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Finally going to build powerwall
#1
I am going to finally launch myself into building a larger powerwall battery.  It is going to be a 7S24P using new Boston Power Swing 5300 batteries that I have.  I will also be installing a Batrium BMS on it and plan to lay it out so at a later date if I want to, I can more 7S24P's that can be wired in parallel and use the existing Batrium BMS setup.  I am going to fuse the battery at the cell level for safety reasons, but the batteries are all new so I am trying to decide on the size of fuse.  I want the safety but also don't want to limit the battery too much if the load calculation increases in the future.  Each cell is capable of 13A continuous discharge so this is potentially a lot of current capability.  Total aH is 127.2 giving me around 6.6kWh at the nominal voltage of 3.7 and more when charged up.  

So right now the question is the size of fuse .....I see what everyone is doing when utilizing reclaimed cells ...but what about new cells like the ones I am going to use?  Current load calculation is around 60amps.  Just using this means 2.5A per cell which the batteries definitely can handle.  But what if I go to a larger load calculation in the future and don't add another pack in parallel yet?   If I put a fuse that blows at 2.5A at the cell level, then a larger load cannot be handled even if the batteries themselves can handle it.  But if I put a fuse at the cell level that is really in the range of the 13A these cells are capable of I may not have the safety level for unknown disasters we all want to be prepared for.

Thoughts on this are appreciated.  As I move ahead on the project I will update with pictures and progress.
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#2
Each level of fusing has a different purpose, pick fuses for the maximum current rating of your cells and packs configuration.

At the cell level you are trying to protect against a shorted cell allowing current from the rest of the pack to flow through it. Look for something in the 10-15A range. The pack level fuse is protecting against excessive loading of the pack. In your case a 24P pack with 13 Amp cells should support 312 Amps, so I would get a 250 Amp fuse/circuit breaker that could be replaced/reset if it blows. At the battery level you could go with another 250 Amp fuse/breaker, but I'm not sure it's necessary, if you have done the pack level fusing. In fact it might be cheaper to skip pack level fusing and fuse the final battery assembly. I would be curious what others, with more experience think.
Headrc likes this post
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#3
For new cells, you would probably be fine not putting a fuse on every cell. However, I would still recommend sections having a fuse. Such as, have 14s80p, and the 80p is sectioned into 4p sections, then those are fused to the main buss. This is more for protection of the whole pack more than for a rogue cell (which cell level fuses on reclaimed cells are really for).

This is the path HBPowerwall and I think Mike are doing on their new cell packs.
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#4
Thanks that helps.  I thought about just doing sections such as the 4p, but fuses are so cheap I thought that doing them at the cell level was extra added protection.  But as stated in my first post, what size fuse is the question.  I still might just do them at certain parallel sections.  But if I do them at the cell level, and the cells are rated for 13 amp then a fuse that blows at 13A seems appropriate I would think.  Unless that is then too little protection.  I am mainly concerned about natural or manmade disasters not the cells themselves being defective.  I have used these Boston Power cells for a lot of projects and they are pretty robust and deliver on their specs.
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#5
I watched several youtubes on this topic. In my case, I soldered my 18650 cells to buss bars as in many pack building videos... so it was natural (and recommended) to use fuse or fuse wire as you wire/connect the positive to the buss bar. Since I now have over 5,000 cells I settled on 30AWG Remington Tinned wire - e.g. 7-9'ish amp range - as a general solution/practice to solder positive to bussbar. As I understand it the goals are:
1) Need a practical solution - you don't want to spend large $$ or large time as number of cells grow.
2) You don't want fusing too low amps - so be sure to have a maximum amp/cell in mind as you design your packs.
3) As I understand it - its OK to have a litter higher amp fuse wire than cell rating.

In my case, the 30AWG = 7-9'ish amp, while some of my cells only have 4amp max discharge specs and some have 10amp max discharge specs. This is OK as the fuse wire is for *catastrophic shorts*, rather than some kind of 'protection' again short term overload of individiual cells. In other words - I'm not trying to protect a 4amp max discharge cell from 5amp draw... but rather protect against dead short / catastrophic failure of a cell from damaging the entire pack.

As far as fusing/circuit breaker of whole battery - you want to keep in mind the wire size. Its not just about the max the pack can deliver, but also that you fuse so that the wire won't breakdown in case you use smaller wire than battery can deliver. In my case, my battery 'could' deliver 700amps but I fuse at 250amps as that's the wiring I use.

I should add - I'm offering my best interpretation/experience from many youtube videos. I am NOT an expert electrician and make no claim that what I'm writing is the best advice.
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#6
(05-13-2019, 02:41 PM)Headrc Wrote: Thanks that helps.  I thought about just doing sections such as the 4p, but fuses are so cheap I thought that doing them at the cell level was extra added protection.  But as stated in my first post, what size fuse is the question.  I still might just do them at certain parallel sections.  But if I do them at the cell level, and the cells are rated for 13 amp then a fuse that blows at 13A seems appropriate I would think.  Unless that is then too little protection.  I am mainly concerned about natural or manmade disasters not the cells themselves being defective.  I have used these Boston Power cells for a lot of projects and they are pretty robust and deliver on their specs.

Being close to starting my powerwall project myself albeit with reclaimed laptop cells (just need another ~400 or so) I am also looking at the fusing issue for my 200p packs.
My fusing philosophy has always been a fuse is there to protect the system from an overload or short not to impede the circuit with resistance.
So I generally will add another ½ of the expected current to the fuse size. Example if I am expecting to draw a maximum of 10A from a particular "battery" I would use a 15A fuse. Enough headroom for an occasional spike but low enough to quickly interrupt the circuit in case of an overdraw.
In your case I would still do single cell fusing as fusing a 4p pack means a 52A fuse ( if you are going with 13A). I'm not sure a nice spot weldable glass fuse exists in that amperage range. Even if you are going with a conservative 2.5A per cell  thats 10A for a 4p. I'm not sure you can get an Axial glass fuse in that amperage. The biggest ones I have looked at are 5A. Doesn't mean they are not available just haven't found any larger than that.I haven't looked either to be honest. Of course you can always go with another setup as the do make some pretty large amperage regular glass/ceramic fuses. 

Personally I would probably go with the 5A Axial glass fuses as we know they mostly really blow at twice their amperage on each individual cell.
I know its over the ½ threshold that I initially said. But in your case your possible expansion would warrant that and if you are pulling 5A out of that battery (5300mAh) its only going to last you an hour anyway. 2hrs at 2.5A.
If you are building a powerwall for storage I would want to keep my amperage use for each battery to a logistical limit (in other words no single 500p packs) to maintain a steady low amperage draw for a long time. I mean if you look at a 5300mAh battery and just draw 500mA from it you got 10.6 hours.

That's my suggestion/thought.

Wolf
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#7
Thanks again.  Actually I have found the glass axial up to 15A on Ebay.  I ordered some 5A and 6.3A and will test with the same thinking that they will probably blow at double those values.
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#8
Also remember that as a fuse gets closer to its rated amp, the wire will start to create more resistance (which is true of any wire, really). So if you plan to pull 5A, you wouldn't want a 5A fuse, but 7A, so as to not waste a lot of power to heat generation, and also shorten the life of the fuse (it will eventually blow even if never gone over the rated amp, but ran high for a long time; think automotive light fuses that 'randomly' blow for no apparent reason)
Headrc likes this post
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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#9
So ..instead of making the choice by when the fuse will actually blow I should use it according to its value?  Looking at all the tests I have seen it seemed to me that folks had a focus on when the fuse actually failed.  Of course those have been reclaimed cells, not new.
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