disconnect switches and fusing...

[WattsItToYa]

Hello! Awesome forum...so much information and great teachers here! TIA
I'm working on expanding my RV's system and have a question on battery disconnect switches and fusing...

Back story-
My RV came factory installed with a Xantrex 1800w inverter, no solar and FLA batteries. The inverter currently only powers a few receptacles- front/rear TV's and passenger work station. I've added 800w of flat mounted rooftop solar, replaced the FLA with (1) 300ah Wattcycle (so 280ah) Lifepo4, added a Victron 150/45 Smart charge controller and Progressive Dynamics 60a lifepo4 converter/charger. Now I'm looking to add a second 300ah mini.
(I will be upgrading to a 3000w inverter and powering the whole coach soon).

I've seen Will's review of the Wattcycle 300ah mini and I now know I need to install class T fuses for each battery. I'm getting ready to order some of the components for the next phase of my upgrade and wanted to confirm my plan.....

This will be a 12v system so I will be paralleling the two 300ah mini's- I want to run each battery to its own 250a type T fuse with Blue Sea 5502 fuse holders, link both of the fuse holders together and to a single battery disconnect switch that will then be busbar linked to (hopefully directly connected to) a Victron Lynx Power In module (that will be converted into a fused Lynx distributor).

Todays (first of many) question is- can I get away with a Blue Sea 9003E 350 amp battery switch or do I need the more expensive 3000 HD 600 amp switch? Or is there a different/better way to go? (keeping in mind I'm on a budget!)

 

[robbob2112]

Read this thread post

Post in thread 'WattCycle 12V 300Ah Mini inside look'

Jan 29, 2025

In my opinion -

See the pictures above -if the top is bulged up like the one with the level on top then I regard that as dangerous and a possible fire waiting to happen.. the reason I say that is the red cable I had that was riding on top of the screw...

Second photo in post 24

Second photo in post 76


If the case top is flat then there is little danger (cables next to the bms, very low risk of...

• robbob2112

• 

It you are going to use victron lynx consider this

Victron Lynx Power In

A modular DC busbar, used to connect batteries to the Lynx DC distribution system. The Lynx Power In contains a positive and negative busbar with 4 connections for batteries, loads or chargers and a ground connection.

bluemarine.com

And I would put a MRBF and holder on the positive post of the battery.

3000w/12.8v * 1.25 = 292amps

So the 350amp switch will work. Just realize they are not intended to be turned off with a load on them. If you want to do that get a midnite solar breaker or a MCCB breaker to use as a switch.

 

[WattsItToYa]

From what Will (and others) indicated MBRF fuses are not recommended for Lifepo4 due to their low AIC rating (compared to class T fusing). The potential current flow from a direct ground or shorted battery could exceed the AIC of the MBRF’s. No?

Yes…Lynx Power In module-
Below graphic is what I’m thinking BUT there would be 2 battery feeds to the switch with (2) 250a class T fuses between the battery cables and the battery switch.

I was thinking because the current into the switch (in a catastrophic failure) could potentially be 500a (momentarily) that may require the 600a switch vs the 350a switch?
Although I just noticed the 350a switch is actually rated for 600a for 5 minutes- so it would probably be fine.
Also just found a BMP Marino switch that is rated for 400a continuous /600a intermittent. Might be a better way to go.

 

[robbob2112]

MRBF fuses have 10ka AIC at 12.8v. The class T has 20ka at 12.8v. I am not suggesting you use the MRBF instead of the class T, instead do them in addition to the class T. If the absolutely did happen and the BMS was shorted the battery would output 8000 amps. It wouldn't last long but that is the max possible.

The fuses that won't work for them are ANL,Mega, CNN, and a few others with AIC around 3000a. Always good to know why someone says something so you can make an informed decision.

The reasonI suggest them is in case of a short from a dropped wrench or a wire that got away from you. They are a lot cheaper to blow than the class T.

The Lynx powerin comes empty but uses mega fuses. If you can put the class T connected to the positive post with a very short wire it works.

The problem with these wattcycle 300a mini, other than they are 280s, is the BMS overcurrent protection is high enough that it may as well not be there. So the battery really does rely on the post fuse to prevent problems.

Whichever method you choose to use for class T the series you want is the JLLN or the ones listed for semiconductor.

 

[WattsItToYa]

Copy that....the MBRF's would be in addition to the class T.
Blue Sea 5118 is the class T's I was going to use. Not sure of the JLLN series?

 

[robbob2112]

Copy that....the MBRF's would be in addition to the class T.
Blue Sea 5118 is the class T's I was going to use. Not sure of the JLLN series?

Those are JJN which is a slight variation and is custom made for Blue Sea... most Blue Sea products are relabeled from other vendors... sometimes to higher specs and other times it is the identical product.

 

[Supervstech]

In a 12V setup, AIC is far less important.
Arc is negligeable with 12V setups.
Yes, LFP can output stupid crazy amps when things go badly, but 12V setups just dont have the energy to create much in the way of arc flash.
Quality fusing, properly mounted, and you are golden.
24V and above, absolutely Class T is recommended, but 12V banks are very low arc flash issues.