Fuses for Balance Wires? Yes or No?

[OffGridForGood]

I have been building DIY 8s and 16s packs for awhile now.
During the build on my first pack, I wondered about installing small inline fuses on the balance leads between the wire and the bus-bar connections. We always say "fuses protect wires" and we pick suitable fuses based on wire sizes.
That all said: What is protecting the BMS balance lead wires? And, more importantly what is protecting cells if the BMS were to fail?
This seemed a moot point when I started on DIY packs, however we are seeing higher active balance currents associated with BMS's and occationally I see some concerns raised about the potential for a BMS to fail and consequences to the cells if the failure is closed, instead of open.
This leads me to consider adding small fuses on each balance lead in my DIY packs.
Questions:
1. Is this an over-abundance of caution?
2. Do any of the commercially built packs use fuses on the balance leads?
3. If I did add a fuse to each balance lead, would I use a small 5-volt, 3Amp fuse? (depending on active balance current, times how much?)
4. What potential down-falls could result if a balance fuse fails during normal opertion of the BMS and pack?
5. If a balance lead fuse fails, how will the BMS respond, does this shut down the BMS? - how would we know the fuse has blown inside the pack case? (cell lead resistance is shown on the JK BMS's data, so an open would show here if one were to look for it.)

{I pondered if this discussion should be posted in DIY batteries, or Safety Check, - fundamentally I feel this is a question of safety, and thus posted here.}

 

[BentleyJ]

Actually an interesting question many have probably thought about. This is my opinion only, no testing to back it up. Just some observations made during bench top tests/mistakes!! For example a typical JK BMS with 2A balancing built in. The wire gauge is so small, maybe even undersized, I would consider the wire essentially is a fuse. It will melt away almost immediately if shorted out thus opening the circuit. Same is true for all the low price Chinese balancers.

Once you move up to 5A+ of balancing capability which would imply 16ga and larger wiring to keep resistance low, serious heat can be generated before wire failure, seems like a fuse would be a good idea.

 

[time2roll]

Those tiny wires are considered to be self fusing.

 

[OffGridInTheCity]

The Batrium Cellmate K9 uses inline fuse holders (pic below from @LithiumSolar k9 install vid) on each balance wire - but as stated above, it seems like overkill to me. I use longmons and attach the + and - wires directly to the packs - no fusing. For my Chargery(s) I also attach the balance wires directly to the packs.

 

[rhino]

I added 3.15A fast blow inline fuses on every balance lead of the 3 JK BMSs I use. There seems to have been quite a few "accidents" caused from balance leads that appear would have been prevented had there been fuses on the leads. Most recent one being this incident: https://diysolarforum.com/threads/melted-wiring-in-gsl-battery-any-idea-what-went-wrong-here.74821/

Those tiny wires are considered to be self fusing.

I'm not sure people have thought about ALL of those balance leads since at some point you usually have them all bundled together. Seems like even if one balance lead "self fuses" when it is bundled together with 16 other leads there is possibility it starts chain reaction of burning/melting other leads.

 

[rhino]

i.e.

 

[OffGridInTheCity]

Yes, it's true that physically running longer wires, and bundling together can use some thought. For example, I have 36" long sense leads into DIY batteries inside my APC 3000s... which as all metal. And at first, just scraping the wires against sharp metal edges as I routed them caused some sparks and I wound up encasing them in flexible mesh conduit.

My point - I was taken by surprise at how easily the insulation seemed to scrap thru to a short on the metal edges - did not expect it. Also, exposed wires can be hit by tools or snagged on a tool or squished if something drops on them or runs into them or other unexpected events.

 

[rhino]

My point - I was taken by surprise at how easily the insulation seemed to scrap thru to a short on the metal edges - did not expect it. So extra precaution can help protect against things like this.

Interesting you should say that since I just finished putting together another battery and decided to try a high abrasion resistant balance wires. I don't attach the JK BMS lead wires directly to the cells. My balance lead construction is ring terminal->inline fuse wire->3.15A fuse->inline fuse wire->butt splice->18AWG FEP insulation wire->insulated ferrule->feed through terminal block with one connection to two connection->JK BMS leads. My first battery I built like this is about 2 years old now.

 

[Skypower]

I hate glow worms over lifepo4 cells. Lots of good points here. If you’re using those little mini fuse holders there’s a possibility of intermittent connection. Check if the contact points are brass or nickel plated. The plain brass contacts against a nickel plated fuse end cap can setup corrosion. Unplug the balance leads from the bms if there’s any chance of a fuse contact touching a terminal. A quick buff on the brass with 3M cloth and a small dab of silicone grease or No-Ox-Id A special on the contacts is a permanent fix. Be aware that the edges of buss bars, including flexible and the two bolt terminals have sharp edges, so route away, use caution.

 

[OffGridForGood]

Some interesting thoughts, thanks guys.
I am not so sure about the idea of the wire being 'self fusing' - as a test I put 5x the amperage through a scrap chunk of balance lead wire, and it got pretty hot, but didn't 'self fuse'.
I wouldn't want my BMS leads to be allowed to carry 5x the normal amperage - ever - and I don't want a wire to melt in half to act like a disconnect. A fuse inside a fuse holder pops and this remains safe/ doesn't create a brand new path to a short.
I will have a look for some small inline fuses, and be mindful to watch for nickel plated vs brass. Anyone have a link to ones they have used?

 

[400bird]

This is what "self fusing" balancing wire looks like:

Melted Wiring in GSL Battery - Any idea what went wrong here?

Power went out a few nights ago. System is grid tied with a pair of Outback Radians and Iron Edison/GSL 10kWh LFP wall batteries. Opened up one of the batteries and was shocked to find melted wired and evidence of smoke. The other 3 batteries seem to be in similar shape (though I am honestly...

diysolarforum.com

It's debated on the root cause in that case, but still.

1. Is this an over-abundance of caution?

Not in my opinion

2. Do any of the commercially built packs use fuses on the balance leads?

Factory EVs do, but I'm not sure about stationary energy storage.

3. If I did add a fuse to each balance lead, would I use a small 5-volt, 3Amp fuse? (depending on active balance current, times how much?)

3 amps is probably fine as long as you balance current is less than 1 amp.

4. What potential down-falls could result if a balance fuse fails during normal opertion of the BMS and pack?
5. If a balance lead fuse fails, how will the BMS respond, does this shut down the BMS? - how would we know the fuse has blown inside the pack case? (cell lead resistance is shown on the JK BMS's data, so an open would show here if one were to look for it.)

A failed would present as a 0 volt cell, so yes the BMS would open/shut down.

Also, gah! No matter how many times I click, this is stuck in italics.

 

[Hedges]

Silicone insulation, or sleeve over wires, would be good to contain them as "fuse".

Although regarding automotive fusible links, I don't find mention of silicone. Might just be rubber, not plastic.

 

[Quattrohead]

What happens to the battery if 1 fuse blows ?
In other words, does anyone run their batteries with a balance wire off ???

 

[OffGridForGood]

Silicone insulation, or sleeve over wires, would be good to contain them as "fuse".

Although regarding automotive fusible links, I don't find mention of silicone. Might just be rubber, not plastic.

I have worked with a number of DIY packs, one thing I noticed with the balance lead wires, it is pretty springy. I have made a habit of using a hockey puck (Canadians, eh) to hold the wires from touching the other cell bus bars while I work on installing the balance lead wires.
This leads me to consider: if a wire was to self-fuse and melt at a point, the loose end could curl up and touch a near-by bus bar = bad.
The wire loom may prevent this, or may not. A fuse on the other hand, will not allow the wire to move when it disconnects, and will not present a potential hazard.

 

[400bird]

What happens to the battery if 1 fuse blows ?
In other words, does anyone run their batteries with a balance wire off ???

I put the wrong size fuse (0.6 amps) on a few balance leads and have blown fuses until I found them all. The BMS just sees a cell at 0 volts, faults, and stops current.

 

[OffGridForGood]

I put the wrong size fuse (0.6 amps) on a few balance leads and have blown fuses until I found them all. The BMS just sees a cell at 0 volts, faults, and stops current.

And this would bring the user's attention to the problem, or at least we would wonder what is going on with one pack, and look at the settings.
One pack would fail-safe, other packs would continue to opperate normally. No down side then.

 

[rhino]

I have made a habit of using a hockey puck (Canadians, eh) to hold the wires from touching the other cell bus bars while I work on installing the balance lead wires.

When building my batteries I put a piece of tape over the inline fuse holder without the fuse in it to hold the two pieces together then I don't have to worry about shorting anything. Then I go back after I have all the leads wired up to put the fuses in. Also works to show which holders need fuses still.

Check if the contact points are brass or nickel plated. The plain brass contacts against a nickel plated fuse end cap can setup corrosion.

I'm using these which are not nickel plated: https://www.digikey.com/en/products/detail/mpd-memory-protection-devices/BF303/2330519 .. not sure how much of an issue that will be?

Using these fuses which are ceramic: https://www.digikey.com/en/products/detail/optifuse/FCD-3-15A/12090372 and says the end caps are nickel plated brass.

 

[toms]

When I use an active balancer in combination with a BMS, I use a fused heavier gauge wire to a DIN rail terminal block, and then take off the sense/balance wires from that.

That keeps the install nice and tidy.

 

[Geoffersb]

I was going to post about the same thing! I have a set of 4s CATL (out a Hyundai) batteries, I've split one so I have a 14s2p setup, used dsub cables cut in half and stripped back, these go to a junction box I made as each pair of cells share the same input to the BMS and there is a ntc sensor on top each battery I want to use

Made me think I ought to fuse them to protect everything, I was considering using those little mini blade fuses and putting a four fuse block on each battery?

Sounds like I might be setting up a problem later on with any corrosion problems?

 

[rhino]

I was considering using those little mini blade fuses and putting a four fuse block on each battery?

I specifically chose ceramic fuses because they have a high interrupt current rating.