21700 battery build

Built a 48V 30Ah pack with Tesla 21700 factory second cells. Total cost was A$660

  • 13S6P (total 78) Tesla 21700 5Ah factory second cells - A$540
  • Cell holders (21700 staggered) - A$15
  • Nickel strips (0.2mm thickness) - A$10
  • 40A BMS - A$45
  • G10 fibreglass sheets - A$30
  • PVC heat-shrink - A$20
  • 12 AWG copper cables + XT-90 connector - A$5

Tested cells with 4-probe ESR meter and all with in spec. These seconds are pretty decent and pretty good value, they are rejected due to defects in pack assembly, not due to any cell defects. The cells are rated to 2C so the overall pack is capable of delivering 60A (or 3kW) which is the max power my system needs to take-off.

Use staggered cell holders so the pack would fit in my waterproof box and leave enough space for the ESC and other electronics.

The “K-weld” proved to be a reliable spot welder. First tried with a cheap spot-welder but that failed. This is also a good point to consider electrical safety, once the cells are wired in series the 50V can create a danger.

Also good to test the nickel strips are genuine nickel - a quick test is to scratch up a small test piece and leave in a glass on salt-water over night, if there are signs of rust then you have steel not nickel.

This is a cool frame from a slow-mo video of the spot weld being performed. The melting temperature of nickel is 1455C, and the spot welder puts through around 1200A for 20ms.

Notice the slots in the nickel strip, they are to help direct the current through the battery terminal rather than through the nickel strips to improve the weld. (Diagram credit: https://www.electricbike.com/introduction-battery-design-2/)

BMS and battery terminals soldered to nickel strips. I chose a 40A BMS as my cruising current is 30A. The over-current protection is 2.5x the rated stead-state current (so 100A). The Tesla cells also have 15A internal fuse which is handy.

Note it is important to evenly distribute the terminal wires to the terminal strips to avoid sinking too much current through the nickel strips.

All wrapped up in G10 fibreglass sheet. This is to protect against anything puncturing and shorting the cells. It also adds structural strength to the pack. Note this stuff is pretty difficult to cut.

For the final step the battery is covered in heat-shrink. It fits snugly in the box.

In the finished product, and ready to ride :slight_smile:


Nice battery pack! Great work, I like the G10 protection you added. Have you had a chance to test it yet? Any issues with heat or battery voltage sag? Thanks. The Tesla cells look great and I am considering using them but I am a bit worried about the heat management (or the lack of it, in an efoil).

Thanks :slight_smile:
Yes gave it a good test yesterday and no heat or voltage sag issues.
I don’t think heat management would be any different between 18650 and 21700 cells as they’re ending up encased in a similar sized box. The 18650 cells would have better heat dissipation if you have heat sinks for each individual cell.

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Hey nice battery, where did you get the cells from?

Thanks :slight_smile:
We’re lucky to have a good supplier in Perth - https://www.lithium-power.com.au/

Looks like a great build, with good cell capacity. What sort of run time are you getting?

I’m also building a 21700 pack, 13s6p with molicel p42a cells which provide 20-30A each, for a tow boogie. I’m going to use a JBD BMS for charge only, and put a big fuse on the battery
output. I’ll put fast blow fuses on the cell end of bms balance leads, as they seem a likely cause for shorting between series cells and causing the pack to burn if they wear through from vibration.

It’s hard to find a premade box for the battery in the size I’m after, around 310*160mm, might have to make one from g10 or acrylic, I want the cells to have a waterproof box.

Thanks. Haven’t run it for a full discharge yet, still shaking out a few bugs with some other parts of my build and learning how to ride it properly. My cruise current is around 30A, so I expect to get around an hour ride time.

Sounds like your battery will pack some serious power output. How much current are you targeting for the tow boogie?

Good idea for putting fuses on the BMS balance leads. What type of fuses will you be using?

Yeah I pretty much made the battery to fit in a sturdy waterproof case that was available. I wouldn’t trust my skills in making a waterproof case from scratch.

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I’ve ordered 2A pico fuses for the balance leads.

Valhalla’s tow boogie seems to be using up to 120A peak from his battery, cruising at only 25-30A. I’d like to be able to tow behind me, which will need some power.

I’ll put the battery in a waterproof case, and then have that inside a pelican case, so hopefully it will have double water protection.