Comment by joshvm
2 days ago
You would be amazed how many battery packs are multiple 18650s in a trenchcoat. Even EV battery packs use them. Though it does raise the question - wouldn't an old EV battery be a better solution than stripping apart laptops?
There's a lot that goes into manufacturing battery packs beyond the cells. How's your thermal path to ambient in your home wall battery? How is the inter-cell thermal isolation? Is there a path for gas discharge in the event of a cell failure? Is the pack appropriately fused at the cell or module level? When a cell fails, does it take the whole pack with it, catch someone's apartment building on fire and kill a family of 5, or merely become stinky with a hotspot visible on IR?
How good is your cell acceptance testing? Do you do X-ray inspection for defects, do ESR vs cycle and potentially destructive testing on a sample of each lot? When a module fails health checks in the field, will you know which customers to proactively contact, and which vendor to reassess?
Yeah lots of batteries are 18650/26650 in a trenchcoat. The trenchcoats run the gamut from "good, fine" to "you will die of smoke inhalation and have a closed casket" in quality and I think that bears mentioning.
Where would you put this battery in that trenchcoat gamut? Inside a server rack, fwiw. https://signaturesolar.com/eg4-lifepower4-v2-lithium-battery...
Was definitely one of the harder parts of our solar install to get comfortable with.
Bigger, fewer, more chill cells, fairly robust trenchcoat.
(IIRC, these packs are 16 100ah LiFePO4 cells in a steel case w/ built-in fuse, breaker, and BMS that monitors individual cell health and pack temperature, w/ automatic cut-off if any of that goes out of spec. The weakness is primarily the MOSFETs on the BMS potentially failing shorted. Fortunately, they've added some sort of additional fire suppression beyond just "steel case" in recent-ish versions of these packs)
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I can't see what the construction looks like but the mention of 'fire arrestors' gives me a lot of hope. If you haven't designed a battery that can take a cell runaway safely, you haven't done the work, and clearly they've done at least that much.
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I get that the trenchcoat needs to be well designed and tested, but I am still flat out amazed that you both agree with “meh, most battery packs are made up of rechargeable domestic batteries you find in a kids toy”
I just assumed there was … special stuff in there
For a highly engineered battery like a premium EV, there are coolant channels, temp monitoring, voltage monitoring, etc.
Soldering some connectors onto some random cells and knowing they shouldn't go over 4.2v is one thing, but measuring cell health via internal resistance, programming a controller to do temp shutoff and wiring up temp sensors, keeping cells balanced, is a lot of extra work, but critical if you at all care about not potentially burning down wherever they're stored.
Keeping the cells small and just using a hundred of them in parallel (and a hundred of these parallel packs in series to get up to the hundreds of volts needed), thus using ~10,000 cells, in EV batteries limits the maximum damage from one cell going worst-case, assuming your enclosure can contain it.
That being said, it seems there is a slow movement towards larger cells, from 18650 to 26650 or similar. But each cell on its own is still a dumb can of chemicals ready to go boom if you mistreat it.
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I used to joke with my buddy back when he first got his Tesla that we were driving around on "over 7000 vape batteries!", as that was the fad at the time and where most normal consumers recognized them.
There's some optimization that happens in the chemistry and construction details for specific uses.
Also with bigger packs inter-cell consistency is really important (good cell integrators will test and bin them by ESR even if they're from the same lot, and using a really reliable cell mfg/vendor is critical because you're selling expensive systems with a number of failure points that scales with the number of cells and you want their process development to be super mature.
There's a lot of risk in creativity when you're selling crap to the public at scale. Way better to just use what everyone else is using.
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Probably, but EV batteries are large enough that there might be an industrial recycling process for them, while old laptop batteries are basically free because it's too much labor to extract useful value from them.
I'm pretty sure most industrial recycling methods for lithium batteries involve grinding them up, so pack size isn't as much a factor as sheer volume. I think there just wasn't much juice for the squeeze until demand from EVs made recycling worthwhile.
Here's a video inside a recycling plant: https://www.youtube.com/watch?v=s2xrarUWVRQ
> You would be amazed how many battery packs are multiple 18650s in a trenchcoat
Also laptop batteries used to be many (usually three or six) 18650s in a plastic trenchcoat.
You could literally rebuild your battery when it died, and pick the cells you liked the most. In theory you could pick higher-quality cells than those you find in the batteries sold on ebay from chinese stores. In theory.
>You would be amazed how many battery packs are multiple 18650s in a trenchcoat
$50 of 18650s in a $500 trenchcoat with DRM protection. So wasteful.
When battery packs that have a non-zero chance of literally killing your users are commonplace, it actually does make sense to vendor-lock the battery. Believe it or not there is actual engineering that goes into making batteries beyond spot welding them to an interconnect and stuffing them into $.50 of ABS enclosure.
The "actual engineering" you are referring to is a $1.00 BMS board.
We are well past the point where we should have standardized batteries. We have bunch of standardized wall outlets that accommodate an array of "non-zero chance of literally killing your users" end products. No reason for battery packs to not be standardized (other than vendor lock in).
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> When battery packs that have a non-zero chance of literally killing your users are commonplace, it actually does make sense to vendor-lock the battery.
Linus from Linus Tech Tips made a few episodes on building a battery out of individual 18650 cells, and one of the thing he stressed (as in, underlined) a lot on is that spot-welding cells is extremely dangerous and there aren't easy ways to put out a lithium fire.
Water is not only not going to help you, it's going to make things worse.
You __have__ to have a bucket of sand with you and if anything goes even slightly wrong you just toss everything in the bucket of sand and bring the whole bucket outside.
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"Cheaply made, not cheap to buy!"
That depends on the problem you're trying to solve. If it's only to build a home power system, sure, but if the goal is "I want to prevent these laptop batteries from ending up in a landfill" then using an old EV battery doesn't really help you much.
FWIW a lot of EVs use prismatic cells, not cylinder cells. Tesla, Rivian, and Lucid use cylindrical cells. Hyundai, Volkswagen, BMW, GM, Ford, and BYD all use prismatic cells.