Molicel P42A 20s8p (Part 3) - Buying The Parts Thread

Hello guys. I’m buying the parts now. I’m starting with the battery cells. Since it’s going to be 20s8p, that’ll be 160 cells. My friend told me to buy 200 cells just in case there’s bad cells. Do I really need to order 40 extra cells or is he just telling me to buy extra so he has some for himself for a range extender or to add an external power booster? LOL. I would understand 170 or even 180 but 200? LOL.

This is my first time buying cells so I’m not sure how much of the cells are usually DOA. So yeah. Thanks.

Here’s the parts list:

Cell: Molicel/NPE INR-21700-P42A 4200mAh 21700 Battery - Authorized Dealer – Liion Wholesale Batteries


Wires: 8 AWG Bulk Roll Silicone Wire Priced for Per Foot - Black


Soldering Station:

Solder Flux:

Kapton Tape:

BMS: JBD Smart BMS 6~22S 6S 8S 16S 20S 22S 250A Li-ion Lithium Battery PCB for Freeshipping - Jiabaida BMS

Some of those, I already have. The ones I don’t have are the BMS and the Cells. Let me know what you guys think of the BMS and how much cells I should be buying.

Thank you.


I forgot about the shrink wrap and the fish paper. I’ll look for it some other time but when I find them, I’ll leave the link here for future people who are planning on building their own batteries.

I’d probably grab a handful extra - it’s not like doing flooring in your house where you need 20% extra.

I’d also recommend consolidating this build into one thread instead of spreading it over many


I see. Thank you. I’ll order 170 cells.

What do you think about the BMS? There’s 2 different websites where I can see the same model.

I’m guessing they’re both the same but there’s a big difference in price so I’m assuming there’s a difference between the two.

I’m sure it’s capable but I want to know what you think of the spot welder.

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I agree, I usually buy 2-5 extra cells and base it off of how close i am to the threshold for lower price per cell by quantity. If going up a few gets enough of a discount to pay for the extra that just makes good sense.

I dont get many doa cells, mostly i want a few to test and mess up. Weld settings and tear off tests on separate non important cells is a nice little peace of mind.


What’s your use case? Most bms are used in bypass for diy eskate

Is this for an electric vehicle? I would not start with the battery.

If it’s not for an electric vehicle, carry on…

Its the same bms. JBD is the mfg, hence why it is cheaper from them. Xiaoxiang is just adding their markup to it.

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My current battery is started cutting off at 73v. Recently, it started cutting of at 76v.

I planned on upgrading to a better battery in the past but I’ve put it off for so long since my reasoning was “I got a working battery. Why bother?” but now, since it’s broken, I finally got the push to build it.

I see. JBD it is then. Thank you.

I already have the rest. Battery is the last thing it needs.

Why 250a bms? Is it because you are not using vesc or similar discharge voltage protective esc?

Spot Welder:

You don’t need a 250a bms. You aren’t going to use the discharge port anyway.

Save your money and get a much smaller bms. I have a 40a jbd in my 20s board. It’s quite small.

Cells DOA? Probably extremely rare that happens. I wouldnt buy an extras. If you need 160 buy 160. 40 extra cells is nonsense. Someone’s trying to rip you off if they’re telling you that.

. If you get unlucky and have a few cells under voltage, just buy a cheap charger. Or if they’re over voltage throw them in a 21700 flashlight for a few minutes.

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DOA is unlikely but cells that have self discharged more than the majority definitely happens. I always toss those cells.

I always order 2+% more than i need and it’s paid off plenty of times

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I built a pack out of only low cells purchased from a battery builder, and it’s stayed perfectly balanced. I’ve also used those cells in in regular packs and also saw no imbalance. Ymmv ofc, I just buy +1 cell in case I drop one like a dumbass

Yeah but if you were building a brand new 160 cell battery, would you risk compromising it by putting a questionable cell into it?

I sure as hell wouldn’t.


I’m not suggesting putting questionable cells in a new battery. I said buy a battery charger so you can charge up any low voltage cell before assembly.

Imo it’s just wasteful to order extra cells if you only have plans to build 1 pack. More likely than not those cells will be unneeded. Just order a few more cells if you end up in the unlikely scenario that you need them.

Op is talking about ordering 40 extra cells and that is ridiculous.

How low is your cutoff for new cells?

You have no idea how much of a cheap bastard I am

When i get a batch of a 100 or so cells and every one is at 3.52v but one is at 3.47 im not gonna use it other than for testing welds.
Personal preference of risk mitigation of having to repair a pack down the line.