Possible 12V LVB Disconnect Switch Approach

[louibluey]

Some FORD writings discourage removing the clamp lug on the 12V LVB (low voltage battery) terminal, which is also the small BMS computer. Apparently, some FORD people think that loosening the BMS clamp 10mm nut and removing the BMS clamp from the negative battery terminal can damage the BMS and/or the battery terminal (color me skeptical).

The FORD instructed alternative (e.g. WSM, newly posted MME GT Police note) is to to remove the ground lug from under the 13mm ground jump post (where the same ground cable is still connected to the LVB negative terminal). That is apparently the suggested procedure, even for a momentary disconnect to cold boot MME. IMHO, that procedure is not efficient (removing the lug under the negative jump post), and not even that safe (because the cable is still connected to the LVB negative terminal).

From past DC work with a Chevy Volt, I still have a BUSS (Eaton) MRCB breaker, marine applications, very nice quality, near industrial if not. I have a 135A, but they trip very slowly as shown in curve below. For example, the actual 135A trip is over 400 to 600 seconds. (I actually got a smaller one for the Volt, once I realized the trip curve.) The threaded studs are large to accept industrial quality #2 cable crimp lugs. I need to pull the covers later to see if there is room for the breaker. If so, I am thinking this might make a good quality inline ground switch. The switch could be used for installation work (e.g. amplifier work, adding a sub woofer fused power line, etc.) or just for cold booting MME, like last month when I lost my driver's speedo display, and lifting the negative terminal reset it near instantly.

I guess I should try to measure the resistance to see what it would add to the ground leg. This would be an unconventional use, usually these devices are used positive side, such as for DC distribution. Note that the DC/DC converter path too (+) and from (-) the 12V bus would be unaffected.

Still need to check to see if there is room. Otherwise, what do you think about this possible approach? (most people will not have access to Greenlee heavy cable shears and industrial crimpers for industrial quality lugs, so it is not an easy universal solution).

Also, a slight irony is that to install a switch or breaker, I will need to take the ground cable completely out, so at least one more 10mm loosen, BMS lift off.

Sponsored

 

[JamieGeek]

You could always just use a marine battery disconnect:
https://www.amazon.com/Disconnect-V...fix=marine+negative+disconnect,aps,70&sr=8-11

Not a breaker, just a switch so low resistance.

 

[louibluey]

You could always just use a marine battery disconnect:
https://www.amazon.com/Disconnect-Vehicles-Waterproof-Isolator-Position/dp/B07HT19T5V/ref=sr_1_11?crid=BKP0Z2Y64X9B&keywords=marine+negative+disconnect&qid=1642610018&sprefix=marine+negative+disconnect,aps,70&sr=8-11

Not a breaker, just a switch so low resistance.

That approach could work. I would prefer a more industrial U.S. or EU version, but I think it would be the same. The resistance of a 100 to 200A breaker might not be an issue, but I do need to measure it later. Eaton (BUSS) or similar manufactures might offer a switch too.

There are of course any number of negative terminal quick disconnects, but we need the BMS to mount on the terminal, plus the quality of many of those auto store products is probably dubious at best.

 

[JamieGeek]

That approach could work. I would prefer a more industrial U.S. or EU version, but I think it would be the same. The resistance of a 100 to 200A breaker might not be an issue, but I do need to measure it later. Eaton (BUSS) or similar manufactures might offer a switch too.

There are of course any number of negative terminal quick disconnects, but we need the BMS to mount on the terminal, plus the quality of many of those auto store products is probably dubious at best.

I put one in my RV seems pretty solid even after about 5 years. (Granted not that specific one I linked to.)

 

[louibluey]

Another possible approach might be a single pole Anderson plug/socket (the famous dual +/- 12/24/48V forklift connectors), but it might not work because we are (apparently) trying to avoid putting stress on the BMS package, and the relatively short #2 ground cable is very stiff (smaller number of wires, more like the stiffness of building cable, much finer wires than #2 building wire, but still very stiff), not like welding cable.

picture of the BMS ground cable, hmm, there might just be enough room for a series breaker or switch just before that bend, so it should be accessible by just removing the first panel below the windshield.

Note how the structural stiffening cross bar prevents an easy lift off of that large ground lug with tab through the chassis at the negative jump post. The combination of that tab and the structural stiffening bar makes me think that the original designers did not intend that to be the first way to lift the negative battery terminal connection. It literally takes about 10 seconds to loosen the 10mm nut at the BMS and lift that clamp off the negative LVB terminal post.

Probably, as mentioned in another thread, techs were breaking BMS and someone at FORD said, what if we have them lift the other side. But, if you don't take the bar off, it takes some serious manipulation of that end of the short stiff cable to get the tab out. So, to my thinking, there is still some serious force placed on the BMS package. Or, you can take the cross bar off too, making the job orders of magnitude more difficult and time consuming. They want us to also remove the cross structure bar, for a momentary disconnect to cold boot MME? REALLY?

Sponsored