How do you chose cell fuse value?

Joined
Jul 22, 2017
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75
Hi folks,

I've looked through the faq, and posts, and can't seem to find info on the factors that go into choosing a fuse rating for cell-level fusing. What needs to be considered when decidingtheamperage to protect the cells from.

Cell C-rating or other related specs?
Expected draw/cell?
Maximum acceptable current (spike or sustained)?
# of cells in a pack?

If anyone could briefly explain the 'hows and whys' of their fuse choices, that would be helpful too.

Thanks,
Eric
 
Cell level fuse is to protect the cell from the pack. What ever your pack size is, you use a fuse according to that. However, 5A is a good medium between low resistance and quick fusing if something goes wrong.

That is if 5A is the rating at which you want it to start failing (which means it usually will fully fail around 8A)

Now, if you have 10 cells in parallel, then it will take longer for 5A to fail than if you had 20 cells in parallel. That's really the part you take into consideration as to which size fuse to use. The smaller the pack, the lower the fuse rating. But, there is a point at which it becomes increasingly difficult to size the proper sized fuse. But generally no smaller than 5A is needed.
 
Lets turn it a bit around. Your pack... How much per cell do you intent to run max? If 1A the fuse should be atleast 2A. But... how many cells will you have? If lets say 20 cells and 1 cell can easily hand out 10A burst then you got 200A out of that pack in worst case. At low end SOC then you get perhaps half of that so 100A.

Do you choose a cell fuse at 2A or 100A then? As Korishan the fuse at the level is to protect the pack from hitting the cell with massive current if something happens. So the fuse should be smaller or as small as its doable without loosing to much. I would say that a 5A fuse wire is more than enough for normal sized pack. They tend to blow at up to 10A and thats ok for the cell to take for a small amount of time.

If you want to have smaller fuses you need to go glass fuses. I personally prefer the fuse wire: its easier to work with and goes faser. It will give enough protection in my scenario and the price is nothing.
 
Average Joe did a YT video on different amp ratings of glass fuses. He uses glass fuses (as am I when they arrive) and he found that even the .5 amp fuses would only blow when hitting 2 amps.
 
Almost all fuses need higher rating to blow. Household 10A fuse wont blow quickly before hitting like 16-20A atleast. Meanwhile running the 10A at 13A for 1 hour will blow it. This is exactly same with the smaller ones
A 0.5A fuse is rated 0.5A to be able to sustain 0.5A continuous for days! Due to its small form factor to get that to blow you will need 1-3A depending on the time you wait.
Just because a 0.5A fuse blows quickly at 2A you cannot use it for a continuous load at 1A. If you do that you will have to redoo all fuses in some time :p

Thats why its important to dont go above its rating even though it blows at higher rating. Because if you do you just create other problems. If you need tomsething that can do 1A but not 1.1A you need to look for active sollutions or something better than cheap glass fuses from Ebay.
 
daromer said:
Just because a 0.5A fuse blows quickly at 2A you cannot use it for a continuous load at 1A. If you do that you will have to redoo all fuses in some time :p

I'm using .5 amp fuses, like Average Joe because I don't plan on going over 1 amp per cell. My first 48V battery will be for an e-bike with ~70 cells and becausethe BMS can only handle 60A, I'm easily under 1 amp per cell and even then, I plan on lowering the current on the controllerto 10-20 amps because of the local e-bike laws not allowing more than 500W. If the.5 amp fuses blow too soon, I'll order the 1 amp fuses but based on Average Joe's testing, which was pretty thorough, the .5 amp should be enough.
 
If your max amperage will be 1A you have way to small fuses and its just to beg for issues. I would change that. The lower current of the fuse add no extra protection Im afraid. If your max is 1A you should have atleast 1.5A fuses or even 2A. Ignore the rating where it blows that Joe tested because that is fast blow current and should NOT be compared to long term protection.

0.5A fuses running at 1A is just causing extra resistance and will blow after a while since you wear them out.

Neither less its not causing any damage more than more work for you in the end :p
 
We'll see in the summer when I actually get to use the ebike.

If the fuses blow too quickly, I can always solder 2 per cell.
 
Here's the thing with fuses. If a fuse is rated at .5A, and it easily blows at 2A, and you pull current at .5A continuously, you are weakening the fuse. So, let's say you pull .5A for 12Hrs straight. This is done almost every day as there is a certain amount of load on the system. Eventually, that .5A fuse will not hold at .5A because it is constantly running at rated amps. The fuse wire will weaken, the voltage drop will begin to increase across that wire, and instead of supposedly by design fail at 2A, it will start to get closer to .5A until it finally does blow.
 
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