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Fuses: Do I need them? What kind/type to use? Where to use them?
Fuses: Do I need them? What kind/type to use? Where to use them?

There are several layers of fuses involved in the process of building a powerwall, or power storage and usage system. Any time you are going to be dealing with potentially life or property damaging energy storage and usage setups, it is highly recommended to use fuses at all levels as possible. This is to protect your batteries, equipment, building, house, apartment, vehicle, someones life, dog(s), cat(s), other critters, liability, and the DIY community as a whole.
The reason for the last one is because if someone doesn't properly fuse their system, it catches fire or causes something else to catch fire and burns down a building or kills someone, the news reporters, gov't, and who ever else, is going to harp on the fact that the system that caused the issue was due to using recycled used batteries, especially if they were lithium based batteries. This is something we don't want as a community as it could encourage legislative's to work on laws that would keep us from doing these types of projects legally.

Fuse Types: There are different types of fuses. Some of the most common are a bare wire, either using very thin wire or actual "fuse" grade wire. These can generally be as little as 1A to blow. The are not shielded in anyway, so there is a "possibility" of melted metal getting someplace where it doesn't belong. Another type is axial fuse. This type of fuse is a piece of glass with two wires on either end, and the fuse wire is inside the glass tube. The gas inside the fuse does not contain oxygen, so it will not make the fuse blow quicker than rated, and also will not corrode or rust the fuse wire. These fuses are very fragile and should be handled with care. It doesn't take much to break the glass.
Another type is the stand blade or buss type fuses commonly use in the automotive industry. Some of these fuses are rated to 48V. I have even seen some buss fuses rated to 200VDC.

Do I need to use fuses and where to use them??
In a word, yes. At some point you need to use a fuse. There are several types of fuses and several locations where fuses are used at.
• Cell Level
• Pack Level
• String Level
• Inverter Level
• Charger Level
• House Level

These are pretty much self explanatory, but I will go over each one to further explain the needs in each location and why to use them there.

Cell Level: This is a very thin wire that is applied from the cell (either Pos or Neg, or on both) ends to connect them to the busbar. The amount of current this fuse wire is to handle is very minuscule. In most applications the current ratings are in the mA, and quite often <200mA per cell; as the current being drawn across the whole pack is shared across each cell that is in parallel (50A draw on an 80p pack = 625mA per cell; on 180p pack = 278mA per cell). It is mostly soldered to the cell, but there are those who spot weld them on.
This is probably one of the most controversial locations to apply a fuse. Some people think they don't need to fuse the cell as the cell will never see any high current draws (<1A). Which is way below manufacturer specs of what the cell can do safely. However, there are 2 main reasons to fuse at the cell level.
1) Human Error: Putting the pack together and some metal object falls onto the pack making contact with both terminals. Without fuses, the pack would rapidly heat up, potentially to fire igniting levels; definitely hot enough that one would burn their hands touching it. Worse case is the cells blow up, or become like a solid fuel rocket, from super heating (especially if this happened with 80 cells or more; 80 * 5A = 400A, or more)
2) This is really the main reason for cell level fusing. As the majority of the packs are made from recovered cells, the cell condition is not 100% known. Its health can be estimated, but not fully accurate. So, after some time in service, it is possible a cell will degrade to a point of shorting out. This usually starts with self discharge; first slowly, then starting to do so rapidly. If it happens to fast, it will cause heating up of the pack, and it is possible to have the same situation as above; fire, smoke, burns, explosion. A cell level fuse is not to protect the pack, equipment, house, etc from the cells. It is to protect the pack from itself. If a rogue cell goes short circuit, the other cells will start dumping large amounts of current into that one cell. If there is a fuse, the pack will be safe due to the fuse blowing at a relatively low current threshold.

Also note, there are those who will straight connect (either solder or spot weld with nickel strip) several cells, then fuse those cells to the bus bar. With this method, you would use a larger fuse to handle the higher possibility of higher amp flow. But usually having only 4 or 5 cells connected together, if one goes rogue, probably won't cause as big of an issue as if 80 of them. Still, there is the potential of 80A (if each cell can do 20A and 4 cells) flowing into the 5th cell, causing significant heating. Is it enough to cause an issue? Maybe, maybe not.

Pack Level: This is a fuse that is designed to go between the pack and other packs. This level fuse is not always used between packs. If used, it is to protect one pack from another for some reason. Maybe a piece of metal (such as wrench or ratchet, or dangling wire) made contact on the power lugs of the packs and main bus bar (I've seen this, and there is a video of it happening. You can watch it here: The ⚡Incident⚡ about 30 seconds in). But, as long as the pack ends are safely covered, there shouldn't really be any problems without using a fuse here.

String Level: This is a fuse that is designed to go between strings or banks of packs. This is usually at max operating voltage of the batteries (especially fully charged). So, if you had 2 banks of batteries that were connected in parallel together (at their most extreme Pos/Neg ends), there would be a fuse between the banks; and actually, have 2 fuses as there'd be one on each bank before they connect together at the main busbar connection. Again, as with Pack Level, this isn't required, as in most cases these ends would be covered and mishaps are unlikely. That is, as long as the ends are super close to each other.

Inverter Level: This is a fuse that is between the string(s) (or main busbar) and the inverter. This is a highly recommended fuse to have. It protects the inverter from any oddity that may occur on the batteries. If there is a sudden surge for what ever reason, the inverter (often a very expensive piece of equipment) is saved the trash bin. At this level, it also noted that this can be used in conjunction with a breaker. The fuse isn't a replacement for the breaker, or the breaker for the fuse. They should be used together if possible.

Charger Level: This is a fuse that is between the string(s) (or main busbar) and the charger. This is a recommended fuse to have as well. It protects the batteries (more so than the charger) from being over charged via current inrush. This can happen if the charger is incapable of keeping a surge from coming through charging source (ie Solar, Wind, etc) such as a Lightning strike, or some other reason.

House Level: This is a fuse that is between the inverter and the house breaker box. This is super highly recommended, almost on the order of required. In most locations, if this is not installed and an inspector paid a visit, it is possible you will receive a huge fine for not being in compliance with electrical codes. This also acts as a safety switch or maintenance switch (or a switch can be added separately). Note: Sometimes this is just a heavy duty breaker that is easily accessible to be switched off without much effort to get to.

Please note that in these I mention the word fuse. This is a general term and can be replaced with breaker in most of the above applications. The one that it would not be is at the Cell Level fuse. This should still be a fuse that once it fails, it is done and needs replacing. All other locations can be a standard DC-rated breaker (except for between Inverter and House, this is AC-rated).

Underlined text were additions, except for headers (11/13/2018)

Quote:DISCLAIMER: I am by no means an expert on any one or all of these fields/questions/topics. The results of this FAQ is a collaboration of multiple different members to come up with a common list of questions that would be asked and we have tried to answer. I was the member who was chosen to post the FAQ. If you have question that goes beyond the FAQ, please post your questions in the relevant section pertaining to your inquiry. Thank you and have a nice day
Hans Tuchel, CBlack, wim And 8 others like this post
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