PSA: Be careful assembling Li-ion Battery packs

I personally haven’t been okay with leaving batteries charging unattended for a long time, just too paranoid about something going wrong (can’t sleep peacefully) and destroying most of my stuff and possibly somebody elses. Too high risk vs reward.

New pack build has been going quite nicely though.

IMAG1050 IMAG1049 IMAG1052 IMAG1054 IMAG1058 IMAG1055 IMAG1057 IMAG1064 IMAG1065

I did the first submersion test with the XT90-panel connector attached to the back of the pelican case (empty insides) and found it to without any extra sealing material to not leak any air out on it’s own, but after pressing the sides of the pelican case in, it did produce some air bubbles meaning that water was getting in and air was coming out. I then redid the test with some hot glue added onto the mating surface of the connector and re-heated it before bolting the connector back onto the back panel. Absolutely watertight, couldn’t get any air bubbles to come out even with rigorous pressing on the case.

Going to visit a hardware store today to get some foam tape to add around the cell pack’s exoskeleton to insulate it from vibration and cushion any harder hits from the pelican case. I personally started to work on this new pack with ruggedness in mind. It will most likely also house the first FlexiBMS and work as a test platform for it.

@evoheyax , live and learn, this is the mentally I have developed as an engineer. You lost one pack, improve on the next one! No bodily harm was caused to anyone, just monetary loss, so no reason to not keep going forward and learn from past mistakes.


BTW never use bullet connectors on batteries if they unplug they will cause a big short, it happened to me before. Use XT60 or X90s.


If I were you I’d disassemble the all P groups by cutting the nickel strip between them in order to add fish paper between them, them stick them together with hot glue or natural cure silicone.

Remove those balance wires and put fish paper between the cells and wires, fix wires with kapton tape.

Buy 200mm heat shrink and wrap the pack together.

I’m sorry to say this but, in it’s current state, your battery does not look safe to me, please remember I’m not being a jerk just want you to avoid a possible fire in the community.


So @Eboosted do you think it would be safe to reuse the cells after pulling off the nickel strip if I sand down the terminal ends of the 30q with a Drexel like in this video at 2:20

What nickel strip width do you reccomend? This 12mm wide seems a bit much.

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This PCB is cool, where can people get that? image

The pack construction and case look really nice. But no fish paper rings for the positive ends? How thick is the white plastic ring in there? image

PCB is custom design by me for this specific pack. Although I don’t see any reason why it wouldn’t work in other packs, if the pack geometry allows for it. I can share the ready-to-order gerber .zip file if somebody wants them.

The cells are fresh virgin Samsung 35Es, So they are what the manufacturer made them to be.

Personally I don’t see the need to use fishpaper with this pack, because everything has been spot welded with minimal thermal stress applied to the cell terminals, so the plastic rings insulators are completely fine. They are very unlikely to mechanically break, because the whole pack is welded first in parallel, then in series, then bolted into the 3D-printed exoskeleton, which is then foam pad insulated and then placed into the pelican case. Very, very rigid build with vibration dampening. Only possible way I see of having them be broken is by melting them with too high currents running in the nickel strips, but the strips are hefty 10x0.2 mm size with multiple parallel series strips to avoid hot spots and carry the current better. I don’t plan on running such high discharge currents that I believe this would become an issue.

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A longboard vibrates a fair amount so something is bound to get loose after awhile. I’ve torn down all packs I’ve built atleast once and saw welding spots gone loose on atleast one spot on all of them. Been Lucky enough to ever penetrate a cell tho knocks on wood

Really appreciate you story and sharing this

I’m no expert in batteries but as an electrical engineer I see a lot of dangerous battery packs created over here.

few examples

  • insufficient nickel used for power distribution
  • insufficient insulation or physical distance
  • no mechanical strain relief between the cells
  • battery packs not or improper secured
  • improper soldering
  • improper routing of balancing wires
  • no vibration dampingen

Highly recommend for our application to use: exoskeletons + fuse wire

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Just my two cents: The 0,15mm nickel strip is rated for 15A. So if you use it for electric skateboarding, i recommend to weld 4-5 nickel strips on the battery.

We all have our own experience I guess Bro:

XT90s are good if you will always be under 90A, I use AS150’s on all my lipo batteries as the charge connectors as well as my board, and as the connector to my esc’s (my default connector, but I unplug to charge with my icharger), past that I either use some 400A 6.5mm bullets I got from Europe or 8mm Castle bullets…

I expect my mountain board to pull more than 90a and I feel more secure in big over rated connectors.

I guess where I am going is I bet the bullets you had problems with were not as big. eg. if you push my 400a 6.5mm bullets together the tolerances are so tight your going to need a serious grip to pull them apart by hand, most are going to reach for a tool.

So I think the rule would be better or more accurate for the layman: at least use XT60/90 or some solid bullets * rated for higher current.

*because those solid bullets have a interference fit that is very tight in order to carry the rated Ampacity

TLDR: 6.5mm-8mm bullets rated for high current are the cats whiskers :cat: the bee’s knees :honeybee: and the ducks nuts :duck: !

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What are the chances of over stressing the battery in regen? Max charge is 4a for 30q cells so that is 16a total, or 4a per Vesc, that doesn’t seem like much and makes me think you must have it set higher.

If I’ve got the regen wrong please correct me as I’m new to using vescs

I use a total of -16A on my pack (10S4p 30q) I think it’s maximum 4A per parallel group.

Yes that’s pretty much what I said, the thing that’s makes me wonder is that he has 4 motors so he must only run -4a per Vesc.

I think you mean 4a per cell, 16a per parallel group

Soldered copper strips, then folded up to on top of the battery, where they were connected to the next p. There was 2 layers of electrical tape between the cell and copper though. I used the battery heat shrink on each p so they weren’t actually touching the next or previous p…Only cells in the same p could touch, which is fine.

Might not work. Battery university said it might work for small lithium fires, but I think were over that threshold.

The fire department used a special extinguisher for lithium fires. It worked really well, but was not your tradition foam, looked more like a clear liquid (not water though).

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Or throw it in the backpack.

Yeah but I don’t like anything in a backpack that can hurt me if I fall on it

This is completely false. 0.15mmx7mm nickel is rated for about 5A continuous. Above that the nickel will heat up, leading to premature failure of the strip, or melt insulators and definitely reduce pack efficiency due to heat. I’m not trying to be a dick but simply correct the number so that others don’t think that a single 0.15mm strip can handle 15A and lead to a pack fire like the OP. You definitely do want to layer 4-5 nickel strips depending on the current load you will need, so no argument here.

Yup regarding putting out the fire the only thing I’ve heard works with lithium fires really is removing the source of oxygen so it can’t burn (completely covering it with sand is the best technique I’ve seen to contain the smoldering fire). Still probably doesn’t hurt to have an extinguisher on hand to suppress it or stop it from spreading to anything near by. I’ve considered keeping a small sandbag in my backpack but lots of extra weight if it were to be enough to actually cover 2 5S 5Ah (let alone something bigger)

Regarding charging I imagine most people are just plugging in a BMS and assuming everything will be good I made my lipos removable so I don’t have to worry (as much) about it when charging can put them in a lipo safe bag at the very least or if you want to go gung ho and feel really safe do something like this:

or buy something like this


This is the issue with nickel stripping. It’s not the best material due to the low amp ratings it can do. if it’s 5a con, then I need what, 20 layers, to do 100a con safely… This is what pushed me away from them in the first place and to copper. But copper is hard to spot weld without damaging the cell also.

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@Eboosted can you clarify what you mean by “isolate the charging port”? I’m not yet building my own packs, but trying to gain enough knowledge to someday consider it.

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