DIY 14S7P battery build in a Pelican case

Hi

I would like to share my battery build. I assembled the first version of my eFoil this summer, and I am planning to upgrade it during the winter. I will paint and clearcoat the board, tidy up some electrical stuff etc, but the battery will mostly remain in it’s current form since it works flawlessly. I might add an external charge port in the future…

Anyway, at this point I can share the pretty much finished battery. My board is space constrained, so I went with a Pelican 1085 case. I calculated that a 14S7P battery will fit nicely inside, and with about 18-22A current consumption during foiling, my cells should see less than 5A each. This calls for high energy, medium discharge cells to optimize run time. LG released their M50LT cells recently, so I selected those for the job. Their maximum current output is 14.4A. That’s easily enough to support the 65-75 battery amps I usually use to take off (about 10A per cell). I am 65kg. The cells don’t warm up much, but if you are a heavier rider, go with a high performance cell. I observed max 34c, logged with LTT BMS. The run time is about 1-1.5 hours.

Starting off with the cells

I hot glued the cells into packs of 7

Found myself some 50 mm and 30 mm wide nickel. The 50 mm wasn’t annealed, so I had to use some annealed 30 mm wide that I had bought earlier. Annealing makes the nickel softer, so I can form dimples for the LG M50LT positive ends. LG decided to recess the cell tops in this model. Most other cells don’t require dimples.

Cutting the 50 mm wide nickel with 3D printed guides. These were for the negative ends.

Forming dimples with 3D printed guides. I only did it for the positive cell ends.The negative cell ends are flat.

Spot welding. The first 30 mm piece is visible here. I welded another one to the cells as well, offset so I can later bend it on top of the flat cells. See pic below.

Bent the tabs on top of the cells and started constructing the pack with fiber reinforced tape

Series connections completed

Balance leads added

Shrink wrapped

Added BMS and glued the pack to the case with structural adhesive

Pack completed!

The Amphenol surlok connectors fit, but barely. I had to cut 2 out of the 4 mounting ears off.

Added battery strap rails to the board

Battery strapped in

That’s it! I hope that someone finds this useful :slight_smile:

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A very nice job ! We’ve not been used to slim battery case designs because of the possible heat dissipation that could dammage the former 2500 or 3000mA cell generations. Do you have a way to monitor the temperature inside the case ? Would you have a link for the case and for the cells ?

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Yeah the LTT BMS has two temp sensors that I taped on the top surface of the pack. The highest I have seen was 34c, at about 20c ambient temperature.

Cells LG INR21700-M50LT 4890mAh - 14.4A - 21700 & 20700 - Li-ion - Rechargeable batteries | NKON

Case 1085 HardBack Laptop Case | Pelican Official Store

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I think your choice of battery cells is wrong.

Why is that? I think it’s perfect. At less than 5A continuous for each cell, the LG M50LT is one of the best choices, as tested by Mooch. Max current rating is easily sufficient for takeoff. The pack does not heat up even when running for long periods of time.

I typically foil at 20A battery current, so I need about 2.85A per cell on average.

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Nice esk8 construction. I considered and decided against the high capacity cells for tow boogie, and with around 16-20A cruising current they would be fine for just foiling along. My hesitation is that the 80+A for water starts would cause major voltage sag, and reduce the wh output. Heavy voltage sag would hamper the lower part of the battery capacity. I guess it depends on the use case, with how many water starts you’re doing. If you’re mostly foiling on flat water without falling off, then they’ll be perfect.

I’d be interested in what a VESC shows for your actual wh power consumption after a foil session when you’ve drained the cells to 3V, compared to the rated wh for the pack. On my 13s8p 40T3 pack the VESC shows around 1600wh used.

Interesting temperature related limits for the M50LT in the manufacturer specs, only allows 0.3C max charge rate if under 25 deg c, and discharge only 0.5C max if under 10 deg c. Max discharge is only 7.2A above 25 deg c, ie a maximum discharge for the 14s7p pack of 50A if the pack temperature is above 25 deg c (which it usually would be). The 14A max discharge rating is only for temps between 10-25 degrees c.

Mooch measured 13.1wh at 10A discharge, which is pretty solid. Good cycle count if you look after the temps.

Great build.

Where did you get the Amphenol surlok connectors from? They look great!

Thanks for the post.

I wouldn’t have personally chosen those cells, but its still a great build with off the shelf case!

Hello, which one would you have chosen? I’m in the same case, I want to make a 12S7p and I weigh 95 kg

Molicell P42A or Samsung 40T. They’ll give you 210A burst current.

2 Likes

Thank you, they are the two models that I have in the shopping cart. I like to know your opinion. With P42A it would be 315 A, right?

I think I’ll go with Samsung INR21700-40T (40T3) 4000mAh - 35A, i think it will be enough

thanks

Great discussion! Definitely go with higher performance cells if you plan to use more power than me. For an optimized build, these cells will give more run time than 40T or P42A. With M50LT you need to be able to run the cells well below 5A each to keep the pack cool and efficient, but if you can, the run time is amazing. The pack is barely warm to touch after a long run time, even with my friends learning, falling and taking off constantly. It takes a bit of time to swim back to the board after falling, so the average consumption during learning isn’t actually that bad. This surprised me, I was actually a bit concerned about battery temps when watching my friends try the board, but when I checked temps after the tests, it was all good.

Here are the connector specs

Battery connector Model number
Battery box pos socket Amphenol SLPRATPSO
Battery box neg socket Amphenol SLPRATPSB
Battery box pos plug Amphenol SLPPA16BSO
Battery box neg plug Amphenol SLPPA16BSB