Do I need to keep manual disconnect switch with BMS that has shutoff?

[offgridliving456]

I'm in the process of revising a 6kWh vehicle build. I've replaced my batteries with a single larger that has a JBD BMS with app controlled mosfet disconnects. The system is protected with a class-T fuse immediately after the battery. Now that I can software control charge/discharge mosfets, do I really need to retain my manual disconnect switch?

In the case of an emergency need to disconnect I'd be relying on the fuse as the manual disconnect is not quick to reach typically and of course if very close to the potential source of the problem. The switch, a Blue Sea, 300A continuous rated switch works fine but is my most significant source of resistance and now that I can use the app to disconnect for system maintenance I'm considering removing the manual switch. I realize opinions will vary, please share yours.

Thoughts?

 

[offgrid-curious]

is my most significant source of resistance

Is it so significant that it's causing a problem? Could you improve the resistance with better crimps/lugs on the switch terminals?

As a software developer, I would definitely not get rid of the manual switch. In an emergency I want the simplest path possible to fix the problem, and using software as a disconnect is just adding more friction to the process as well as more points of failure (where's my phone? the app needs to be updated, etc).

 

[offgridliving456]

Thanks for your thoughts. It's certainly not a significant problem just entertaining the idea of increasing efficiency as much as practical while not jeopardizing safety. I've done all I can to improve the connections, re-crimped lugs, polished and added Caig contact enhancer. At full load, the lugs and cables are increasing in temp about 30F over ambient. Well within the safety range but loss nonetheless.

Agreed on the safety front however it may very well be faster and safer to use my phone to disconnect the mosfets as the switch is 'in the trunk' if you will...exit the vehicle, access the rear compartment and assuming the switch isn't blisteringly hot because of the fault turn it off. It may very well not be that useful in a true emergency.

In an ideal world, I'd relocate the disconnect making it accessible from the 'living space' but that would entail a redesign and of course longer conductors...

 

[offgrid-curious]

IMO I'd keep both. If your phone is faster and safer to do the disconnect that's great. But also good to have the manual back up just in case.

 

[hwy17]

The switch, a Blue Sea, 300A continuous rated switch works fine but is my most significant source of resistance

What led you to this conclusion? I mean I don't disagree there is resistance in it, but how do you conclude that it is significant enough to change?

 

[offgridliving456]

What led you to this conclusion? I mean I don't disagree there is resistance in it, but how do you conclude that it is significant enough to change?

By measuring the temperature under multi-hour full load of all connections. It's the most significant source of resistance in the system however it's not necessarily significant.

I would argue that it's not significant but if the switch isn't accessible enough when I need it then what's the point in keeping it.

FWIW, this was more of a thought experiment, to learn and gather others opinions, and discuss.

 

[hwy17]

By measuring the temperature under multi-hour full load of all connections. It's the most significant source of resistance in the system however it's not necessarily significant.

Oh, yes that is a decent test.

As long as it didn't get over 75C I would lean towards not removing it. Your real world usage will not be as intensive anyway.

 

[offgridliving456]

Temps remained below 45C, consistent over two full capacity tests. No issue there. And you're right, I don't stress the system to this degree normally. My typically full-load only lasts 20 mins. to heat water to shower and not everyday (water is the limiter not power).