Construction of the battery units - wouldn't it have made sense to make cells replaceable?

[FrankR]

I don't pretend to know anything about this, so those of you much more in the know - please be gentle.

It's my assumption that inside these battery units, it is all hard wired and soldered. If that is true, wouldn't it have made sense to make these units more like a consumer electronic battery compartment where you open a door and can pop cells in and out?

I believe these these units use fairly standard obtainable cells. 18650, 21700.

It would be great to pop cells out, put them on a checker, and swap out if necessary.

What am I missing?

 

[tomjasz]

What am I missing?

The ease at which a hobbies can create a disaster. Been there…

There are a couple of kits to do just that but it’s still expensive. Getting batteries is expensive. And replacing individual problem cells in an older pack can also be a disaster. Think about how powerful these packs are. I have all the equipment but gave up when I evaluated the risk welding.

I have these kits.
Vruzend.com and,

Agnius - AMtech

18650.lt

DIY 18650-cell battery-pack building KITS, five options for 2018

Recently, we have seen several kits for assembling a battery pack out of 18650 cells. Here are some examples, so you can see if that is something that might interest you. And remember, these aren&#…

www.electricbike.com

 

[JES2020]

The common advise is to replace ALL the cells when one goes bad.
The reasoning (I guess) is that if one is bad the others are right behind it.
So it really doesn't save you much, if you have to replace all the batteries.

 

[retiredNH]

The motor batteries are subject to high current draw and tough, vibration prone conditions. Neither is conducive to friction or pressure contacts as normally used in replaceable battery compartments. You'd need terminals that have a lug on them, but that would skyrocket cost and space needed.
On top of this, you have battery balancing issues if all the batteries aren't the same, not just capacity but also in internal resistance, you have cell balancing problems with big negative consequences. Good battery packs are built up of cells specifically matched for capacity, voltage and IR, all of which change as a battery ages, so if you replace one bad battery in a pack with a new one, leaving the rest, there's no way the pack will be matched...

 

[AHicks]

The other piece is that even just decent battery packs seem to be holding up really well. I have a '17 RAD City that was purchased in late '16 that's still going strong. It was a daily rider until about a year ago, and is now on "extra bike" status for use by friends. That bike has been charged countless times. Enough where if it quit today it wouldn't owe me anything. Point being, individual cell replacement, or the ability to do that, may be overkill....

 

[theemartymac]

Modular construction costs a lot, and doesn't makes business sense for most manufacturers. Lithium-family cells are also not very tolerant of mix and match ages and brand/quality. Here is a video from a very tech saavy youtube blogger from up here in Canada who thought he could save money rebuilding his own batteries for his company. Results are pretty clear, not for the faint of heart, and likely just not worth the risk...

LTT Battery Repair Adventure

 

[rich c]

The most common way of building a battery pack is with a spot welder and nickel plated strips. Every once and a while we get the "did I fry my battery pack" questions. Some start out with the owner admitting he shoved his key into the charging port. You really think they should be inside the battery pack? Then we get the speed builders that are making huge packs over 72 volts. I'm not smart enough to know when those guys short out a pack, just when will they get burnt or shocked. I have read that an automotive pack will kill you. But when you describe a battery test with a "checker", and admit you know nothing about battery packs, I hope there is more support for not getting inside the battery pack. Especially since the machine only has 2 wheels, and how difficult could it be anyway?

 

[retiredNH]

Automotive packs put out several hundred volts, so yea, lethal. DC voltages start hitting the lethal range around 40-50 volts, depending on other factors.
When it comes to ebike electricals, given lethal shock potential of many battery packs and the explosion risk if they're mishandled, I get a bit frightened when I see some of the questions on this board. It brings back memories of one of my early auto mishaps, burning the tip off a screwdriver trying to get a car battery terminal loose. Lesson: Always take the ground lead off first...

 

[fuyume]

I don't pretend to know anything about this, so those of you much more in the know - please be gentle.

It's my assumption that inside these battery units, it is all hard wired and soldered. If that is true, wouldn't it have made sense to make these units more like a consumer electronic battery compartment where you open a door and can pop cells in and out?

I believe these these units use fairly standard obtainable cells. 18650, 21700.

It would be great to pop cells out, put them on a checker, and swap out if necessary.

What am I missing?

What you are missing is the risk of catastrophic failure of Li-ion cells. As other commenters have pointed out, multi-cell battery packs need to have the cells closely matched, and if they get out of balance or there is a cell failure, the good cells will start charging the bad cells, with a high risk of overheating and spectacularly bad consequences, including risk of fire and even explosion. The chemicals emitted by burning Li-ion cells are *extremely* caustic, like hydrogen fluoride/hydrofluoric acid and if you breathe them in, you will be lucky to survive.

Note that this issue arises when Li-ion cells are connected in series, but as they have a characteristic voltage of about 3.6-3.7 V, you obviously aren’t building a 36 V, 48 V, or 52 V battery pack with wiring many cells in series. It only takes one cell failure to potentially cause a meltdown.

 

[tomjasz]

The most common way of building a battery pack is with a spot welder and nickel plated strips.

Most common if we're talking about those $300 Amazon, eBay, Aliexpress, and direct sales junk packs.

Quality builders use PURE nickel welded strips and the welds are consistently machine-made, and cell level fuses with a BMS with sensible features and data links.

Lots of kit builders actually build batteries far better than to aforementioned sources.

overkill....

Poor judgment unless the pack cells were matched. ie. a new battery with one bad cell. But that's not a simple job for most.

I'm anal when it comes to shipping "dangerous goods". Failure to follow legally required parameters could be a costly perhaps even a guilt-ridden nightmare.

Nah, off to recycling.

I'll have all my battery-building, cases, pieces, and parts soon for sale. No more welding for me...