diy solar

diy solar

Horsefly's New 160Ah Build

Are those DIY bus bars? What size screw did you use for the balance leads?
Just realized I didn't answer these questions.... Well, actually I kind of did in other posts. The bus bars are the ones that came with my 8 x 230Ah Eve cells last summer. I ended up making my own for that build, using copper bar stock cut to the right length, drilling the holes and then nickel plating all the bus bars. For this smaller build I decided to just use those old bus bars, as the 160Ah cells are the same size except shorter than the 230Ah cells.

The holes that I drilled and tapped for the balance leads are M4. I bought a big jar of 100 or more shrink-wrap ring terminals for M4 for the previous 3 builds, so that decided it. I think I saw that Andy of @Off-Grid-Garage used M3, which would be fine too.
 
Nope, that was true with many / most of the older JK / Heltec BMS's, but this one runs directly off the 8S battery itself. I think if you want to use any of the current JK BMS's on a 4S 12V battery you would still need a buck converter, but I'm not sure. Supposedly they are coming out with a BMS designed for 4S 12V, which would not require a >12V power source.

It does have an annoying "kick-start" requirement though. To start the BMS when power is first applied, you have to either (1) use the optional little LCD device, and press the button to start, or (2) bypass the LCD button and rig together a resistor and push-button connected to two of the pins where the LCD would connect to the BMS (@upnorthandpersonal figured this one out), or (3) apply a relatively small voltage across the B- to the P- connectors a moment (apparently a 9V battery will do). You only have to do this the first time, or if you ever actually turn off the BMS. I went ahead and bought the LCD with mine (it was cheap), but I may only connect if/when I need to start the BMS.
Great info! Thank you.
 
Just realized I didn't answer these questions.... Well, actually I kind of did in other posts. The bus bars are the ones that came with my 8 x 230Ah Eve cells last summer. I ended up making my own for that build, using copper bar stock cut to the right length, drilling the holes and then nickel plating all the bus bars. For this smaller build I decided to just use those old bus bars, as the 160Ah cells are the same size except shorter than the 230Ah cells.

The holes that I drilled and tapped for the balance leads are M4. I bought a big jar of 100 or more shrink-wrap ring terminals for M4 for the previous 3 builds, so that decided it. I think I saw that Andy of @Off-Grid-Garage used M3, which would be fine too.
Shouldn't all those leads be fused or are they and I missed it?
 
Shouldn't all those leads be fused or are they and I missed it?
While I think there are some people who fuse the balance leads, I think that is a real minority of us that do so. I've not put fuses on any balance leads of my builds, and I think if you look at most of the builds posted on this forum, they don't include fused balance leads.
 
While I think there are some people who fuse the balance leads, I think that is a real minority of us that do so. I've not put fuses on any balance leads of my builds, and I think if you look at most of the builds posted on this forum, they don't include fused balance leads.
Do you know what the intention of why some advocate/insist that you do this? If the wires are thin enough I'd think it would self fuse but I am wondering if when you bundle them tightly together to make the install look clean there is risk of cascading failure with all the wires heating up/catching fire before the initial wire with the short even "self fuses"?
 
Do you know what the intention of why some advocate/insist that you do this? If the wires are thin enough I'd think it would self fuse but I am wondering if when you bundle them tightly together to make the install look clean there is risk of cascading failure with all the wires heating up/catching fire before the initial wire with the short even "self fuses"?
I really don't know. The purpose of these wires is to (1) measure the voltage of each cell, and (2) provide some relatively small current into / out of a cell to help them balance. Neither of these is a high amperage thing. If something goes bad enough that it would open a fuse on the lead, something is pretty bad in the BMS or between the leads. I would also be a bit afraid of the balance lead fuse adding some resistance, although that also may not be a big deal.

If a fuse did blow and disconnect a balance lead, the BMS would at that point see a cell voltage of 0V, so it would do a low voltage disconnect of the BMS. I suppose that is what you want to happen? Depends on how conservatively you size the fuse.

Maybe someone who does do fused links in the balance leads will see this and respond. I just don't know - off the top of my head - anyone here on the forum. Otherwise, you could post your on topic with the question, maybe in the DIY LiFePO4 Batter sub-forum.
 
I see one of Andy video showing fuses on his balance wires. @9:50 mark.
You kinda sent me down a rabbit hole there @Bud Martin. I went skimming through that video and several others weeks / months after. In the video you linked he was still thinking about using the fuses, and not sure if he was going to. He had decided he needed 10A fuses instead of the 5A fuses he had. Then in a later video he said he was still not sure about using the fuses, and the he was going to do it one one battery as an experiment. I didn't make it through all the videos up to present (I'm way behind on his stuff), but it sounds like he isn't sure yet.

I just don't know that I see it being worth the trouble, but I guess I understand the idea of it.
 
You kinda sent me down a rabbit hole there @Bud Martin. I went skimming through that video and several others weeks / months after. In the video you linked he was still thinking about using the fuses, and not sure if he was going to. He had decided he needed 10A fuses instead of the 5A fuses he had. Then in a later video he said he was still not sure about using the fuses, and the he was going to do it one one battery as an experiment. I didn't make it through all the videos up to present (I'm way behind on his stuff), but it sounds like he isn't sure yet.

I just don't know that I see it being worth the trouble, but I guess I understand the idea of it.
Yeah, I'm looking at doing basically the same as he did with fuses on my next build. But they sure won't be 10A rated!!!! I'm going to size them to be appropriate for my BMS sense leads. They can do double duty by fusing the little shunt monitor too-- an often overlooked detail

Edit: A lot of times people talk about the AIC rating of fuses when they are involved with lithium batteries, how does that factor in when you're looking at wiring that will vaporize at the low hundreds of amps anyway?
 
Also interesting to check this table that shows fusing current for the different wire gauges. Puny 18AWG can withstand shorting 83A for 10 seconds, or 250A for 1 second.
 
@RobertGreen and @rhino - The two of you have pointed out the two components of a pretty good argument for not bothering with balance lead fuses.

The little fuses that Andy of @Off-Grid-Garage used will not have much of an AIC rating. If the problem arises that somehow a balance lead is shorting between two cells (like I said, something went really bad), LiFePO4 will attempt to send thousands of amps through that lead. The fuse itself will burn out, but the current will continue - briefly. The size of the balance lead won't care that current for long before it melts and breaks the circuit. I can't say if that melting of a balance lead will cascade to fire, but...

So I don't see a situation where a surge happens that is enough to blow a small glass fuse but small enough that the fuse actually stops the arc. So what's the point?
 
So I don't see a situation where a surge happens that is enough to blow a small glass fuse but small enough that the fuse actually stops the arc. So what's the point?
Batrium at least does specifically state to use a ceramic sand filled fuse which is supposed to extinguish the arc.. I assume by their specific recommendation that those types are better than the regular glass ones in this case.
 
Well, then. I guess I would say I don't see any harm in putting balance lead fuses, but I'll stick with my "probably not worth the trouble" position.

I quickly asked the Google about sand filled glass fuses, and they seem to be a thing in Britain, for some reason. The only sites I saw selling them wanted British Pounds. Who knew?
 
To add on what @SparkyJJO said and since I thought it was somewhat interesting.. I found this question/answer relating to glass and ceramic fuse differences. Basically you should never be using a glass fuse with anything connected to a LiFePO4 battery (and relying solely on that fuse for the safety of the wire it is attached to) because its "high rupture capacity" is too low.. A 10A glass fuse is likely to continue to support the fault (until the wire itself fails or something else) whereas a ceramic one will extinguish the fault because it will have a very high rupture capacity (like 40kA).
 
All those automotive blade fuses would also be a no-no connected to a LiFePO4 battery.
 
I'm not trying to derail your thread by going completely offtopic, Horsefly, but now I'm really thinking about this fuse thing! ?

Using the parametric search on digikey shows a decent few ceramic fuses with sensible looking specs for this.... I would be looking towards something in an axial package like Andy used but with a high AIC rating (maybe a few thousand A) and a price under a dollar. With the weird shortages and stocking issues it is hard to find something that checks all the boxes and still comes in under dollar a piece.
The attached pic comes pretty close.
fuse.jpg

I assume that with smallish ~22+awg wire used, that between the resistance of the crimped connections and the wire itself after a few inches in length, the maximum current you could see would be limited well under the maximum possibly delivered from a large lifepo4 cell. In order to see ~60v across one of these theoretical shorts, the circuit would need to include all 32 crimped or bolted connections across all the cells and their bus bars or cables, which would present a certain non-trivial resistance when added to the IR of the cell stack. I would think that the maximum current that could be developed in a small wire lead in that kind of circumstance would be limited to a few hundred or lower thousands of amps in a liberal estimation.
 
I assume that with smallish ~22+awg wire used, that between the resistance of the crimped connections and the wire itself after a few inches in length, the maximum current you could see would be limited well under the maximum possibly delivered from a large lifepo4 cell.
I think you are looking at it wrong. The 22AWG wire does have some resistance, but all that will do is cause heat to be generated in the wire, and lots of it. It won't be enough to really limit the current appreciably. What will limit the current is the fuse, and or the melting of the wire.
I would think that the maximum current that could be developed in a small wire lead in that kind of circumstance would be limited to a few hundred or lower thousands of amps in a liberal estimation.
I'm not certain I follow your logic, but if I accept your premise then the fuse and wire will be toast within something less than a second.
 
I think you are looking at it wrong. The 22AWG wire does have some resistance, but all that will do is cause heat to be generated in the wire, and lots of it. It won't be enough to really limit the current appreciably. What will limit the current is the fuse, and or the melting of the wire.

I'm not certain I follow your logic, but if I accept your premise then the fuse and wire will be toast within something less than a second.
What I'm saying is this; the wire and connections in the circuit have a certain resistance. This resistance will ultimately limit the amount of current that can flow, even if they were connected to a battery of infinite size that was able to source infinite amounts of current.
I doubt that you could develop 20 kA through one of these sense wires because if nothing else, even the resistance of a few inches or a foot of small gauge wire like that would limit the maximum current.

I could be way off base here, I'm certainly no expert-- but even one of these small ceramic fuses with an AIC of a few thousand amps would make me feel better about the sense leads on my next build.

edit: I definitely tend to think that if there's a short on one of these little wires, it's not going to last very long whether theres a fuse or not! ?
 
What I'm saying is this; the wire and connections in the circuit have a certain resistance. This resistance will ultimately limit the amount of current that can flow, even if they were connected to a battery of infinite size that was able to source infinite amounts of current.
I doubt that you could develop 20 kA through one of these sense wires because if nothing else, even the resistance of a few inches or a foot of small gauge wire like that would limit the maximum current.

I could be way off base here, I'm certainly no expert-- but even one of these small ceramic fuses with an AIC of a few thousand amps would make me feel better about the sense leads on my next build.

edit: I definitely tend to think that if there's a short on one of these little wires, it's not going to last very long whether theres a fuse or not! ?
I get what you are saying, and you may not be far off. So I was jumping to conclusions. Let's look at the math...

22AWG stranded wire has a resistance of 16.14 ohms per thousand feet. So let's say we have 1 ft of such wire, so the resistance on that wire would be 0.01614 ohms. Now let's be optimistic and say it only has a short across one cell at 3.6V. So the current through that one foot of 22AWG wire is 3.6V / 0.01614 = 223A. Now to figure out the power dissipated by the wire in watts, it is the volts times the current. 3.6V * 223A = 802W. That's quite a bit, and it will certainly melt the wire fairly quickly without a fast fuse, but it isn't enough to need a real high AIC rating.

Let me know if I got that wrong. Maybe a regular glass fuse if fine, if you really feel you want to fuse. For me.... I still don't think it is worth it.
 
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