LFP for AGM Battery Swap

[Phil Mac]

The Blue Sea MRBF 125 A terminal fuse I’ve installed for each battery is listed with an interrupting capacity of 10,000 A @ 14 V DC. From what I understand ABYC’s recent guidance for lithium banks uses ≈5,000 AIC per 100 Ah of capacity, so a 200 Ah equivalent bank (2 × 100 Ah in parallel) has an AIC requirement of about 10,000 A. I understand that conventionally class T fuses are used but I thought MRBF were OK for a small bank and met requirements. I hope that isn't incorrect but I can certainly revisit.

 

[Phil Mac]

I agree - I didn’t mean to sidetrack us and broad brush all AI use cases as untrustworthy. I’ve seen it make unfortunate mistakes like conflating volts with amps for fuse ratings before, but with care it can be useful.


Upon more reading of the new ABYC standard based on ABYC publishes updated battery and electrical standards your AIC sounds reasonable to me. Panbo quotes the standard as requiring 5KA AIC for each 100AH of lithium. If your batteries are connected to a positive bus then you have two 100AH batteries and they’re each protected by a 10KA AIC fuse at the terminals, so you should be fine. If your batteries are daisy chained then the “second” battery terminal has a 10 KA AIC fuse and 200 AH of lithium behind it, which is still fine.

You may want to check the size of the cables between wherever they are paralleled and the next downstream overcurrent protection. Since the batteries are in parallel and each one is protected by its own 125A fuse you could put 250A on the cables that carry the total current from the bank. If the cables are big enough to handle 250A then you’re fine. If you thought they only needed to be big enough to carry 125A you may want to reconsider.
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[colemj]

fuses are there to protect conductors and equipment.

That is a general description of the purpose of fuses. But in your case, you want to size the fuse to protect the wire it is attached to. Downstream of that, there will be additional fuses to protect equipment or sub-circuits if those are needed.

You do not want to have too small of a fuse on your battery in the hopes of protecting equipment attached to them. Your only concern should be the ampacity rating of the wire attached to the fuse, and that ampacity should be sufficient for all of the devices connected through it. Otherwise, you will risk nuisance fuse blows, which can actually damage downstream equipment.

Mark

 

[Stu Jackson]

That’s almost all.

but I wanted to not oversize the fuses

My That's All assumed using the correct type of fuse for the application and was based on the OP's confusing discussion.

Oversize the fuse? How.......?

 

[Phil Mac]

So what’s wrong with my fuse choice? Do you know how to use ai?

 

[Phil Mac]

I have a small house bank and trivial power draws. The system is designed with all of that in mind. It works great for my use and I have more power than I need. I’m not powering a big inverter or any other high demand devices. I appreciate simplicity and that what this was all about.