Bradley: As C.Johnson suggested, please check your current carrying conductors. If your batteries are not secured against motion, the connections in the lugs can work loose, the lug connections to terminals can work loose, etc. You could also have corrosion. How you secure the lugs to your battery terminals can cause enough heat to melt battery posts. And that leads to my story from last Summer:
We were driving my 1920 Milburn Electric to the top of Mount Tabor in Portland at the request of a friend who was getting married there. On the way up the hill, we started smelling smoke. It didn’t take much time between the smell of smoke to the appearance of it, then “boom” and the car coasted to a halt. Checking the rear battery pack revealed what happened: One of the battery terminals had vaporized and 2 other ones appeared ready to melt down (fissure crack horizontally along the terminal’s side and the encased terminal lug bolt was loose). I was able to bypass 4 of the 14 batteries & proceed on to the event. None of the 8 batteries in the front pack had any issue. The next day I looked for root cause. My inter-battery cable lug connections & wire gauge were to blame somewhat. But in peeling apart the 2 battery posts that had almost melted down and had loose bolts, I found a disturbing design feature with these battery posts. The threaded battery post is little more than an inverted bolt with its head cast into the lead. Ordinarily, this probably isn’t a problem if battery cables are attached by pulling tension with a nut. But what I had done in installing these batteries was I secured the cable lug by sandwiching it between 2 nuts on the threaded battery post. This had been referred to by EV folks for a long time as a preferred way to get a solid connection and eliminate the risk of causing the “stud” to become loose by stripping threads---i.e. it may twist, but with all those threads, it’d still have a good connection in the battery, and between stud & cable would be secure. While that worked well for batteries with traditional “studs”, some or all of these newer batteries don’t have “studs” and hence there is no thread-interface with the lead, only the “bolt head”. So here’s what can happen: vibration or heat causes battery terminal inverted bolt to mechanically disconnect from its lead-encasement, then suddenly what was a low resistance connection inside the terminal goes high-impedance. With steady high current (e.g. my 100amps+ with my Milburn), lead starts to melt from the inside of the battery terminal outward. This accelerates, terminal gets super-hot, cables get hot & within seconds the terminal can melt down.
I wondered why my front pack hadn’t had any overheating let alone a single meltdown. Looking at the cable attachments, the truth revealed itself:
ALL of the connections in the rear pack were as described above (no tension with the battery post): Nut, Lug, Nut
ALL of the connections in the front pack were in mechanical tension with the battery post: Lug, Nut, Nut
I was able to save the 3 batteries with melted lugs by drilling, tapping & installing traditional “studs” in the primary lead area as described by others.
Then, I made sure that only those 3 connections used Nut/Lug/Nut (ironic, but appropriate given they now used “studs”) and all the rest of the connections pulled tension on the post.
At this point, my ol’ car still has 2 lug interfaces that I need to replace, but all the rest are solid & without worry.
It’s great that you did this test at the dock and discovered this before having a problem.
I don’t think your shunt itself is at all to blame. I’d guess you weren’t anywhere near drawing the 250amps rating for your shunt, but were pulling what, 100 amps at the dock? Your 50mv/250amp shunt has resistance of only 0.2milliohms. At 100 amps, the shunt power will only be 2-watts.
Even at 250amps, the shunt would only draw under 15-watts…not enough to be the cause of the temperature rise you saw, let alone needing much in the way of ventilation.
Let us know what you find out J!
-Myles Twete, Portland
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Julian Webb
Sent: Sunday, January 12, 2014 12:06 AM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Hot shunt...
Hi
It sounds like either your shunt is too small (current wise) or if its been working OK before now then your motor, controller or wiring etc has developed a fault.
If you have a tong tester (goes around the outside of an insulated cable and let's you measure the current flowing through it) you could turn it on starting at the motor and work your way back to the batteries and you should soon see what's drawing too much current.
A motor or controller drawing enough current to make a shunt of the right size hot enough to make wood smoke should be smoking itself I would think.
Shunts do get hot when under full load and most manufacturers say they have to be ventilated.
Have you run the motor for this long before? If not then maybe it is too small?
Cheers
On 12 Jan 2014 00:20, "semicolonsutra" <no_reply@yahoogroups.com> wrote:
Hello,
I have a ME0913/Gen4 controller powered by four 12V batteries connected in series and off the negative lead of the 48V pack I have a shunt (I believe it is 50mV/250AMP) to measure the voltage. I was down at the boat today and ran the motor while tied to the dock and after about 30 minutes of run time I noticed a small but distinct smoldering smell (the smell you would get if you left a hot iron on a piece of wood). I turned off the motor, disconnected the battery and searched around for the problem. It turns out the lead from the negative terminal to the battery pack leading to the shunt was quite hot--probably about 100C. The shunt is mounted to wood (it is a wooden boat) and that from the negative terminal to the shunt gets hot when the motor was running. The shunt mounting screws were hot and heated up the wood. Alas, that is the situation. Now this does not seem right but I'm not sure what the problem might be. Any ideas? Thanks in advance.
--Bradley
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