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I would define "homemade" as assembling the batteries without any sort of regulation or safety standards.

I'd be a bit frightened if I knew someone in my apartment building was assembling their own batteries -- poor quality ones can burn down the building.

It's an increasing problem here in San Francisco. I know at least one friend who had to move due to smoke damage caused by a neighbor collecting these batteries. https://sfstandard.com/2024/01/09/san-francisco-e-scooter-bi...

Yeah this is incredibly dangerous. It's not at all like installing a power outlet. You can do that relatively easily and safely. These batteries will catch fire in the middle of the night, burning down apartments and they will kill many people.

So can store bought: https://www.youtube.com/watch?v=JCaDz_2YcGQ

So can galaxy notes, hoverboards, vapes, etc. So can poor electical installations, bridged fuses, cooking oil fires, etc.

These packs (18650 based) are a lot safer than "baggied" packs found in many fpv drones and RC cars, and those packs are factory assembled.

Firewise, i'm more afraid of an alcoholic with a cigarette, than of batteries.

What I find concerning is most people here would have seen expanded seams on a phone or laptop and known about the "puffy" battery within.

The rate of proliferation of gadgets with such batteries is crazy. So much of society already has so many of these little flashpoints in phones, tablets, toothbrushes, kids toys (electric scooter anyone?) - statistically your concerns are already valid without anyone being an electronics hobbyist.

So, compare the number of battery fires to eg. gas explosions, cigarette fires, oil fires due to cooking, etc.

Doing anything with mains power, water pipes and gas pipes is still done by many people themselves, and the risk is minimal.

Also most of the battery fires happened with factory installed batteries, be it in vapes, galaxy notes, or other devices.

Also: https://www.youtube.com/watch?v=JCaDz_2YcGQ

They are even matching the internal resistance of the cells, I can't think of much more a professional manufacturer would be doing. As long as they are getting their cells from reputable sources (big if) I don't see anything wrong with a boutique battery industry for niche applications.

A professional manufacturer would not put any cell in series but instead would have a charger/discharger with voltage per cell, with current and temperature sensors as feedback loops. The danger of cells in series differs with the cell chemistries.

Using a BMS is another indication they are not professional or even had an engineer design them.

Sadly I know of only a very few small professional battery pack manufacturers, certainly none of the EV or bike makers.

> Using a BMS is another indication they are not professional or even had an engineer design them.

What is the proper alternative? Do any ebike packs conform to the standard you expect?

By "series" do you mean "parallel"?

Note that Tesla famously used a 6-series/74-parallel configuration in the Model S's battery pack: https://circuitdigest.com/article/tesla-model-s-battery-syst...

Each pack only had a single BMS connection for every clump of 74 cells! Presumably they only got away with doing this by doing extensive binning/characterization of the cells before building them into packs.

> A professional manufacturer would not put any cell in series but instead would have a charger/discharger with voltage per cell

My Bosch e-bike has 4 cells in parallel and put 10 of those in series. This so called 10S4P is a very typical setup for e-bikes. Each individual cell does definitely not have their own current/temp sensor. The BMS will check voltage for each of the 4 cell blocks.

> I can't think of much more a professional manufacturer would be doing

Safety equipment, in a safe environment, not exposing others to fire risk?

There's a fair bit of design, validation, and certification that goes into making sure a pack is safe. The spot welding and wrapping is just the final result.

Thermal design to make sure the pack doesnt overheat under normal conditions. There's the mechanical side to make sure that the pack can survive shock and vibe.

Getting packs certified to UN38.3 or IEC62133 are no small feats. Unsurprisingly, the certification process is not cheap. For 62133, we had to provide a ton of samples.

> same as with replacing an outlet at home, but more of a fire damage instead of a shock risk

Not a good comparison at all. The battery can burn down the entire building and kill everyone in it - or just burn them alive and leave them breathing. Don't forget all their property.

Your wall outlet will just knock you on your butt, afaik.

> You can also make them yourself at home, although it can get relatively dangerous if you don't know what you're doing... same as with replacing an outlet at home, but more of a fire damage instead of a shock risk, so definitely not for everyone, but with some basic precautions, can be done safely.

Did that for my semi-diy bike. Got an older Sparta with stupid DRM in it, removed all the sparta parts, made a custom PCB for the motor/hall sensors and had the NiMH battery replaced it with a custom 7s9p pack (in frame so was limited in options) resulting in a nice 650Wh battery instead of the 200Wh it used to have. Using recycled cells from a crashed scooter resulted in a (relatively) cheap, reused ebike.

Is this really a common practice outside of FutureMotion/OneWheel?

Bosch is known to pair their hardware such that you can’t replace a bad element, e.g. replace a bad Bosch battery with a 3rd party unit. The entire Bosch ebike hardware ecosystem is locked down and only approved dealers can repair.

It’s pretty common. They’re far from the first ones to have started that practice, which began in the 2000s. The concept of the battery management system (BMS) killing the pack for safety reasons came with modern BMS chips that can do state of health (SoH) measuring, along with state of charge (SoC) measuring.

Some laptop manufacturers do that... once you disconnect the voltage from the BMS, the BMS is efectively dead. If you know it, you can use a power supply to keep it energized, if not, tough luck. You can still buy a 3rd party battery to use with that laptop, but the original laptop cannot be "renewed" with new cells anymore.