lol my apple is tastier then 
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Iâm thinking through building some p42a packs.
12s2p ( 4x3 pgroups in tayto ) and 12s5p (separated/flexi p groups in prototipo)
With 30-40A discharge per cell Iâm hung up on the current required at various points.
any examples to point at of similar builds?
If iâm gonna connect tabs from cell to a bus bar type thing, how big a tab? how big a bus?
seems like 2 layers 0.15mm x 12mm wide nickel is just enough to handle the current from one cell to some kind of bus.I know the series connections can be parallelized and the current flow from cells through the bus towards the series connections is kinda divided amongst the series connections. so I can reduce the amount of current on the bus connection. for high amperage cells like this though what kind of bus connection is appropriate?
presumably if I had 2 cells and 1 series connection then does any part of the bus see the more than a single cells worth of current? or just the series connection?
what else should I look at / read?
Howcome everybody (OK, most) sells these extension cables without the balance end on?
what do I need to attach the balancing end? a crimping tool?
Theyâre designed to be soldered. Theyâre for either building your own packs, in which case youâre soldering already, or repairing broken packs, in which case youâll probably be soldering too.
Basically those arenât âextension cablesâ, theyâre just cables. If they were extension cables, theyâd have connectors on both ends, one male and one female.
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haha,
I was looking for just the connector part and wondered why they only seem to sell these with the damn wire already on it, lol.
Yeah, a crimp tool for JST is definitely in my battery toolkit. Mostly because I build packs for charging with hobby chargers often. BMS work rarely needs the crimps, unless you have to repair a broken balance lead.
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Use this table.
The tabs carry the current for only one cell. The bus carries current for the whole group, so naturally needs to be bigger. Me? I would use .2mm thick nickel as wide as you can get your hands on. Series connections with some husky braid or better, and arranged so that current travel through nickel is minimized. 225 amps continuous for the 5p is no joke, if you actually intend to pull that much from the battery. Whatâs it pushing?
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Do you plug main then balance or balance then main?
Main lead first, balance second. And remove balance, main.
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One of the advantages of using the hobby charger is that they can detect errors in wiring without blowing up. Iâve plugged shit in whatever order and itâs fine. The charger will throw a âbalance lead errorâ if thereâs a problem, often before I can see it. Like a wire about to break. I had a a balance lead error once and thought that was strange. didnât see anything wrong, so I jiggled some cables. and sure enough, once came loose. Also I tried plugging it in backwards with alligator clamps and it throws a âreverse polarityâ error instead of poofing. Good times, lol.
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Best TIL of the day for me.
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yep. already was. and those are easy to think about in terms of tabs and series connections.
itâs the bus bits that were giving me some thoughts.
right, only starting with a unity pushing a pair of 6374 or 6396 and a heavy rider, so If I can I will work my way up to max the unity at 120Amps from the battery. but weâll see I"ll probably have thermal issues. I want to minimize voltage drop. .
but should i design for what the pack can deliver? or what I will ask it to deliver? I had been assumign the former.
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This is a great question.
I often design for what the pack can deliver because it will be near what I ask it to deliver, plus some 10-20% head room (thinking 30qâs here). These high amp cells kinda change that game up. In a perfect world, I would choose a cell that balances power and capacity for the application, and build accordingly, but often we donât get that choice. If the wiring is too small, it will heat up, slow shit down, and show itâs love for you by glowing red in extreme cases. Too big means you carry some extra weight and wasted money on more copper than you actually needed. Going too big also makes it more difficult to build, and cold joints can result if you use a puny soldering iron against something like 3/4" braid. The cost vs benefit equation changes depending if you own a factory making scooter batteries or maybe just a homejobber hacking together an esk8 out of random used parts.
Did this beer fueled, rambling excuse for an answer help?
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So Iâm going to build a 12s10p pack for the Flux motion deck.
Gotta be 18650S
are there any cells that would make more sense than 30Q?
Going to run 8 inch Kendas off or TB 6380 170kV motors on a Unity. So I think Iâll be ok setting motor amps at 70 and battery at 40âŚ
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MJ1 will give you 10a and 3500mah per cell. Makes sense for a battery that big, especially if you arenât gonna ask more than 80 battery amps. 1/6th more capacity if thatâs what you are after.
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Iâm not sure I will need more capacity⌠But maybe I can use those and make a 12s8pâŚ
Iâm a little worried about they weightâŚ
Damn I didnât know this. Makes me wanna move over to hobby chargers.
What number of A would you recommend in a charger for an 8s 2p p42a battery?
Maximum charge current is 2C, so 16.8A. Really though, unless youâre dead set on having your battery charged from dead to full in 40 minutes, a 4 or 5A charger is more than sufficient, and much smaller and cheaper.
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thanks
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