Have I built a bomb?

[tanlars]

Hey guys I have a somewhat serious question and I’m hoping some of the battery experts can weigh in for me. I recently purchased a used Stealth electric bike with no battery so I did some research on battery options. Not wanting to shell out for a $2k stock battery (and having some experience building smaller packs with BMS), I decided to go the DIY route. Now that the pack is built and ready to use, I’m having a “What was I thinking?” kind of moment. I have spent about $700 to build a 21s3p 30ah lipo pack with a bestech 100amp BMS. My question is this: Now that the pack is built, is it safe to use if I am cautious and take the necessary precautions? Or would I be better off safely discharging it, throwing it away, and building a smaller, more expensive one with 18650s or Lifepo?

I fully admit that I was stupid to not pose this question before spending all the time and money that I did. I accept that and now I just want to proceed in the most intelligent and safe way possible.

I also acknowledge $700 is a lot to throw down the drain, but I am more than willing to take the loss if it means saving the bike, my home, and most importantly, human life.

Here are the battery details:

To get my desired voltage and capacity to fit in the battery compartment (as well as my budget), I chose to permanently parallel individual 1s Lipo cells taken from new HK Multistar 10000mah batteries. I first assembled 21 packs of 3 paralleled cells. I connected them by running a nickel strip through the pouches’ tabs, bending the strip ends over one another on top of the tabs, spot welding it to form a compact loop, then adding solder to ensure each tab had a good connection to the others (I tried to keep heat buildup to a minimum, but of course the solder was very hot). I then connected the 3p packs in series using more nickel strips (enough to stay within conservative ampacity limits) to get 21s3p. I then wired up the BMS, and added a few JST-XH balance leads that run to the front of the pack to be able to check individual cell voltages at any time. After that I added charging and discharge plugs, a few protective plastic panels, and taped it all together. I wish I had taken pictures of the process, but here is the basic schematic for my design (BMS not shown):



Here are my main concerns:

1. I foolishly did not discharge test the 63 cells that went into this pack beforehand. I now realize that if there are any bad cells in the pack, they could at best drag down the other cells they are paralleled to, eventually killing them (best case scenario), or start a very nasty chemical fire with 30ah of lipo to feed it (worst case scenario).
2. The battery is a very tight fit in the bike’s battery compartment, leaving no room to add foam or any other shock absorbing material. I’m worried the first crash on the bike could damage or puncture the cells.
3. I’m currently storing it in a vented steel toolbox on the cement floor in my garage. I plan to do all my charging there as well with an 84.7v lithium charger . In the event of a charging failure, would the toolbox be enough to contain the fire? I’ve installed a fire alarm in the garage as well and plan to closely monitor the battery anytime I charge, but I don’t know if I’d be able to do much to extinguish the fire if the battery did go up in flames.

I'd greatly appreciate any advice, opinions, or past experiences that could help me gauge the danger level I’d be assuming if I decided to use the battery. If you do feel it's ok to use as long as it's with a wary eye, I'd appreciate any safety tips or items of concern specific to my design (I won't ask you to list off all the items of general lipo safety, unless there is one point you feel I should be particularly aware of). Or if you’d just like to comment about how big of an idiot I am, that’d be fine too. Don't hesitate to let me know if you really feel that my battery attempt belongs miles underground.Thanks in advance, I hope you’re all enjoying some good riding weather.

 

[ScooterMan101]

Long post so from what I have read , you did not even use the pack ?
or used it just a couple of times ?

From talking with people at the RC airfield , who all use Hobby Lipo Bricks, most do , not, do extensive discharge testing before using hobby lipo's , they Do , however always use balance chargers, not the bulk chargers that we e-bike people use on the 18650's '
They only use one pack ( at a time/per flight) , and the packs are so cheap, that they can just discard , after a number of uses , a bad pack if need be.
They also charge in open spaces and keep close by when charging.

As far as getting rid of those batteries,
There are plenty of people that would want to buy them, there is a classified section here on E.S.
perhaps not all the cells at once, but still plenty of potential buyers.

Mostly people like me that live in Apartments, where you can get evicted at anytime for causing such a fire, need to worry allot about Hobby Lipo's , and even then only because it is nearly impossible here to find another place to live, unless you are making over $ 100,000 a year so you can afford move , to a different insanely overpriced House/Apartment.

What part of the Country do you live in ?

In a House, or a part of the Country that does not have a tight/Overpriced Housing Market ?
then you just take the right precautions. ( Charging 6 feet or more away from any other combustible materials , Making a metal Sand Box that the batteries go in while charging, a big enough box and put many inches or a foot of sand on the bottom and all sides, just in case of Thermal Runaway. etc. )

 

[tanlars]

ScooterMan haven't used it yet. Thanks, I really appreciate your advice on the charging box and safety precautions. I live in the Phoenix area and I live in a house I own. I'll definitely consider selling the cells on the classifieds if I can manage to get them apart without damaging them. Thanks for your help!

 

[John in CR]

While a short of the mains vaporizes metal instantly, when battery packs go the process is too slow for an actual explosion, so no not a "bomb". All of our batteries represent a fire hazard, and RC lipo significantly moreso since their chemistry inherently makes them prone to fire starting and self-feeding within the pouch containing each cell. All we can do is mitigate the risks.

It sounds like you're taking the steps to prevent fire getting started due to internal damage from over-charging/discharging. My bigger concern is the physical structure and mitigating risk of cell or wire damage initiating an event, and other than the layout you haven't told us about the structure of the pack and wiring. That includes both physical barriers as well as compressio.

FWIW, next time flip the even number cells around the other way to reduce those long diagonal series connections down to very short straight tabs.

 

[tanlars]

John, sorry I guess my post title was a bit dramatic. Since the bike's battery compartment is so small, I don't have a lot of room to add protective foam or other shock absorbers, but I share your concern and I'll see what I can do to protect the cells and wires as much as possible. Thanks for your help. Also thanks for the tip about the tabs! I'll keep that in mind for my next build.

 

[John in CR]

The pack requires compression, and be sure to use some kind of end plates to keep the end cells flat, especially at the corners (where the shrink use on RC lipo packs falls short and ends up deforming the cells, which are made of many layers of alternating materials.) Compression will shrink that dimension by a surprising amount, and the end plates also help protect the battery from damage. Into your packaging also include a means of securing the battery to the bike, because if it can bounce around, it will, and that's sure to result in problems.

 

[fechter]

Most Lipo fails I've seen were due to physical damage to the pouch. Some kind of armor and careful packing to prevent denting the edges is important. Compression is also important as pointed out. This keeps the internal resistance lower.

That is a lot of Lipo, though. It would suck if one cell decided to go pyrotechnic. It's really like a bunch of little gasoline filled plastic bags with a built in igniter. If the whole thing is in a metal can that keeps oxygen out, it can only burn so much, but escaping vapor could ignite outside the can.

 

[ScooterMan101]

If you are able to compress the pack , there is a type of foam that is very shock absorbing,
It has been 15 years now since I used some on a seat of a Microlight that I had, so I forgot what it is called.

The selling point of it , with a video, 15 years ago was that the designer of this special foam put a 2 inch thick piece of it
on a concrete floor, then he jumped onto it landing on his knees on the foam .

They use this foam in the Aerospace Industry, it is / was light blue in color.

If you do sell the Hobby Lipo and make your own, or have someone make a 18650 cell pack for you,
you can also use this foam for shock absorb.

If you do use 18650's , be sure to make a pack that is at least 6 p or more, you want allot of Parallel Cells for any, Powerfull/High Speed hub / mid drive motor .

 

[MadRhino]

As long as you built your battery pack so you can monitor each and every cell, it is not dangerous as long as you do monitor them. I don't trust a BMS with RC Lipo, I prefer monitoring by myself and charging fast to make sure that I am never tempted to leave them charging alone. Lipo fire is slow to start and the faulty cell does puff and smel before catching fire. As long as you are with them watching, you will not start a fire because you will have plenty of time to stop charging and take appropriate measures.

Then, I would not build a battery assembling single LiCo pouch cells. RC Lipo bricks are making a battery quick and easy to maintain, assemble and disassemble. Using a standard that is easy to source anytime has a neat advantage.

 

[wesnewell]

If I understand what you did, you went to a lot of unnecessary trouble, that may end up in a lot of maintenance troubles later on if you happen to have a bad cell. By keeping the 3s packs in their original form, it would be easy to just replace one should the need arise. There's a wring diagram showing a 24s3p pack here. That's how I'd have wired it up in minutes, and then added to bms.
https://endless-sphere.com/forums/viewtopic.php?f=14&t=39666
But what's done is done. so now all you can do is check each of the cell groups and rewire and replace it should one go bad. I ran my 24s2p rc lipo pack for over 4 years and 15K miles without a problem. Most of them puffed up to a degree, but I never worried about it and never had a problem other than voltage sag towards the end. All I use is rc lipo and wouldn't use anything else with the current chems available. I would never ever use 18650's. Just wring them up would be a nightmare. And from a maintenance standpoint, intolerable.

 

[fechter]

See what a failure looks like:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=56305&start=50

The battery box was able to contain things. If this had been indoors, the smoke would stink like crazy but at least not burn the place down. All metal battery box is good.