Battery Cut-Off Switch - Positive or Negative Terminal

[mjrendon]

So why do switch manufacturers show the switch connected to the positive terminal? 

Wouldn't it be safer to place the  cut-off switch on the negative terminal lead as it makes sense to disconnect the negative terminal lead first whenever working on the electrical system. 

Has anyone installed their cut-off switch on the negative terminal lead rather than the positive lead in an Oliver?

 

 

[John E Davies]

I don’t know the exact reason, but why would you not simply follow accepted practices? Guessing here, in a multiple battery installation (bank 1, bank 2, starting, bow thruster), like in a yacht, you have separate systems connected to a common ground. Most of the diagrams you see from makers like Blue Seas are what is recommended for a more “strenuous” marine environment.

How many folks individually fuse their batteries? Hands up. Hmmmm. Nobody, not even Oliver, even though it is strongly recommended, especially with a lithium battery installation. I am going to be wiring my new lithiums with terminal fuse blocks. Otherwise there are several feet of completely unprotected large gauge cable.

Victron mandates individual battery fuses, though they show them downstream a ways, which is not as safe, since it leaves some of those cables unprotected.

 

John Davies 

Spokane WA

 

 

 

 

Edited December 27, 2020 by John E Davies

[mjrendon]

1 hour ago, John E Davies said:

I don’t know the exact reason, but why would you not simply follow accepted practices?

If the switch is installed on the positive terminal lead, then any positive leads that are between the battery and the switch are a "risk" since anything in the RV that was grounded is still grounded.  Working on any positive lead between the battery positive terminal and the cut-off switch could be dangerous until the negative battery terminal lead is also disconnected.  I will suggest that it might be safer to do this work if the cut-off switch were placed on the negative terminal, but I could be wrong too.

NOTE - My concern comes from something that I read where lithium batteries are supposed to be balanced annually, which includes charging each one when isolated from the others. So at least annually I would expect to be working in the RV battery compartment.

1 hour ago, John E Davies said:

How many folks fuse their batteries? Hands up.

I had thought about this quite a bit when you mentioned Victron's recommendations in a previous post.  I read that Victron uses an external BMS (battery management system) that is incapable of providing isolation for overcurrent situations, where as some other manufacturers do use internal BMS with this capability. I believe that Victron recommends fusing each of their batteries individually because of this. Fusing a Victron's battery would be the primary protection for a short circuit event and would be a back-up for other batteries that already have this protection built-in with an internal BMS.

Edited December 27, 2020 by mjrendon

[Jim and Chris Neuman]

I use the Blue Sea's terminal fuse blocks on my sailboat.  In a boat the primary function of fuses is fire prevention as fire at sea can be disconcerting ... maybe less so in a trailer but this is cheap insurance. 

Kind of spendy but these fuse blocks are very well made and a snap to install.

 

 

[mjrendon]

10 minutes ago, Neuman's said:

I use the Blue Sea's terminal fuse blocks on my sailboat. 

Neuman's,

Do you fuse each battery on your sailboat and if so, are you using Victron lithium batteries or another manufacturer? 

Thanks,
Mike

[Jim and Chris Neuman]

Hi Mike,

I have two battery banks.  A single starting battery (105 - AH AGM) is fused between the alternator / battery charger and the ships power bank (3 - 105AH AGM's) is also fused between the alternator and battery charger.  This is done per the recommendation of Blue Sea's as well as Balmar, the alternator manufacturer.

Battery charger is for use when tied to a dock, is also a Blue Sea's unit.  They make really good stuff.

Jim

[mjrendon]

1 hour ago, Neuman's said:

I have two battery banks.  A single starting battery (105 - AH AGM) is fused between the alternator / battery charger and the ships power bank (3 - 105AH AGM's) is also fused between the alternator and battery charger.  This is done per the recommendation of Blue Sea's as well as Balmar, the alternator manufacturer.

Jim,

This is interesting.  The AGMs do not use a BMS and I suspect that your fuses are placed between each system component, where as with Victron lithium batteries they recommend fusing each battery individually.  So if you are using multiple Victron batteries in a single bank, then each battery should have its own fuse for over current protection.

Mike

 

Edited December 27, 2020 by mjrendon

[Jim and Chris Neuman]

Mike,

Correct - on my boats 3 battery  ships systems bank (all 12V 105 AH AGM's) the Blue Sea's fuse bar is placed on the + stud at one end of the bank and cables from both alternator and battery charger are fixed to the blocks fused terminal stud.  Negative cable is attached to the last battery in the bank and is not fused.  No need to fuse each AGM battery ... the manufacturer of your lithium batteries may have a different take.