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I guess I need to make 72v 35ah battery pack

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Sep 26, 2013
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Texas
I've been trying to buy one, but when I describe what I need I never hear from the builders again. I guess I'm going to have to make my own. I have no experience building battery packs so I expect to need a lot of help. I'm going to need to buy everything I need, spot welder, supplies, battery cells. I have a good soldering iron thanks to Amberwolf.

I plan to use Samsung 50S, 25 amp drain cells 20s 7p. I don't expect ever running over 100 amps, but I'd like to have a 120 amp capable BMS running at less than full capacity for longevity. Please recommend a good spot welder, and BMS.

This is a BMS I found, but I'm not sure It's what I need. I might end up running the BMS external of the pack to save space. I have a cavity in my frame just above the battery where the BMS can go. Can anyone explain the difference between common port, and a separate port BMS, and which one is better for me??
 
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Common port, charging and discharging are done on the same port on the BMS. Separate port, there are two ports, one for charging, one for discharging.

Situations when common port is best include using regen and when parallel connecting two batteries as this allows the BMS to still be able to shut itself off if overcharged. Whereas with a separate port bms, regen current and current from another battery pack that is coming in through the discharge port cannot be shut off even if the pack is overcharged potentially causing a dangerous situation.

Separate port is only really a thing because it's cheaper I believe, if anyone knows more reasons please correct me.

Edit: I have a 14s60a common port Daly smart BMS. Similar to the one you linked but with the Bluetooth adapter. I'm quite happy with it. The app is clunky at best, but still it's nice to be able to change all the settings with relative ease and even just being able to see a readout of all the cell voltages without measuring them with a multimeter is pretty nice. I'm also pleased with the BMS itself, it's fully potted and seems robust.
 
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It's nice how you are never without regen with a separate port BMS too, though. Regen doesn't charge very much anyway, so not blocking it on a full battery isn't that unsafe. Meanwhile my controller turns itself off and needs a software connection from an app or laptop to reset the over current faults if it can't regen on a downhill. Probably doesn't matter unless you live on top of a hill, though, and you could avoid the issue by not charging to full.
 
I don't recommend starting with such a large project. It's going to cost $700 for 140 Samsung 50S and a mistake will mean you blew a lot of money. Get a Kweld. I don't have one, but it will minimize weld failures as well as let you deal with the connections required for a 100A battery. Practice on scrap cells til you learn not to blo holes in cells or make welds that fall apart during a ride.

Don't follow any tutorials where they glue the cells together. Only safe for small batteries that are stationary. Use cell forms.
 
It's nice how you are never without regen with a separate port BMS too, though. Regen doesn't charge very much anyway, so not blocking it on a full battery isn't that unsafe. Meanwhile my controller turns itself off and needs a software connction from an app or laptop to reset the over current faults if it can't regen on a downhill. Probably doesn't matter unless you live on top of a hill, though, and you could avoid the issue by not charging to full.
I'm not currently set up for regen, though I hope to be someday. When I do, I want both protection in the controller and the BMS. Plus I only charge to 80% typically. I definitely lean on the over cautious side though. If I could only pick one of those three options (while I won't preach needing all three, I will preach needing at least one!) I'd probably choose protection in the controller so in theory it protects both battery and controller.
 
Appreciate your helpful replies. I'm going to have to show my ignorance by asking some more questions. It sounds like if the charge wires are connected to the battery output terminals, like with a Common Port BMS, wouldn't that bypass the BMS when charging? Would it be correct to say most common eBikes sold online have a BMS that would be considered a Separate Port BMS?
 
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I don't recommend starting with such a large project. It's going to cost $700 for 140 Samsung 50S and a mistake will mean you blew a lot of money. Get a Kweld. I don't have one, but it will minimize weld failures as well as let you deal with the connections required for a 100A battery. Practice on scrap cells til you learn not to blo holes in cells or make welds that fall apart during a ride.

Don't follow any tutorials where they glue the cells together. Only safe for small batteries that are stationary. Use cell forms.
Thank you, that's very good advise.
 
Appreciate your helpful replies. I'm going to have to show my ignorance by asking some more questions. It sounds like if the charge wires are connected to the battery output terminals, like with a Common Port BMS, wouldn't that bypass the BMS when charging? Would it be correct to say most common eBikes sold online have a BMS that would be considered a Separate Port BMS?
No it doesn't bypass the BMS protection when charging. It's more than just connecting the charge wires to the discharge port, the circuitry inside a common port BMS is different such that the BMS is able to shut down current in both directions on it's one output/input. Whereas in a separate port bms, it can shut off current flowing out of the battery on the discharge port but can't block current flowing in. (So something like regen could overcharge the battery since it can't block the charge current) Then vice versa on the charge port, it can block current flowing in, but can't stop current flowing out.

Can't say for sure which most bikes have but probably separate as most bikes don't have regen or any reason to need a common port and separate port is cheaper to make.

Edit: One still does typically attach a separate set of charge wires onto the discharge port with a common port bms, just for convenience otherwise you'd have to unplug your controller anytime you wanted to charge.
 
This is what I want to replace. I'm thinking I can get five 21700 across the frame, and stack two high with room to spare, I hope.battery measurments 2.jpg
 
No it doesn't bypass the BMS protection when charging. It's more than just connecting the charge wires to the discharge port, the circuitry inside a common port BMS is different such that the BMS is able to shut down current in both directions on it's one output/input. Whereas in a separate port bms, it can shut off current flowing out of the battery on the discharge port but can't block current flowing in. (So something like regen could overcharge the battery since it can't block the charge current) Then vice versa on the charge port, it can block current flowing in, but can't stop current flowing out.

Can't say for sure which most bikes have but probably separate as most bikes don't have regen or any reason to need a common port and separate port is cheaper to make.

Edit: One still does typically attach a separate set of charge wires onto the discharge port with a common port bms, just for convenience otherwise you'd have to unplug your controller anytime you wanted to charge.
Ok, that works out better because most of the BMS I see online are Common Port.
 
A question that's come up is, if the battery pack is 20s 7p, can you use a BMS that's 20s 10p, and just not use the last 3 charge wires, or does the BMS have to exactly match the battery pack?
 
BMS doesn't have a "P" number, only S, because all cells in P are already connected together and thus use the same balance/sense wire from the BMS.
 
Cells in parallel don't count as individual cells to a BMS. X# of smaller cells in Parallel is the same as a single large cell.
 
Kweld is $237 USD? They were about $150 in 2021 when I started looking. I'm pulling my endorsement.

After a lot of wasted money on junk welders. I use a $20 welder that does .15 mm nickel well enough for my needs, and hasn't blown up after several thousand welds. Runs off a $50 high c-rate drone battery which needs a $75 RC hobby charger to keep it at storage voltage when not in use. I found these Lipo's puff up in a few months if you leave them charged near full. The battery could power a K-weld, but now that they are $237, kweld is off my list.
 
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KWeld is great, but you pay to play... I tried a couple of cheap Amazon/Ali spot welders before going to KWeld. I wish I had followed "buy once cry once." For the currents you are trying to get, I would think you want to be using copper which would rule out most (all?) cheap spot welders.
 
After a little more research on BMS, I see that the one I referenced in the op is a passive BMS. Active BMS seem to be preferred. Would this one be a better choice? The physical size is good.
 
After a little more research on BMS, I see that the one I referenced in the op is a passive BMS. Active BMS seem to be preferred. Would this one be a better choice? The physical size is good.
Yesterday I ordered one of these BMS, and a K Welder. A 100 amp BMS is the biggest one of these that will fit in my frame so that's what I ordered, and that's the same size as I have now. I chose this one over the Daly BMS because I don't like the idea of balancing by burning off high cell voltage with a resistor. I live in Texas and my battery already has to live in a hot enough environment without making even more excess heat.
 
Looks like even with the holders, 5 cells will easily fit across my frame, and there's more than enough room for 2 layers high, and 7 parallels long. If I knew what I was doing this is probably a very easy pack t build.
Cell holders.jpg
 
I'm in the process of ordering supplies. Heat tape, Fish paper, plastic cell holders. The welder is expected near the end of the month.

All the videos I watched say use pure Nickle strip 0.15 x 8mm. Watch out for cheaper Nickle plated steel. Are we in agreement? Any problem with something like this?
s-l1600.jpg
 
All the videos I watched say use pure Nickle strip 0.15 x 8mm. Watch out for cheaper Nickle plated steel. Are we in agreement? Any problem with something like this?
This depends on your pack layout. Perhaps you can draw it, or at least describe it? I just skimmed through the thread quickly, so perhaps you've stated it and I just missed it, but it seems like you're planning a 100a capable pack. Meaning your total series connections need to be able to handle 100a. How many strips of nickel will be in the parallel connections? I couldn't tell for sure from your descriptions in the above posts whether you're planning 7p or 10p.


If you follow this chart, you'll see that .15x8mm nickel is only good for about 5 amps. To safely carry 100a, you'd need 20. If you're planning 10p, and there's 10 series connection points between all your parallel groups, then it's "possible" to get what you need by stacking 2ea .15x8mm strips per nickel connection. If you're doing 7p, I wouldn't recommend stacking 3 strips.
 
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