MRBF fuses attached to battery switches

[s/v Jedi]

This has been discussed on the forum before. Today I got two types of switches and a bunch of fuse blocks to see how this pans out.

See attached pictures. On the right is a simple on/off battery switch. The input is on the right side of it and has a fuse block.
On the left is a 4-position switch for two batteries. The inputs are on the left side and underside of the switch and have fuse blocks.

The outputs of the switches are facing each other in the middle. For this setup I would want to link them, plus have a cable towards the busbar. There is a special link plate for this which I have ordered. I will update this thread when I have it.

Of course you can also rotate the switches but this is right side up.

I am redesigning a house bank which has two LiTime 24V 200Ah batteries with 200A BMS which connect to the left switch. I want to disconnect all batteries when the boat is in “storage mode” and use a small 24V 50Ah battery with 50A BMS instead, which I will connect to the right switch.

On my boat the positive cable coming from these switches first goes to a remotely controlled latching battery solenoid switch before it continues to the busbar. These manual switches are for isolating batteries for testing or when one develops a problem etc.

Interestingly, next to these switches will be a Victron shunt for the negative conductors. The two big batteries will connect to the battery-side of the shunt but the small battery will connect to the load-side of the shunt so that it isn’t counted. It has a Bluetooth BMS so I can always check it that way, plus it’s only in use when the boat isn’t used.

I have completed testing this small battery to run the boat while in storage. I tested for a period of 7 weeks and it was flawless. It is a mini version LiTime trolling motor battery at 20V 50Ah.

[rslifkin]

Is there any reason for putting the fuses at the switch rather than at the batteries before the switch? I'd want that cable run protected, plus it keeps the switch setup less cluttered.

[s/v Jedi]

Originally Posted by rslifkin

Is there any reason for putting the fuses at the switch rather than at the batteries before the switch? I'd want that cable run protected, plus it keeps the switch setup less cluttered.

To stay within ABYC guidelines, these cables can be a maximum length of 18” as is, or 72” (all from memory, hope I’m correct) when this cable is sheathed and fastened.

The reason for doing this is that the fuse blocks have a hole for a 3/8” stud while many batteries have smaller (or larger) terminals, while these switches have 3/8” studs.

Also, fuses are often easier to access at the switches than at the batteries.

[rslifkin]

Originally Posted by s/v Jedi

To stay within ABYC guidelines, these cables can be a maximum length of 18” as is, or 72” (all from memory, hope I’m correct) when this cable is sheathed and fastened.

The reason for doing this is that the fuse blocks have a hole for a 3/8” stud while many batteries have smaller (or larger) terminals, while these switches have 3/8” studs.

Also, fuses are often easier to access at the switches than at the batteries.

Makes sense to me. It has always bugged me a little bit putting an MRBF with a 3/8" hole onto a battery with a 5/16" stud, but with an appropriate washer under the nut I've never seen it cause an issue in practice.

[Tin Tin]

Why not just put a Blue Seas 187 200A Circuit Breaker on each Battery lead. This allows each battery to be isolated and cable protection at same time.


Whilst on the topic sort of, have you considered attaching battery temperature sensors to each battery connected to a Cerbo GX that if excess temp could trigger a remote battery latching relay on each battery such as the Blue Seas ML-RBS to isolate the overtemp battery and send a warning to you via internet, assuming boat is near wifi. I know the BMS should do this too, but with the Cerbo you can set a lower trigger point.

[PippaB]

Originally Posted by s/v Jedi

To stay within ABYC guidelines, these cables can be a maximum length of 18” as is, or 72” (all from memory, hope I’m correct) when this cable is sheathed and fastened....

Had to go look it up to check my memory, but unprotected length is limited to 7" in E-11.10.1.1.1.

[capt jgw]

Right. ABYC requires battery cables (except engine start cables, no fuses required) to be fused within 7" of cable length from the battery, unless the cable is in a sheath or conduit, then it's 72". They're concerned about the cable insulation chafing through and causing a short circuit. A sheath would prevent that. Assuming the OP's switches are between 7 and 72", putting the fuses at the switches would be ok if the cable is sheathed. Cheap and easy to do so no big deal. My main battery cable is about 9" to the fuse and is in about 50 cents worth of split loom. Best practice would be to put the MRBF fuses directly on the battery positive terminal as they were designed to be, but if they're truly hard to access I guess the switch location would be acceptable.
And in practice, a fuse should only blow if there's a problem. If that happens you've got more work to do to determine and correct the cause before replacing the fuse. Access to the batteries would be a minor issue in that case.

[MerMike]

Originally Posted by Tin Tin

Why not just put a Blue Seas 187 200A Circuit Breaker on each Battery lead. This allows each battery to be isolated and cable protection at same time.

Circuit breakers have inadequate AIC ratings to be used with LFP batteries.

[Tin Tin]

Originally Posted by MerMike

Circuit breakers have inadequate AIC ratings to be used with LFP batteries.

Fair Point.

[s/v Jedi]

Yes, 7” is unlikely so a sheath is the way to go. I like the new type: https://www.amazon.com/gp/product/B07FW5ZTVH

At least I got the 72” right

With lfp batteries having a BMS that protects against overcurrent I think they could relax that 7” rule a bit.

[rslifkin]

Originally Posted by s/v Jedi

Yes, 7” is unlikely so a sheath is the way to go. I like the new type: https://www.amazon.com/gp/product/B07FW5ZTVH

At least I got the 72” right

With lfp batteries having a BMS that protects against overcurrent I think they could relax that 7” rule a bit.

I think that 7" rule is part of why Epoch decided to put a fuse inside the battery case on some of their batteries in addition to the BMS. Makes it easier to meet that rule, as assuming you use adequately sized wire relative to the fuse in the battery, it's protected right from the terminal.

[capt jgw]

Originally Posted by s/v Jedi

Yes, 7” is unlikely so a sheath is the way to go. I like the new type: https://www.amazon.com/gp/product/B07FW5ZTVH

Yeah, my positive bus bar and fuses are mounted right on the side of the battery box and I still couldn't get the feed under 7".

[Hartleyg]

Jedi said: "Yes, 7” is unlikely so a sheath is the way to go. I like the new type: https://www.amazon.com/gp/product/B07FW5ZTVH"


Thats the sheathing I'm using and it's quite easy to work with. I splurged and got some of the red to do the positive cables


Hartley
S/V Atsa

[s/v Jedi]

I received the link to couple these switches. You get two of them which makes the price is bit more bearable.

I think it’s better to turn the fuse blocks backwards like in the new pictures. This requires a 7/8” stand-off behind the switches to create room for the fuse holders.

I think I’ll design something nice for the 3D printer for this

[AndyOz]

Originally Posted by rslifkin

I think that 7" rule is part of why Epoch decided to put a fuse inside the battery case on some of their batteries in addition to the BMS. Makes it easier to meet that rule, as assuming you use adequately sized wire relative to the fuse in the battery, it's protected right from the terminal.

Wow. So if the fuse blows, you have to either replace the battery, or get out your multi tool, eye and other protection and cut the battery open!