Is there a difference if I daisy chain batteries or connect them to a buss bar for parallel connection?

[AC/DC]

There are trip curves (you can look up) for breakers and for fuses. The idea is the fuse allows all current up to your max operational but doen't allow excess current - indicating a fault or deadshort. This is to stop a serious problem from escalading into uncontrolled runaway event.
The BMS's will cut off current if loads exceed max for a short duration, however they are not able to contend with a huge current flow such as if you dropped a tool directly across the terminals. The intent of the Class-T is to be the ultimate safety shut off for a catastrophic short that if un-interrupted would escalate into serious trouble.
In my set up, I put a mega fuse between cells #8 and #9 as an added 'internal' pack safety device. I used 200A for the internal fuse.

I'm gonna use my very basic beginner setup to illustrate, because I'm literally building my first things and want to bake safety in from the getgo:

I currently only have two (2) 12.8V 150Ah batteries in series, but getting 2 more soon because, are you even alive if you're not on 48V? So I will use that example, even though 2S would still illustrate it. OK, so assuming I have 4 x 12.8V 150Ah batteries in series to make a 51.2V 150Ah battery.

What is the problem we're trying to avert by fusing each individual battery? And is it visible in a Series setup or only in a Parallel setup? My understanding is that if one battery shorts somehow, "there will be an ungodly amount of current that can melt your wires." Thinking about it some more it seems like this could only happen in a parallel setup, is this correct? Because if one of the batteries shorted, it would literally be a direct connection between the entire system's + & — and all of the capacity would "be dumped" at once. In this case, both a breaker and a fuse could technically individually help, no?

Does this mean a Series setup does not suffer from this problem? If a battery in series shorts, in essence becoming a direct cable vs a battery, the voltage of the system would immediately drop by a significant enough amount to trip inverter and whatnot, and even if it somehow didn't the other batteries would only need to "cover" 1/#S the current (so in a 4S, +1/4th) of the load, which would likely not be the max load rating of the battery. So a 150Ah with a 100A load, if one shorted the other 3 bats would need to cover an additional 25A (1/4), in a madeup world where the voltage wouldn't drop to an unusable level. But the point being that there wouldn't be a huge inrush of current to melt one's cables.

So my initial question was, thinking of the internals of each battery, each one of my 12.8V 150Ah batteries contains cells behind a BMS (probably 4x150Ah cells). I was worried that the problem at the battery level could also exist inside each battery at the cell level. And, indeed, if the above is true, it would still exist for batteries with internal configurations which include cells in Parallel, but not for ones (like mine) which are just in series — is this correct?

Thank you for the very interesting and useful info.

 

[OffGridForGood]

I'm gonna use my very basic beginner setup to illustrate, because I'm literally building my first things and want to bake safety in from the getgo:

I currently only have two (2) 12.8V 150Ah batteries in series, but getting 2 more soon because, are you even alive if you're not on 48V? So I will use that example, even though 2S would still illustrate it. OK, so assuming I have 4 x 12.8V 150Ah batteries in series to make a 51.2V 150Ah battery.

This is series connection - the thread title was referring to parallel.

What is the problem we're trying to avert by fusing each individual battery? And is it visible in a Series setup or only in a Parallel setup? My understanding is that if one battery shorts somehow, "there will be an ungodly amount of current that can melt your wires." Thinking about it some more it seems like this could only happen in a parallel setup, is this correct? Because if one of the batteries shorted, it would literally be a direct connection between the entire system's + & — and all of the capacity would "be dumped" at once. In this case, both a breaker and a fuse could technically individually help, no?

If you short a battery in series, only the one(s) in the short circuit path are at risk, however you would (should) still have protection. The BMS on a LFP battery can't be relied upon to cut off a dead short and protect the system.
Imagine four car batteries on a bench, connected in series with short battery cables pos to neg to pos to neg all the way down the line. A fuse just at the very end main pos. Now by accident you drop your open end wrench across battery #3 and the wrench lands across the neg and pos on that battery - bam sparks shower you, the wrench hits red hot in seconds and the fuse at the end of the row on battery #4 doesn't do a thing.

Does this mean a Series setup does not suffer from this problem? If a battery in series shorts, in essence becoming a direct cable vs a battery, the voltage of the system would immediately drop by a significant enough amount to trip inverter and whatnot, and even if it somehow didn't the other batteries would only need to "cover" 1/#S the current (so in a 4S, +1/4th) of the load, which would likely not be the max load rating of the battery. So a 150Ah with a 100A load, if one shorted the other 3 bats would need to cover an additional 25A (1/4), in a madeup world where the voltage wouldn't drop to an unusable level. But the point being that there wouldn't be a huge inrush of current to melt one's cables.

The shorted cell/battery will dump all the current it can, and in LFP this is a staggeringly high amount. The other cells outside of the shorted path will do nothing, and your inverter or main fuse will not trip, it will do nothing to protect you from the 20,000A flow the dead short is unleashing either.

So my initial question was, thinking of the internals of each battery, each one of my 12.8V 150Ah batteries contains cells behind a BMS (probably 4x150Ah cells). I was worried that the problem at the battery level could also exist inside each battery at the cell level.

I does, however inside the pack, it far less likely to experience a dead short, if the pack is sealed up. An exception could be a wire in a poor location rubs through the insulation and shorts to a cell inside the pack. Hopefully wire management has been well thought out and no risk of short will be present inside the pack. That all said, I put a megafuse inside my packs half way through the pack just to be extra secure, and to be the ultimate safety if the BMS fuses into a conductive mess due to some unexpected short.

And, indeed, if the above is true, it would still exist for batteries with internal configurations which include cells in Parallel, but not for ones (like mine) which are just in series — is this correct?

Thank you for the very interesting and useful info.

With BMS controlled packs, I don't recommend putting multiples in series at all.
I prefer the full voltage to be provided by a pack, and if more current is needed, then parallel up packs.

 

[AC/DC]

This is series connection - the thread title was referring to parallel.

Thanks, I was a bit thrown off by the "daisy chain" in the title.

The shorted cell/battery will dump all the current it can, and in LFP this is a staggeringly high amount. The other cells outside of the shorted path will do nothing, and your inverter or main fuse will not trip, it will do nothing to protect you from the 20,000A flow the dead short is unleashing either.

Got it. Would there be a case other than accidental where this could occur? i.e. other than a physical wire/object shorting things out, could a cell inside a battery somehow become bad and cause a short? I'd think if so, it would be ultra rare.

If you short a battery in series, only the one(s) in the short circuit path are at risk, however you would (should) still have protection. The BMS on a LFP battery can't be relied upon to cut off a dead short and protect the system.
Imagine four car batteries on a bench, connected in series with short battery cables pos to neg to pos to neg all the way down the line. A fuse just at the very end main pos. Now by accident you drop your open end wrench across battery #3 and the wrench lands across the neg and pos on that battery - bam sparks shower you, the wrench hits red hot in seconds and the fuse at the end of the row on battery #4 doesn't do a thing.

A while ago I made a beginner mistake and actually did just that with one of the LFP batteries I have, a wrench across the terminals (big lesson learned). This is an EcoWorthy 12.8V 150Ah. There was a spark and the wrench was glued to the terminals (to the one it touched, the one it was already in contact with was fine). I panicked, hit it with a non-conductive object and got it off, panicked some more, saw that not much else had happened tho and nothing else was happening. Then I measured the battery. Dead, 0V. Besides being grateful that things didn't go really south, I was like, oh, no, I killed the battery. A few minutes later I decided to measure it again, and it was back to normal, like nothing happened. The BMS saved my behind.

What megafuse are you using? If the BMS cannot be relied on, the fuse needs to be before the terminal, easier on an exposed battery, but on a encased battery it must be inside the box. Putting one in the middle seems like a good idea too, I wonder if it does anything with resistance that can affect balancing.

With BMS controlled packs, I don't recommend putting multiples in series at all. I prefer the full voltage to be provided by a pack, and if more current is needed, then parallel up packs.

Yep, that's where I'm headed as I build my own 48V, but right now this is what I've got (I actually only have half, 2x12V for a 24V system — ordering 2 more). But yea will start the series closely matched and keep an eye on individual voltages. If/when I build the big bat, I may break these apart for other uses as either individual 12Vs, or one 24V and two 12Vs.

 

[OffGridForGood]

The BMS saved my behind.

Good thing!

What megafuse are you using? If the BMS cannot be relied on, the fuse needs to be before the terminal, easier on an exposed battery, but on a encased battery it must be inside the box. Putting one in the middle seems like a good idea too, I wonder if it does anything with resistance that can affect balancing.

For interior to the pack I use a 200A Megafuse this is my last resort final line of definence and likely will never be needed.

 

[AC/DC]

For interior to the pack I use a 200A Megafuse this is my last resort final line of definence and likely will never be needed.

Gotcha, I now realize it's literally called Megafuse Super small part right, like this? Seems prudent to pick some up esp since they seem rather cheap. Thanks for yet another useful tip!

 

[OffGridForGood]

Gotcha, I now realize it's literally called Megafuse Super small part right,

yes, basically like a mini fused bus bar.
In my 16s DIY battery boxes I put the Megafuse at the "turn" between Cell #8 and #9 - I set the two rows of cells exactly the right distance apart to suit the size of the fuse.
In my 8s DIY pack, I put the Megafuse between Cell #4 and #5, again at the "Turn" where two rows of cells meet up.
I can't take credit for the idea, I was given this thought from a forum member, wish I could recall who and credit them with the idea any time I repeat it.
I have been running six DIY 16s packs for long enough to tell you the fuse in the pack has zero effects on balancing, or battery performance - and yes, at first I put the fuse in a single pack only, and monitored how the pack performed compared to an identical pack with a bus bar instead of the megafuse. No difference and All good.
Now all my packs have the internal fuses.

 

[AC/DC]

yes, basically like a mini fused bus bar.
In my 16s DIY battery boxes I put the Megafuse at the "turn" between Cell #8 and #9 - I set the two rows of cells exactly the right distance apart to suit the size of the fuse.
In my 8s DIY pack, I put the Megafuse between Cell #4 and #5, again at the "Turn" where two rows of cells meet up.
I can't take credit for the idea, I was given this thought from a forum member, wish I could recall who and credit them with the idea any time I repeat it.
I have been running six DIY 16s packs for long enough to tell you the fuse in the pack has zero effects on balancing, or battery performance - and yes, at first I put the fuse in a single pack only, and monitored how the pack performed compared to an identical pack with a bus bar instead of the megafuse. No difference and All good.
Now all my packs have the internal fuses.

Very cool, much appreciate the info and thanks for paying it forward!

Are the ones I linked at AAP the ones? They have a small sale (10% off) and they're in stock at my local store, thinking of picking up like 10 of them.

 

[OffGridForGood]

Very cool, much appreciate the info and thanks for paying it forward!

Are the ones I linked at AAP the ones? They have a small sale (10% off) and they're in stock at my local store, thinking of picking up like 10 of them.

They look right, check they will fit your cell terminal stud diameter.

 

[robbob2112]

Thanks, I was a bit thrown off by the "daisy chain" in the title.


Got it. Would there be a case other than accidental where this could occur? i.e. other than a physical wire/object shorting things out, could a cell inside a battery somehow become bad and cause a short? I'd think if so, it would be ultra rare.


A while ago I made a beginner mistake and actually did just that with one of the LFP batteries I have, a wrench across the terminals (big lesson learned). This is an EcoWorthy 12.8V 150Ah. There was a spark and the wrench was glued to the terminals (to the one it touched, the one it was already in contact with was fine). I panicked, hit it with a non-conductive object and got it off, panicked some more, saw that not much else had happened tho and nothing else was happening. Then I measured the battery. Dead, 0V. Besides being grateful that things didn't go really south, I was like, oh, no, I killed the battery. A few minutes later I decided to measure it again, and it was back to normal, like nothing happened. The BMS saved my behind.

What megafuse are you using? If the BMS cannot be relied on, the fuse needs to be before the terminal, easier on an exposed battery, but on a encased battery it must be inside the box. Putting one in the middle seems like a good idea too, I wonder if it does anything with resistance that can affect balancing.


Yep, that's where I'm headed as I build my own 48V, but right now this is what I've got (I actually only have half, 2x12V for a 24V system — ordering 2 more). But yea will start the series closely matched and keep an eye on individual voltages. If/when I build the big bat, I may break these apart for other uses as either individual 12Vs, or one 24V and two 12Vs.

This is why when you do more than 2 in parallel you put fuses at the bus bar end. If you short across the terminals of 1 the fuse for it blows verse the current from the other 3 (in a 4 pack) doesnt rush down the wire of the shorted one and melt it.

Calculation for a 4 pack of 200amps each when you short one at the terminal with a wrench you get 600amps down the wire plus the 200amps from the battery shorted. In 0.1 seconds it melts the sheath from the 2/0 wire and in 1 second it melts the wrench. Fuses react between 0.01 and 0.2 seconds depending on what type. Class T or MRBF are both basically the same curves and blow between 0.01 and 0.1 seconds..

I haven't checked mega fuse curves. Just checked the curves and the Mega fuses would hold between 0.5 and 1 second before blowing. So if the other BMS do their jobs and limit current to 200amps and the Mega fuses blow with their curves you still _can_ have melted wires sheath and intact wire that is glowing hot still carrying current and even if the BMS on the shorted battery stops you can still melt the wrench with the 600amps. It just would take about 1.5 seconds.

Fortunatly BMS react in 0.000001 seconds if they work properly. But thet can fail short or fail open. If they short they melt in a few seconds and open. But by that time you have a fire.

So, Ideally you just pop a fuse at the bus bar/victron for the shorted battery.