Finally going to build powerwall

I am going to finally launch myself into building a larger powerwall battery. It is going to be a 14S24P using new Boston Power Swing 5300batteries 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 14S24P's that can be wired in parallel and use the existing Batrium BMS setup. I am going to fuse the battery at the cell levelfor 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 questionis the size of fuse .....I see what everyone is doing when utilizingreclaimed 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 2.5Aat 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.

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.

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.

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 parallelsections. 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.

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.

Headrc said:

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 parallelsections. 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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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 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 availablejust 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 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 storageI would want to keep my amperage use foreach battery to a logistical limit (in other words no single500p 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 fromit you got 10.6 hours.

That's my suggestion/thought.

Wolf

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.

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)

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.

Here is my progress and how I am building the packs for this smallpowerwall. I actually went with 6.3A fuses. They should blow about 13A/dead shortand as such providewhat I want ....which is protectionagainst a dead short or some other catastrophe ....and not impede current in normal use. This is going to be a 14S24P powerwall with Boston Power Swing 5300's, all new batteries and the Batrium BMS. It is meant to be able to be moved around if need be and for allindividual parallelpacks to beable to be removed in case of any problems with that pack. Additionally if I want to expand it I can just add packs in parallel at the bottom of it and not have to invest in more Batrium longmons.

I fabricated brackets out of steel and then wrapped them with silicone tape to prevent shorting with the BatriumLongmons that will be mounted on the top of the batteries with ziptie holders. This is so the packs can be mounted vertically on a wall. I am also building a box for all of them and the batrium. The box is painted with intumescent paint which is supposed to form a foam barrier and be at the very least a fire retardant in case something like that might happen. There is 1/4" ABS plastic in between each pack.I think I may cover the front of the box with 1/4" acrylic and have ventilation ports on the sides of the boxes and probably hook up some fans but I really don't expect this pwallto be experiencing significant heat issues as the normal load will only be about 60 amps.

The packs are spot welded to the batteries on the positive side with 1.5MM pure nickel which is then soldered to the busbars. The negative side has the 6.3 amp fuses soldered to the battery as well as the busbar. I tried a setup whereI spot welded nickel tabs to the negative side of the battery then soldered the fuses toit and the busbar but it was just too messy for my taste so I went with just soldering on the negative side.

I want to implement a shunt trip breaker with this powerwall but still need to either buy the expansion board ....or figure out how all that works with just the mosfets on the Watchmon4.

That's it for now .....comments are always welcome.

Headrc said:

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 5300batteries 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 levelfor 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.

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Sorry for the bad news. Charging up wont give you more energy (kWh). 3.7 isnominal voltage. That means in your case, you will get 6,6 kWh only if charged up to 4,2 Volt.

Do you have specifics on thisintumescent paint you mentioned?

This is a 14s battery. If the cells are at 3.7 volts each then that comes to 51.8V. 127.2aH x 51.8= 6588.96 or rounded off to 6.6kWh. If the cells are charged to 4.1 (which is what I plan to do) then the total voltage is 57.4V. 127.2aH x 57.4 = 7301.28 or rounded off to 7.3kWh.

The intumescent paint I used is by Firetect. Here is their web page with all the information about their product:

https://www.firetect.com/

There are also several Youtube videos for their product. There are other intumescent paint manufacturer's such as Fireguard. But one deciding factor for me was Firetect was the only one I could purchase a small container of the paint from. This stuff is not cheap and I did not need a 5 gallon bucket for this little project. And the shelf life is a maximum of a year for it.

Headrc said:

This is a 14s battery. If the cells are at 3.7 volts each then that comes to 51.8V. 127.2aH x 51.8= 6588.96 or rounded off to 6.6kWh. If the cells are charged to 4.1 (which is what I plan to do) then the total voltage is 57.4V. 127.2aH x 57.4 = 7301.28 or rounded off to 7.3kWh.

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Ah, you start off with 3.7V (or 4.1V), but as you start drawing power, that voltage keeps decreasing. So tocalculate the total energy stored (Wh), you have to use {average voltage} * capacity (Ah).
So according to the spec sheet, the Boston Power Swing 5300 battery has a nominal voltage of 3.65V and a capacity of 5300mAh.
Nominal voltage is essentially the average voltage between the fully charged state (4.2V) and empty state (2.75V).
The capacity of 5300mAh is reached only when the battery is fully charged to 4.2V. If you only charge it to 3.65V, you've only put in only about 2650mAh.
Now, say you have a battery chargedto 4.2V (because you put in 5300mAh). The energy contained is 3.65V*5.3Ah = 19.345Wh. (spec sheet says 19.3Wh)
If you charge that same battery to only 3.65V (because you put in 2650mAh), the energy contained is about 3.2V*2.65Ah = 8.48Wh.
...roughly speaking.

Ok I see the logic in this. Thanks ...always learning. But I think the issue then is exactly what is the mAh if using just the nominal voltage of 3.7 volts. Looking at the discharge graph on the Swing 5300 spec I gotta believe it is more than 2650mAh. And that also is relevant to what C level of discharge is applied. I will have to run a test of the batteries using these parameters and see what I come up with.

But with all that said I appreciate educating me on this ...like I said always learning ...and I enjoy that.

Just a friendly observation/comment if I may. It looks like the busbars on each side of a pack are terminated so that plus / minus are at the same end of the pack - as apposed to opposite ends of pack. That was my first approach as well but as I started handling the packs I started worrying about dropping a wrench on or some metal on them.. and causing a short. This lead me to updated my pack design to have plus/minus bus bars terminate at opposite ends of the pack, which just seems to be safer as I manhandle them around.

I have seen where opposite ends is also recommended for better current flow thru the pack (instead of from the end of the pack) but an engineer friend told me that at low current it won't matter significantly.

Thanks. Although there was a purpose for this for me. It was so I could add parallel packs at the bottom ...or top at a later date and add capacity and not have to add additional Longmons for the Batrium. What I may do is cover the one end of the busbar with silicone tape, essentially terminating it for the safety you mention. Although I was also thinking of using those opposite ends for current flow as you mention. I appreciate the suggestion.

Why comment people whoknow it better anyway.

Headrc said:

Ok I see the logic in this. Thanks ...always learning. But I think the issue then is exactly what is the mAh if using just the nominal voltage of 3.7 volts. Looking at the discharge graph on the Swing 5300 spec I gotta believe it is more than 2650mAh. And that also is relevant to what C level of discharge is applied. I will have to run a test of the batteries using these parameters and see what I come up with.

But with all that said I appreciate educating me on this ...like I said always learning ...and I enjoy that.

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From the discharge graph, 3.7V might seem like almost 80% ofthe capacity, but when calculating capacity from the discharge curveyou have to figure outthe area under the graph. Try counting the number of boxes formed by the graph grid. You'll see then that at 3.7Vit is in fact close to 1/2 the area vs measured at 4.2V.

Yes, C is absolutely relevant. And temperature. And capacity drop due to age/wear/damage. One can calculate all day long, but it'll still just be a ballpark figure.

I'm not sure why you want to charge only to 3.7V. The general consensus seemsthat in order to increase cell lifespan, higher voltages should be absolutely avoided, as should lower voltages. And while there is a huge benefit in reducing the max charge voltage from4.2V->4.1V, much less is gained going from 4.1V->4.0V. Going lower providesever smaller benefits.
Most people seem to be using acharge/discharge range of4.1V ~ 3.0V ?
I'm a little more conservative, using4.05V charge voltageanddischarging to3.4V,which I guess translates toabout 80%usable capacity?

ajw22 said:

I'm not sure why you want to charge only to 3.7V. The general consensus seemsthat in order to increase cell lifespan, higher voltages should be absolutely avoided, as should lower voltages. And while there is a huge benefit in reducing the max charge voltage from4.2V->4.1V, much less is gained going from 4.1V->4.0V. Going lower providesever smaller benefits.
Most people seem to be using acharge/discharge range of4.1V ~ 3.0V ?
I'm a little more conservative, using4.05V charge voltageanddischarging to3.4V,which I guess translates toabout 80%usable capacity?

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Agreed. Looking at the discharge curves can also give you a pretty good example of "where" the capacities are at. You'll see that the curve levels off around 4.1-4.0V, then it'll start to drop again around 3.2-2.9V (depending on the manufacturer of the cells and chemistry used; always check the datasheet for your cells particular stats). The drop off is usually pretty substantial, too, almost like a cliff.

I really do not want to charge to just 3.7V ...this all came about with the discussion of my kwH calculation. I was informed my calculation was wrong if you look at some of the earlier discussion. My plan is to charge to 4.1V.