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

[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.