A new project I'm working on right now is a powerbank that could power a 12v dc to 230v ac converter. The result so far is a 3S 20P 18650 (Lithium-ion) battery pack which has around 517 watt hours. To compare "big" powerbanks (20000mAh ones) have around 74 watt hours. My powerbank will have  7 times the capacity compared to normaly called "big" powerbanks.

I tested around 300 cells from old Laptop batteries. Most of them could not be used because they were under one Volt. One Volt is for me the savety limit, because under one Volt the cells could have a small internal short which causes heat and self discharge. Also I sorted out cells that have got warm when discharged/charged or had a capacity under 1200mAh.

I tested each cell with the opus bt-c3100 and charged them back up with a self build charger containing eight tp4056 charging boards (I expliained them in my other Powerbank post) :

(Red indicates charging and blue fully charged)

Opus:

Open Laptop battery:

Most of the bad Cells:

This got me around 80 good cells with a capacity over 1800mAh and around 40 cells with a capacity under 1800mah. For this powerbank I used 60 of my best tested cells listed below in packs.

I arranged them in 3S 20P packs. This means I have three packs in series and each pack contains 20 Cells in parralel.

Like so:

Used cell capacity:            

            Pack1  Pack2  Pack3

Cell1:  2552 2506 2585

Cell2:  2532 2498 2566

Cell3:  2504 2480 2557

Cell4:  2462 2472 2546

Cell5:  2455 2456 2504

Cell6:  2454 2384 2496

Cell7:  2438 2370 2447

Cell8:  2433 2306 2438

Cell9:  2412 2298 2386

Cell10: 2298 2292 2250

Cell11: 2289 2289 2240

Cell12: 2240 2282 2229

Cell13: 2224 2275 2224

Cell14: 2204 2267 2211

Cell15: 2198 2259 2204

Cell16: 2191 2237 2199

Cell17: 2190 2234 2174

Cell18: 2190 2224 2172

Cell19: 2152 2222 2095

Cell20: 2141 2207 2036

Total Capacity:

Pack1: 46559 mAh

Pack2: 46558 mAh

Pack3: 46559 mAh

I soldert the cells in their packs together. This it not ideal, because the cells get heated which can effect their capacity and  lifespan. Normally you spot weld them together because it doesnt heat the cells, but I dont have a spot welder.

Spot weldet 18650 battery Pack:

How I did it (not recommended):

For savety each cell got his own 5A fuse. Inaddition to that I also used a temperature fuse for 4 cells together that will cut off the load when set off.

Finished battery pack:

In the next part I will probably go over the electronics and the casing for the powerbank.

If you have any questions feel free to ask and please leave feddback if you want.

Have a nice day!