You definitely need to test them all, then keep only the ones that you are comfortable with. Even so it’s best to put them in a shed instead of the house. And to connect each cell with a small fuse.
It’s not a hobby for everyone. But batteries are the most expensive part of solar. Let’s say you reclaim 200 good cells per year. At the prices you mentioned, that’s a value of $800-$1000.
Not to mention that when so many cells work with each other, there is no need to stress them. Powerwalls are the easy life for a cell.
I really disagree. If your vacuum lasted 5-6 years, the batteries are on their way out , even if you test them and they test good. You don’t want to trust these old batteries on something important. If its for a toy for your kid, its a different story. If its something important, your gonna have to spend the money to do it right. Otherwise, don’t do it.
You just aren’t going to find batteries that honestly “are on their way out” that “test good”. It’s quite the opposite, with some brands the circuit board they put in the battery slowly reduces how high it’s allowed to charge based on a counter of how many times you have plugged it in so it gives the illusion you need to buy a fresh battery. Let that sink in. I had an old Dell laptop like this.
As far as “spend the money to do it right”, it depends on your needs. And how fast you need it. But many “new” batteries would fail my testing.
My point is you don’t know their health until you test them. And I’m also saying you can’t assume all cells in a pack are the same health forever.
Tore apart my dyson v6 pack today, what a mess. I was planning to adapt it to use 24v kobalt power tool batteries but I think it’s not worth the effort. Didn’t realize the power switch was in the battery and driven by a lever from the trigger so I can’t just hook up a battery of the same voltage because it would run the whole time. I cant run the power from the battery through the dyson circuit board because the cell monitoring faults out. I tried to connect the power tool battery direct to the terminals on the vacuum but it sparked like it was drawing a lot of current. I dont know if the battery turns on slow to ramp up the voltage but it seemed to draw way too much current hooking it up direct, like it was charging a DC bus too quickly.
The temperature sensor in the dyson pack is glued between 2 of the cells and it pulled the wraps off when i removed them.
At this point I’ll probably let the kids tear apart the vacuum then toss it. Not sure if I ruined it when I hooked up the battery but I’m not going to buy a new battery to find out.
Just going to buy a different vacuum with changeable batteries so I can run a lot longer. I liked the dyson but can’t live with short battery life and not buying a new one at $100. I only paid $150 for the whole vacuum.
For what it’s worth the dyson had LG cells in it which I’ll probably toss. Who needs low capacity cells anyway, let alone year old ones.
I got it working. I added a rocker switch to turn it on and off, it pulls enough current that I didn’t want to plug the battery hot. It would probably have ruined the contacts eventually. Since the old switch controlled the circuit board in the battery to turn the power on to the vacuum the trigger no longer works. It’s not beautiful but it works and now my battery lasts twice as long as a new dyson battery.
I have about 10 of these el cheapo’ yard/side wall lights that are solar powered- they cost like $4 each new (and come with REALLY bad 18650s- that test UNDER 300mAhs… if that). The batteries that come with them are pure (real) junk. Some won’t even charge to 4.2V.
So… I re-use these older laptop, tool (*and today I reclaimed a cordless vac- set of six 2200mHa batteries that currently are testing 1400mAhs at 3.6V- so far, so good!)
I think it’s cool I can “re-purpose” these older 18650s in those outdoor lights. Heck, my wife’s SD-02 holds a battery I got out of a Sony Vaio (bought new in 2003). Still charges to about 1800mAhs 
I have rebuilt all these yard lights using these old batteries and as they are not inside or used in mission critical stuff- and I get yard lights that actually run YEARS- trouble free (well, not all of them 
-but most have run from 5 degrees to 95 degrees here in Kansas and some I haven’t touched in three years).
It’s all about the application and if I could buy 4000 of these (better/older brands) that tested at least 2000mAhs- they would work GREAT for a powerwall- many Utoob vids of folks doing this around the world.
I consider to buy the Dyson V11. Just saw there can be problems with the Torque head ( Dyson V11 Absolute Update & Maintenance Tips - YouTube ). They apparently put in a weak part that should break short after the warranty period. And now the battery…., the power button has broken for some too, a piece that’s difficult to replace. Dust can also accumulate inside the machine. In additional it is overpriced. Dyson was known for quality, their new cordless models seems to be crap. Some say V8 is a better purchase, though it has shorter runtime. Think I will buy Miele instead.
Same is true for Dell, can’t even get a new fan for my old Dell Desktop and the Dell Monitor power button has broken.
3:23 “cheap little plastic obviously designed to break off and force you to get it repaired or buy a whole new monitor”.
