Trimetric shunt/wire size question

Empty Nest, Soon wrote:
Grizzzman – My concern was at what point does the cross-sectional area of the conductor (the bracket) become small enough that it gets hot from resistance and perhaps burns in two? Same principle as a fuse.

But, I suppose if the voltage drop across it is so negligible, then the heat generated from the current passing through will also be negligible?

Wayne

The heat generated is, for all practical purposes, the power dissipated by the connection--after all, the power has to go somewhere, and heat is the only real option. Thus, it's proportional to the current multiplied by the voltage drop. .01 V drop at 100A gives you 1W of power. You're well under that, of course, both in terms of current and voltage drop. (Note that, since the voltage drop is proportional to the current, the power dissipation is proportional to the square of the current. That doesn't really matter here; we're way far below any danger.)

How hot the connection gets in terms of temperature depends on the heat generated and the rate at which heat is dissipated to the rest of the world. As it gets hotter, the rate of dissipation in absolute terms goes up (greater conduction to the air, etc.), and equilibrium is reached at some point. A broad, relatively large copper bar is going to be able to dissipate a fair bit of heat, at least when compared with, say, a light bulb.

Conclusion: your copper isn't going to melt or get dangerously hot unless you do something completely unreasonable like short the battery through it or cover it in extremely effective thermal insulation.

BFL13 – I’ve considered adding a bit of copper to the far side of the shunt for my portable charger clamps. But, as you said, the shunt bolts are too short to take many lugs.

I measure the bolts at 5/8-inch and the threaded holes in the shunt at 1/2-inch. Factor in the lockwasher, the flatwasher, the big cable lug, we’re down to less than 3/8-inch of threads gripping. Add in a flattened copper pipe for clamping the charger and the grip gets down to about 1/4-inch. That makes me a bit nervous.

I’ve thought about substituting a longer bolt, but I don’t know what throwing a different metal into the mix will do the the electrical properties of the shunt, so I decided to play it safe and stick with the original bolt.

What I’ve decided to do is expose the sides of the big lug where it is crimped to the 3/0 or 4/0 negative cable and clamp my battery charger to the lug rather than adding a new piece.

I like your idea of making a bus for fittings and I may do that as one of my first mods to the system.

I may go solar, but first I want to gather more info on what I am able to reasonably do for keeping batteries charged with just the charger and little genset.

DrewE – Thanks for that explanation. I’m able to follow what you’re saying and it makes perfect sense. My father was an electrician, so I learned enough to do basic wiring, and I have a pretty good rudimentary understanding of electricity. But I also appreciate that there are factors that enter in that I may not be aware of, therefore, my inclination toward excess caution when I’m doing something for the first time. My whole purpose in posting this thread was to get validation from someone who knows this stuff better than I do for the solution that I had in mind.

Thanks again to everyone who replied!

Wayne

You only need one copper pipe on the outer end of the shunt, no lugs.

Bang the pipe flat at one end. Drill two holes along that flat part so the first hole is for the shunt bolt. Next hole is for a bolt (you provide) to take some or all of the neg lugs. If too many stacked, drill a third hole and have two bus collections of neg lugs.

Beyond that last hole, the copper pipe is still round out there, and can take charger clamps. Mine is long enough to take several Vector charger neg clamps all at once, for when I used to do that. Now I mostly just use the PowerMax 100amper (blue wires in the photo above) and maybe also the Vector 40amp charger (VEC1093DBD) as well if I am in a hurry, so I don't need such a long pipe anymore but there it is.

Only reason to clamp to the pipe is so I can see the total charging amps on the Trimetric. I could still charge the batts just as fast by clamping to the batts directly, but then it would not show on the Trimetric and its AH counter would be off by a bunch.

BFL13 wrote:
. . .
Only reason to clamp to the pipe is so I can see the total charging amps on the Trimetric. I could still charge the batts just as fast by clamping to the batts directly, but then it would not show on the Trimetric and its AH counter would be off by a bunch.

That was my thinking, too. The whole purpose of me installing the Trimetric is to understand how well I'm managing the batteries with just the 40 amp VEC1093DBD and the little genset.

Wayne

I use the Trimetric for more than that, as you will too.

Eg, it is useful to compare the AH count with the (mostly) resting voltage to tell when it is time to recharge. The limit for me is around 50% SOC ( 12.1v or 230AH down from full), but if the morning voltage is 12.3 and down 170AH in one day I know I can't go to next day, so I recharge "early" from 12.3.