Building a battery pack (cheaply)

[Xist]

Originally Posted by Cobb

Using grant money for college for an EV project? Thats very American of you. Some just blow it on non renewable stuff to get women or just supplement an income cash flow problem.

We were crowded in a classroom in Basic when two civilians came in and handed out AAFES credit card applications. When they left our First Sergeant told us he hated that his superiors required that he allowed that, he thought that offering credit cards to brand-new Soldiers was a terrible idea. He then told us that our Drill Sergeants were going to be upset that he told us this, but he got a credit card while he was in Basic he quickly owed two thousand dollars and all that he had to show for it was a Sega Genesis--he spent the rest on beer.

[planetaire]

Originally Posted by Cobb

...I think the American I know of working with it had to over come PAL vs NTSC for the display, then there was another issue that held everything up.

Hi, Cobb

We only use 70 slaves boards. We don't use the master designed by PP.
Because I already have a PC with data logging and displaying during trip. The same PC receive/send data to the bms during driving. So no need to use his master board and the screen is wider, highter display rate, graphic ability ...
During cell charging from the grid there is a relay connected via usb or bluetooth to the PC.
This relay can control an elcon charger or, cheaper, several switching power devices.

[AlexanderB]

So I got offered ~3000 used 18650 cells, asking price is €500. I'm extremely tempted to go pick them up right away.

I hope I'm not taking too big of a leap here, I was gonna do a small boost pack on a hybrid, now I'm thinking 20+ kWh full EV conversion daily driver. I haven't even picked out a donor vehicle or electric drivetrain yet. In over my head a bit, I think.

[P-hack]

used 18650s are a LOT of work for a diminished return. Basically you have to make a test rig that reconditions them and gives you the stats (AH, leakdown, internal resistance), plus a prequalification charge if they are dead, and plenty of new concerns if they are dead, x 3000. Plus you should "fuse" them individually. And paralleling them takes a little time too. And they might only be good for 1AH.

It may be a good project for you or not, just want you to be aware of the magnitude.

[planetaire]

Originally Posted by AlexanderB

So I got offered ~3000 used 18650 cells, asking price is €500. I'm extremely tempted to go pick them up right away.

I hope I'm not taking too big of a leap here, I was gonna do a small boost pack on a hybrid, now I'm thinking 20+ kWh full EV conversion daily driver. I haven't even picked out a donor vehicle or electric drivetrain yet. In over my head a bit, I think.

There are several type of 18650. From the power type to the energy type.
For a boost pack you can chose energy one.
For a main pack power one are better, because amperage will be probably highter then temperature can be a problem, needing some ventilator at least, temp monitoring probably.

Just for help, two links:
Battery test-review 18650 comparator
AkkuDB

So you can compare with the one you are buying.

For comparaison, I bought more then 936 new LG cells 2.8Ah 2$ each+transport+taxes. Now they are at 2.2$.

[AlexanderB]

Ah, some concerns, yes.

Yes, I'm painfully aware of the magnitude, but the price difference is worth it to me. I can do this stuff at night and during weekends, when I can't work to earn money to just buy new stuff..

I have a selection process in mind though, and building an automatic discharger that can do a bunch of individual cells at the same time shouldn't be a problem.
Not too worried about dead cells, if they take a charge, and don't self-discharge, I'll test their capacity and see from there. Some dud cells are to be expected, especially in the batch of "bad" packs, but less in the batch of "used, pulled working".

I might include a tile and a bag of sand in my (dis-)charging station though, if one catches fire for whatever random reason.
The ones that stay dead, will be brought to the battery recycling station, acceptable losses.

For now, I have 165 laptop batteries (mostly 6 cell?) on the way, and some sample cells from the 3000 cell batch. I'll see how they behave before I buy an even larger quantity.

Fusing the cells individually was the plan already (see first post).

I know these aren't the best cells for high power applications, but at $0.25 each though, instead of $2.20. I'm okay with a little worse specs.
I'll be using active ventilation and temperature probes all over the pack. Don't want it to blow up and set my car on fire.

I might need some other type pack in parallel, to put out enough power for acceleration and to help store regen energy, I don't think these cells like doing much above 1C discharge rate. Perhaps I'll use a Prius pack, who knows.

[P-hack]

I've got a few dead 18650, this was worth watching

https://www.youtube.com/watch?v=aEVxVVET0nE

pwming a power transistor and a shunt could be helpful.

[AlexanderB]

Update

So my first box of "dead" batteries arrived, $30 for 95 batteries (+ expensive shipping), I sorted them a bit, and I picked a handfull to tear down.
Results so far:

about 2/3 of the packs are recent-looking 6 cell Dell or IBM/Lenovo batteries, the rest is HP/Dell/Toshiba 8 cell packs, and a bunch of randoms.

A few of these packs are severely rusted on the inside/ outside. I'm going to assume they're toast. Most of the rusty packs are very old-looking anyway, no big loss there. I might open them to confirm (and take any non-rusty cells) but I don't expect much,

Another few (I think 4 or 5) are not 18650 cells, but prismatic (li-po?) cells. Also useless in my case. I might see if the ones that aren't giant balloons still take a charge, if so, maybe use them for something else.

While disassembling I started ripping the spotwelded tabs off. This might not be the way to go, as I got 1 cell punctured (a tiny pinhole in the bottom) and it started oozing blue fluid, so I put it seperately, I should probably put it outside.

Most of the spotwelds come off easily though, but some just tear, leaving little sharp shards of tab stuck to the cell. (I really should start using pliers instead of my hands)


Other than that, my nickel plated cell tab strip arrived.

And I missed a relatively good deal on those battery pack spacer clips, someone on Aliexpress was selling them for what amounts to $0.13 per cell in the finished pack (against $0.20 regular price on eBay.) but I'm still not convinced, so I let it slide..

[AlexanderB]

A second batch of used batteries arrived, I got nearly 1000 cells, about a third of them I have in a box, 2/3 are still in their laptop packs waiting for me to rip them open.

I'm contemplating buying another ~1000 cells, or first using what I have to build a pack and do some testing with that.

My chargers, cell holders etc.. arrived, and it took some messing about to get it all to work.
Firstly, the battery clips don't make contact with the flat-top batteries, so I added a solder blob in each one to make it so they do. further more, the crimp on the wires isn't really good, so a few of them had bad contact. (Some more solder fixed that too.)

The single cell chargers don't take kindly to reverse polarity, so 3 out of the 10 died when I accidentally put in cells backwards. Whoops.

Also, I was using my laptop and a bunch of USB cables to power them, not really the best way, now I hooked up some LM2596 boards in parallel and set to 5.00v output. I added a little stick-on heatsink on them, because at 3A each they get quite hot.

I ordered more battery clips, chargers and some of those adjustable DC-DC converter boards (I borrowed these from another project, and its nice to have spares)

Next up, building the battery discharging device. The idea is to use a 2.5A capable transistor, a 10W 1.8Ohm resistor, a beefed up battery clip, multiply that by 10, and add an Arduino and an lcd display to make a 10-cell-at-a-time discharger to figure out how good all the individual cells are. I might add some diodes so I don't blow stuff up in there too.