prevailing wisdom about opcd within 18 inches of the battery

[John Frum]

Its common knowledge that you put the fuse or breaker close to the source of power.
I think the reason to put the fuse or breaker close to the power source is to minimise the possibility of a short upstream of the ocpd.
If you have a bms and that bms has overcurrent protection I think this rule can be relaxed.
Let the bms protect the battery and the fuse can protect the wire.

 

[BiduleOhm]

The main question is: how many BMS can survive a dead short? currently we know one who can't: https://diysolarforum.com/threads/deligreen-bms-short-circuit-and-over-current-protection.4146/

It's extremely hard to design the disconnect to handle the current of a short-circuit long enough and being able to cut the current quick enough to not destroy itself before. You need to over build, very good mosfets, very good design and pure hardware (software is too slow, even with interrupts...). So my hopes aren't too high for the usual 50 to 100 dollars BMSs.

 

[John Frum]

I guess if the BMS gets destroyed and protects the battery, that is a win.
I admit I have not tested that feature

 

[fat_old_sun]

Its common knowledge that you put the fuse or breaker close to the source of power.
I think the reason to put the fuse or breaker close to the power source is to minimise the possibility of a short upstream of the ocpd.
If you have a bms and that bms has overcurrent protection I think this rule can be relaxed.
Let the bms protect the battery and the fuse can protect the wire.

aren't you assuming that the battery is lithium type with a BMS? what if it's a lead acid or some other chemistry with no internal electronics to prevent a over-current situation?

 

[John Frum]

aren't you assuming that the battery is lithium type with a BMS? what if it's a lead acid or some other chemistry with no internal electronics to prevent a over-current situation?

If you have a bms and that bms has overcurrent protection I think this rule can be relaxed.

I qualified my assertion.
If there is no bms then old-school rules still apply.

 

[nebster]

With high power systems, just use terminal fuses. Then you are zero inches from the battery and all the wiring is made safe from OC. Don't use a FET inside a BMS, use an external contactor.

You can take shortcuts with smaller setups, but the cost/safety ratio is pretty good on this approach for large packs, especially if you are living with/on top of them.