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Comment by StavrosK | Hacker News Reader

Those batteries are generally 18650 cells welded together, but you do need a spot welder for that, and to figure out their voltage so you know how many to put in series. You may be able to just take the protection circuit out of the pack you have now.

Sony and Panasonic are good 18650 vendors.

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I love to (or used to before we switched to WFH) check in on the battery recycling bins in the office and look for discarded laptop battery packs. Usually they are full of good 18650 cells (except for 1-2 bad ones that bricked the pack).

Oh yeah, that's usually like $40-$50 worth of cells just tossed.

All the laptop salvage cells I have test with high internal resistance and low capacity, though the packs I salvaged them from were not dead, just low in capacity.

They may be worth $5/cell new, but not so much in their current state.

It's not recommended, but I have soldered in replacement cells with a regular 30W iron. It can be as easy as open the pack, identify and replace bad cells, manually balance charge the cells (one at a time), tape the pack back together. It's a bit janky, because the plastic cases are sonic welded and you have to butcher them a bit to get them open and then glue or tape them back together, but it can be difficult to locate batteries for old laptops and I don't want to spend money.

The problem with soldering them with an iron is that you get the cells way too hot by the time solder has managed to melt, even if you're quick about it.

Not if you work quickly and use 63/37 0.6mm rosin core tin-lead solder with a 30W iron. I never need to apply the iron for more than half a second. I've done this more than a few times, and I realize that it goes against standard safety recommendations, but I'm going to keep doing it.

And if you're recycling cells from other packs, they are likely to have remnants of old tabs still connected, so you can solder to the tabs instead of directly on to the cells, which gives a little more thermal wiggle room.

You can also stick the cells in the freezer before soldering. It adds a fraction of a second to coming up to temperature, and gives the cell proper a lot more thermal wiggle-room. Nothing bad will happen if you keep them above about -20° C.

Definitely file this under "bad advice you probably shouldn't follow, and don't tell them I sent you", but 'allegedly' this works...

That's extremely, extremely risky to do if you don't know the cell's composition, especially the liquid electrolyte, and its performance in cold temperatures. Last thing you want to do is manage to get it frozen and physically damage the layers.

Yes, you should look it up, and pretty much any 18650 will have -20° C or lower as the lower end of its range.

It would be extremely, extremely risky to sell a product which could be destroyed by leaving it in the car in the winter.

I think this advice only earns one "extremely" or maybe a mere "very"...

What are the consequences of getting cells too hot by soldering? Immidiate danger or danger while discharging/charging?

immediate danger

I didn't think that the absolute series voltage would matter much - the right number of cells will be close enough for the voltage regulation in the laptop. My big concern with that approach is matching the cells. It's not practical for me to do that myself since I would need to test a lot of cells. Or maybe I am overestimating the negative consequences of using unmatched cells. But I don't think so.

How do you mean matching them? Just get 8 NCRs or whatever and that's that.

The absolute series voltage matters in that you don't want to over- or undervolt it by 3.7V usually.

Matched in terms of voltage and discharge characteristics. Many of the battery guides mention the possibility of the weakest cell in an unmatched pack being improperly charged. And that that would result in shorter life for the pack or worse consequences.

Hmm, I was assuming the charger would have a balancer, if it doesn't you can get in trouble, yes.

Maybe you are right that it is a balancing charger. That would simplify things a lot. Although some of the packs have some of the cells in parallel so would have to be carefully matched.

Eh, sure it matters somewhat. But if you think you're getting matched cells when you buy anything with a battery pack from China you're kidding yourself. So a home made pack isn't going to be any worse off that the cheapest kit you already have. If you go to the trouble to assemble your own pack with matched cells you'll be ahead of the game, if that floats your boat.

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