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I have a bunch of 18650 cells from laptop batteries lying around and I was thinking about making a huge powerbank, which I could use even to charge my laptop. Most of the thing I have an idea how to do it (powerfull step-ups to charge laptop, some stabilized 5V outputs, 3d printed case...), but I don't know much about charging huge number of 18650 cells. My ideas at the end of this post, but first,

Let's sum up what it should do

Buch of 18650 in a box. I much don't care about the total voltage, I could use DC step-ups or step-downs to charge my phone or laptop. It's primary going to be charged from power outlet, so I would like to have it with AC cable and put the main power source inside the box. Of course, discharge/overcharge protection is needed, some charge indicator will be good to have. I could mount a volt meter to it, but I'm not going to explain everybody how to use it (as BFU's will propably still damage the batteries)

Also, it would be great if there will be an option to charge it, even slowly, by 5V 2A power supply, or 12V car outlet (even it there will be step-up to the main charging DC voltage).

So in the end, the point is How to charge about 40 of 18650 cells?

Some of my ideas... As I said before, I have no experiences with charging so many batteries. I usually use 1-3 18650 in paraller, charging them with TP4056-like charging modules from aliexpress. I know something about balancing boards for set of cells, but that's all. And I have almost no knowledge about the overheating during charging.

Currently I think this is the best solution: You can find circuit for balanced charging of 3S or 4S set of 18650 at aliexpress, even with discharge protection, like this one. However they are propably designed for 1 or 2 cells at each segment, so even if I will provide them 12V power source with many Amps, the charging will take too long. So how about connecting many of them in paraller? If you look at the picture below, the first drawing is how regular charge circuits are connected. Will it work if I will connect some more charging circuits in paraller as shown in the second part of the picture?

18650 charging board

Or this charging board claims to be 25A, but there is so little description. Does it mean that it could charge bunch of cells, totally at 25A, or does it mean that 25A is the maximum discharge current and it will still charge the cells at so little current?

Other, propably bad ideas I had before:

The first idea I got was connecting all the batteries in paraller, so I will get batery at 3.7V with huge cappacity, and using buchn of 18650 charger boards from aliexpress, also in paraller(as I wasn't able to find some high current charging modules), but to charge this, I will propably need something about 5V 20A power supply, which is something I don't have lying around.

My other thoughs were about dividing them into, lets say, 10 groups of 4 cells in paraller, and then build some switching board so I could decide if the groups are going to be connected in series or paraller, maybe bad idea. But the main point about the groups of batteries was that I will be able to charge them all at once, and if I will not have the time/power supply to charge it everything, I will be able to unplug some groups and stay with 4 cells, which I could charge with regular phone charger and keep the rest from draining energy.

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    \$\begingroup\$ Before you actually connect anything, I'd recommend buying a fire extinguisher from a source far more reliable than aliexpress. \$\endgroup\$ – Enric Blanco Mar 19 '17 at 22:30
  • \$\begingroup\$ @EnricBlanco I actually never had a problem with anything from aliexpress. For sure, I always buy parts rated at higher current than is going to be in the circuit, but some things are working for years and they never exploded or anything. \$\endgroup\$ – Adam Ježek Mar 19 '17 at 22:36
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    \$\begingroup\$ What @EnricBlanco was saying is have a fire extinguisher on hand and be sure that works because it is very likely you will need it. Lithium batteries are not toys and do not respond well to abuse. Given you do not know the answer to your question that alone says you should not be doing this. Battery packs require certification. Regulations on the safety features that must be implemented in every battery pack like: Built-in PTC (positive temperature coefficient) protects against current surges, circuit interrupt device, Safety vent, Separator inhibits ion-flow by melting process \$\endgroup\$ – Misunderstood Mar 20 '17 at 1:31
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    \$\begingroup\$ I hope the batteries are sorted in 1% bins \$\endgroup\$ – Sunnyskyguy EE75 Mar 20 '17 at 5:30
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    \$\begingroup\$ It is not really a good idea to mix and match old 18650 cells in series and parallel. Very likely some of them are near the end of life. If you do, make sure you use a good BMS. Basically, I think you will end up putting a lot of time and effort into something that will not last long or work well. Everpower makes a 10 cell Lipo chargers. I am sure there are other companies also. Maybe group your cells into packs of 10, and make 10s battery packs with them? Use a BMS and charge with the everpower charger. And buy a fire extinguisher. \$\endgroup\$ – mkeith Mar 20 '17 at 6:07
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You can use a regular balance charger with as many cells in parallel as you like. With mix'n'match batteries you will absolutely need minimum voltage protection for each set of parallel cells, if you can't find a suitable board you could just have detectors wired to one big relay on the output. You will also need to make sure the cells are at the same voltage at the time that you connect them, connecting them through a resistor until they equalise to within 0.1 volts is plenty. I would recommend getting some sort of capacity meter to test each one, add them up so that each parallel set has the same capacity, try discharging the whole pack and see which sets need adjustment.

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