I have a 5Ah power pack from a ferrex drill/impactdriver/anglegrinder that failed after very few charges but about 15 months old. The charger refused to charge it. One of the 18650 cells is dud but the other 5 are fine. I also have a 40 cell pack from a cordless mower that I was given as it didn't work, these cells are also 18650 , probably the same chemistry and 3.7V nominal but lower quality cells. It seems a waste not to use them.
I have a car jumpstart pack with compressor which previously had a 17Ah
12V gell cell and the 40 cell mower pack fits where the battery came from. I can salvage 32 cells to make a 4s 8p pack.
Any gotchas to look out for? I have a cheap 4s BMS though I am unsure if the balancing will cope with 8 cells in parallel.
Has anyone recommendations for a cheapish spot welder for the nickel strips?
Don't forget that some 18650 cells have an integrated control circuit to limit discharge current, so not good for car starter pack. They are identified by being a couple of mm longer.
What current are you planning on drawing from this?
A car jumpstart can be hundreds of amps. For that you need:
- enough current from the cells. Let's say power tool cells are rated at 10-20A each. In 8p mode that's about 160A. Will that be enough?
- a BMS capable of carrying the current, because any BMS worth its salt needs MOSFETs in series to disconnect the load if the BMS decides things are unsafe. Is your BMS rated for hundreds of amps. Also, Chinese BMSes lie about their current carrying capacity: a 100A BMS might only be good for 30-50A unless you upgrade the MOSFETs and the soldering (eg add copper busbars)
- does your BMS have a temperature probe, because pulling hundreds of amps from 18650s are going to get hot (especially if you're over-drawing from the cells)?
- how are you planning on charging? You'll need a constant current / constant voltage supply rated at 4.2 * 4s = 16.8V with the current being your max charging current.
There's a good reason lithium jumpstarters use pouch cells, because they're designed for the peak current. Power tool cells aren't.
If you just want it for running the compressor that might be OK, depending on the peak loading current of the compressor. Would it be happy with
16.8V when the battery is fully charged?
Have a look on ebay and Aliexpress - there are designs there around the £20 mark. The ones with an integral lithium cell look easier to use than the ones needing a car battery, although I have no personal experience. There are no doubt Youtube video reviews of the various designs.
You wouldn't find those inside packs, because the packs themselves contain the control circuit (BMS). It's only if the cells are loose, eg from vapes or similar applications that take individual cells.
Not a good idea to wire them in parallel, as if one goes short then the current between them might make the battery catch fire without protection. Just saying. Brian
I'd have thought his best bet would be to find a good cell and fix the failed battery with it, and keep the others as spares.. I don't think I'd be trying to start cars with those cells, you could bugger the lot up and have nothing to show for it but a pile of charred remains. Brian
I don't know but most of the time the jump back puts a bit into the 12V battery first and then the 12V is in parallel with the jump pack so sharing some of the load. Of course the jump pack is nominally at 16V and I don't know how the two internal resistances compare.
The ferrex cells are rated at 20A continuous and 35A 4 second peak.
From what I see of teardowns of the pouch type the cheap ones bypass the BMS with no mosfets on the output.
No because it is simply a cell ballancer really. Both the original BMSs do have probes, smothered in heat conducting paste and jammed between to cells.
Yes I was wondering about a charger, it can be slower than the max 4A per cell the ferrex cells specify. I was thinking of a circuit going between the pack and a laptop power supply.
Yes I saw that.
Not sure but the jump pack is otherwise scrap as I shall not replace the gell cell.
Brian the issue is that the plethora of other cells I have are not marked with maker's details, so are likely of lesser quality than the LG cells from the ferrex pack. Also my soldering iron did not manage to melt the solder attaching the BMS to the nickel strips connecting the cells, so I cut them instead. I had alsready replaced the ferrex battery for my tools.
It is really an exercise > I'd have thought his best bet would be to find a good cell and fix the
If your car battery is at say 10V, your pack is at 16V and you connect them together, then it's mostly down to the internal resistance and the resistance of the wiring. I wouldn't be surprised if hundreds of amps can flow, since if the car battery has xxxA CCA then maybe it can also accept something like xxxA in the other direction?
18650s will have higher internal resistance, but that would cause them to get hot, which is bad news for them.
That's mostly because it's expensive to build a proper BMS at those currents. So they just assume you're not going to drive them into a dead short, and if you do they die. There's often a lump in the cables with some diodes for reverse polarity protection and they also act as a fuse in the forward direction.
(even a car battery at 0V doesn't stay at 0V when you apply current to it)
I'd look on Aliexpres for a '4S BMS' and pick one rated at roughly the kind of currents you're talking about. Also, a cell balancer designed for a power bank might only have 10s of mA of balance current, whereas you want a lot more.
(active balancing is nice, but passive is probably ok)
You can get '16.8v' or '4s' chargers, often repurposed from other rechargeable devices like hoverboards:
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You can also get adjustable 'CC-CV' modules that convert from higher voltages:
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The main thing is a constant current until the target (16.8V = 4.2V per cell) and then going into constant voltage mode.
If you got an adjustable CC-CV charger, you could maybe charge to say 3.85V per cell. Misses out on a few percent of capacity at the top end, but makes the max output 15.4V, which is a bit more lead-acid compatible. Also should get a better cycle life from the cells.
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