My own experiences fixing a RAV4 EV not charging/fuse blown issue

[dfergenson]

Well, I asked for peer review and I got it. And when I'm wrong, I usually want to know it early. And I was wrong.

I had asked if anyone else had any insight into the resistance of the fuses. The bottom line is that their resistance is in the low numbers of milliOhms. miimura and I just tested one. So, while I was considering changing the fuses in the charger to the same model as the one in the battery junction box, I now know that this is unnecessary and I will keep the same model even as I engineer an external fuse box for them.

miimura and I met up in his laboratory and connected an electronically controlled current sink to a 24V power supply with one of the fuses in-line. Although we don't know the accuracy, the precision of the current sink is given to the nearest 0.001 Amps.

At 3.000 Amps of sunk current, we confirmed a voltage drop of 5.6 mV across the resistor using two multimeters that agreed perfectly to within their 0.1 mV precision. We then reran the test at 6.000 Amps and observed 11.3 mV across the fuse on both multimeters. So the measurement of the resistance was highly linear, as one would expect.

V=IR AKA R=V/I = 0.0113 V / 6.000 A = 1.88 mOhms. This is similar to the other resistors that I had proposed to use so there is no advantage to using them; I'll just keep using the ones that Tesla had put in the charger.

So, how much power will be dissipated at 40 Amps? V=IR=0.00188 * 40.0 = 0.0752 volts dropped across the resistor. P=IV=40.0 A * 0.0752 V = 3.01 Watts.

The specs sheet that alflash posted gave the spec at 3.8 Watts and it could be a matter of our measurement precision or, more likely, that the specs sheet gave a maximum energy dissipation. In any case, the fuses are fine to use and I will continue to use this version going forward.

Thanks miimura and alflash for helping check my work. I'll edit the initial post to point out my error.

-Davio

[asavage]

Never mind; still can't delete a post here.

[alflash]

Never mind; still can't delete a post here.

OFF.
Respected asavage,
You wrote that "a) Fuses don't "go bad" or "get weak": they generally open under reasonably specific circumstances. Fuses "going bad" is an old wives' tale. Similar to one of my favorites: "placing a car battery on concrete will drain it". Ugh."
What other reason do you see for such fuses blowing, if not exceeding their maximum allowable I2T value?
Two large fuses (40A) are easily checked without dismantling DC/DC converter.

[asavage]

Measuring a fuse for open is easy-ish. Replacing it in the DC-DC or Charger isn't.

I'm not sure what you're asking me. I'd said that fuses don't "go bad" or "get weak", two phrases that I've heard a lot over the years, by people who don't understand how fuses work and seem to be genuinely confused by electrical devices in general. You're asking:

What other reason do you see for such fuses blowing, if not exceeding their maximum allowable I2T value?

It seems we agree: fuses open because a current was exceeded for a long enough time interval; that's the purpose of fuses.

I wrote about fuses not "going bad" because I write that every year or so reflexively, whenever the subject of fuses "going bad" for no apparent reason comes up. Absent physical damage, fuses blow due to a circuit overload, not "it was old" or "magic is afoot in the world" or whatever superstitious reason a person might ascribe.

In the old days, fuses blew a lot more frequently, and equipment was designed with that in mind, so fuses could be changed easily; they were "user serviceable parts" in the old parlance. Nowadays, it seems fuses are buried many man-hours into equipment, and what purpose they thus serve is no longer apparent. To protect the wiring? If the fuse is not serviceable, that is (I guess) the fuses' only purpose now.

[alflash]

...
Nowadays, it seems fuses are buried many man-hours into equipment, and what purpose they thus serve is no longer apparent. To protect the wiring? If the fuse is not serviceable, that is (I guess) the fuses' only purpose now.

I'm based/taked on the fact that high-voltage fast-acting fuses, for example,

have a different purpose / function (and internal structure) than the usual ones:

High speed fuses provide superior protection or isolation for components such as diodes, silicon controlled rectifiers (SCRs), Gate torn-Off Thyristors (GTOs) and IGBTs and in light and heavy harsh DC traction applications as high as 4000Vdc and 10,000A. Also used for the protection of DC link/power converters. Optimized dimensions and materials allow for excellent thermal dissipation for high current cycling enviroments, or heavy duty applications.

E.g. https://eu.mouser.com/datasheet/2/87/Bu ... 598476.pdf

[asavage]

i translate that to "these very fast-blow fuses protect wiring and some internal components against cheap/faulty circuit design".