Thoughts?

[u/MyBatteryGuys]

Hey guys, wanted to get opinion on my custom battery build.
I had a client reach out about needing a battery for an old electric scooter.

The battery had specific requirements as he had old battery bay - it was a tight fit. 330x178mm and about 74.6mm of height to work with, so if the layout looks weird it’s because of the dimensions.

Specs:
- 7S8P - 56x EVE 40P 21700s (25.9V)
- About 32Ah, around 830Wh
- DALY 7S 60A smart BMS

Used 3d modeling and created custom holders for tight spacing to fit the measurements so I also Laser cut the copper busbars myself - modeled the pack in Fusion first, pic is the cut strip next to the CAD. The C ring cutouts are the spot weld lands, one per cell.
External battery level meter, Anderson SB50 output, comes with a 29.4V 4A charger
2 year warranty on the build

Charged $475 all in - design, cells, BMS, meter, connector, charger, assembly and full test cycles. One off custom job, not production work.

Two questions - anything you would change on the build? And was $475 reasonable, or should I have charged more or less?

The cost of cells was ~$220 and the charger, bms, anderson connector, and other misc (fish tape) probably another $80. So call it about $300 in parts, which left maybe $175 for the design work, modeling, cutting the busbars, cell matching, spot welding, assembly and the full test cycles.

The hours I put in definitely don't math out at $175, but it was a one off and I ate some of the learning curve. Curious where you all would have landed on price.

[u/BigBoarCycles]

I'm curious what the c ring cutouts are for. Can you explain more?

[u/MyBatteryGuys → u/BigBoarCycles]

Copper is a pain to spot weld - what makes them great (conductivity) is also the reason it makes hard to spot weld. The c cutout essentially limits how much the heat can be dissipated enough yo make a strong connection when welding. I use steel/copper sandwich to give it more strength.

[u/BigBoarCycles → u/MyBatteryGuys]

Why not use nickel like most commercially available copper buss strips?

While you're limiting the ability to dissipate heat, you're also limiting the ampacity, and of course, the ability to dissipate heat. I suppose current would flow through the thickest part... But not if your main contact points are on the thin wing(c part). Another question- how will you spot weld to the button(+) when the c strips avoid the button.

You might not need the ampacity, then why use copper at all? Just use nickel.

[u/uniballrr → u/BigBoarCycles]

The current takes the path of least resistance (copper). The steel is better for sandwich welding than nickel. 

[u/BigBoarCycles → u/uniballrr]

Better for rusting too

[u/uniballrr → u/BigBoarCycles]

It usually falls right off after the weld. And flux helps with oxidation. 

[u/SanguineDrome]

Looks cool, weird shape for the tabs, I think practically could be better, but looks fun.

I'm sure you know this but you can't making only $175 for that and be profitable, even if it's multiples of the exact same pack (aka, no additional design time, and bulk material pricing), it wouldn't really be worth doing unless you've got nothing else to do.

But sounds like you enjoyed putting it together, and wouldn't have had a chance to do so if it weren't paid for, so it sounds like you're happy with the outcome.

You have all the numbers, so only you can decide if it's worth your time & experience. To be a profitable business, you'd need to charge double.

[u/IronMew]

anything you would change on the build?

The scooter.

500 dollars worth of battery are wasted on anything that runs on 7S.

Given the low voltage and that you say it's old I imagine it's probably something like a Razor 300, or otherwise one of those ancient chain-drive boat-anchor protoscooters from before the day of modern brushless drives and hub motors. None of them are worth investing that kind of money to repower them.

[u/MyBatteryGuys → u/IronMew]

Trust me, I said that. But there is significant sentimental value.

[u/IronMew → u/MyBatteryGuys]

Ah, that I completely understand. As long as they understand it too, I see no problems with this.

Only question I have is about the tab design. Why go for that design instead of simple straight strips (with trimmed corners so as to remove sharp edges)? The actual current-carrying part of the tabs seems worryingly narrow.

https://i.imgur.com/BSbzf1n.jpeg

[u/BigFaithlessness593]

What’s the max discharge, those are great high-discharge cells seems a bit overkill for 60a but it’s great for less voltage sag.

[u/Affectionate_Rub5116]

Umm see thos tiny tabs connected to the center... That's your cross sectional area, I wouldn't put this on a scooter.

[u/uniballrr]

Custom work can add up fast. That guy got a real good deal. Ive been looking for 14s5p copper busbars with ss tabs for Polly dp6 (18650) if that's something you do.