so, using the example above, 

R = V ÷ A   = 0.1V ÷ 100A   = 0.001 ohm

Then W = A^2 x R  = 85A^2 x 0.001 ohm = 7.2 W of heating power.

(85A?  That's a lot of amps for a 100A shunt!)

Now, I can do the same calculation for our [five times larger] 500A/50mV shunt, which gives me a resistance of 0.0001 ohm (0.1 milliohm).  And using my highest 12A observed current, I calculate the heating power as:

W = A^2 x R  =  12A x 12A x 0.0001 ohm = 0.0144 Watts or about 15 milliwatts (15mW) of heating power, or 15 thousanths of a watt.

OK, so what?  Now the question is, what does that mean in terms of a temperature rise in the shunt itself?  Well, my physics is a bit rusty, so another search gave me the following page of good basic info, which identified the thermal property I needed to know.   It's called "specific heat", and it relates the heating energy calculated in watts above to an actual rise in temperature for a given type of material..

Since I am not sure exactly what metal the shunt is made of, I will make the assumption that it likely has properties similar to copper, and I found that copper has a Specific Heat value of 0.385 Joules per gram at room temp.  The Victron manual lists the shunt weight at 315 grams, so multiplying gave me..

0.385 J/g  x  315 g =  122 Joules of energy (also = Watt-seconds)

And since I know the number of watts from the previous calculation, I can divide the 122 watt-seconds by the number of watts to get the heating time in seconds..

122 W-s ÷ 0.015 W =  8,133 seconds, or about 2.5 hours.

In other words, while running 12 Amps through the shunt, it would take 2.5 hrs to get a 1°C rise in temperature, or 1.8°F.  It would take 5 hours to get a 3.6°F rise in shunt temp.  This doesn't consider the (+/-) effects of ambient outside temperature, which could be a factor since our shunt is mounted in a box in the outside tub.

My Conclusion:

As expected, this appears to be a temperature change so minor (1 or 2 degrees over several hours), that I doubt I would ever notice it with our 150 AH battery and single 100W solar panel.  Our battery monitor hasn't had any issues for a couple years now.  The protection I've provided for our shunt is putting it in a water-resistant (but not air tight) box inside the tub, and coating the electronics circuit board with a waterproof "conformal" coating to protect from moisture, corrosion or short circuiting.  I haven't made any attempt to drill vent holes, though there are holes with a bit of extra space where cables enter the bottom of the box.

For the typical T@B owner with a Victron battery monitor, I would guess one doesn't need to be concerned about their shunt heating up.  If you've got a battery bank with lots of solar power, or use a high powered inverter etc, then maybe you could share your own observations or try doing the same math for your system & let us know if you come to a different conclusion.

If you have any observations on this issue, I'd be interested to hear them.  This is my amateur attempt at a scientific answer to this question, so feel free to contribute anything further, or please let me know if you see any problems with my calculations or assumptions.

m_lewis

I have used similar shunts in multiple applications, often with current flows approaching 80% of the device's rating. At the higher amps, they do tend to heat up a little. I have not measured their temperatures, but I would guess a rise of less than 5 degrees C. At a mere fraction of their rated capacity, heating is a non-issue. The BMV's 500 amp shunt can easily handle an automotive starter motor's demands. 
In our camper, even with the 1500 watt inverter running near max load (~100 amps), I have not seen a significant temp rise at the shunt. If you are concerned about it, I recommend mounting it such that the fins are vertical, allowing for thermal convective cooling.

ColoradoJon

I have the Victron BMV-712 and their 500 amp shunt installed at my off-grid cabin.  It monitors four 6V batteries (12V system) and a 4000 watt pure sine wave inverter.  I run a full size fridge, lights, television, and other items and the only thing that gets close to warm is the inverter itself.  The shunt itself doesn't get hot at all.  No worse than any of the wire that makes up all of the connections