So recently i've been getting a lot of used, and old, laptop batteries, and taking out the 18650 battery cells. Is it possible to determine how much mah that each individual cell can store, either using some basic math, or by buying something online to measure it (possibly a multimeter constantly connected to it)?

I tried doing a bit of searching but came up empty handed.

If you have windows, there are version for windows 7 and stuff too.

If you take them out... I don't think you can get the actual mAh out of them with a multimeter... just the state of charge. You can however hook them up to a thing that draws a constant wattage and see how long it takes to drain it. If you can find some maths for that you may be able to find the capacity. I don't have my equation sheets with me right now, but I believe it is possible.

The capacity changes over time, wouldn't a more practical way be to just determine which ones are faulty and which ones are good? Dead cells leak and can't hold a charge for long, so by charging a cell, measuring the voltage - wait for an hour or 2 - measure again, if the voltage has dropped by .1 or .2 V then it's a dead cell and good for nothing.

I know mah capacity decreases over time with all batteries because of the cells degrading slowly, however, by getting the exact mah rating, I'm thinking I could mix and match different cells from different batteries, if they have the same Lithium tech inside, and if they have similar mah and voltages.

Ohhh.... or not. I can probably find the watt hours... or work done. And I guess you can take the derivative and something to find amp-hours...? Damn, it has been a while since I had to think about this. Looks like I won't be much use afterall.

I just want to give a shout out to Martin Lorton, a gent from south africa who has a fantastic youtube channel on things like measuring equipment, solar panels, battery banks and much more. He also has a website with a forum where I'm sure you can get an answer to your question. The Channel with the playlist I figured you'd need the most and the forum is at his website http://mjlorton.com/

If you only have multimeter, what VXAce said would work, but you couldn't just hook up a headlight because the resistance is fixed and the current will drop as voltage does. Google "constant current dummy load" if you want to try that method, though I suspect that is more involved than you are looking for.

How are you charging the cells? Even the cheapo lipo chargers will read out mAh. Charge up a cell from a fully discharged state and read what it says on the charger.

I'm charging the cells by connecting them via jumper wires to an old portable battery bank's controller board. It allows for charging and discharging the batteries. I'm in the process of charging all of the batteries, because I don't want them to stay at near 0% for long periods of time, in fear of completely killing them.

Hmm you never will get its true full mah capacity. theres always a set amount reserved by the on board control circuitry. Im assuming you want to discharge to its end discharge level.. which is probably like 5 percent less than total capacity because otherwise you wouldnt be able to recharge it lol ?

You CAN actually get the storage capacity of the battery with just a multi-meter and a resistor.

You can slowly discharge the battery over a resistor and measure the voltage every so often to get the discharge curve. Then, because you know the exact value of the resistor, you can transform your voltage/time graph to a current/time graph. To get the total AH capacity just calculate the area under the curve. (use excel to do the maths)

Oh, I should also say... when doing the discharge, do not go below 3v per cell. This will damage the cell. Only a tiny remainder of the stored energy is left at that point anyway. Don't be worried about missing that tiny remainder in your calculation.

run the experiment on your cell (battery on resistor, measure voltage every 5 minutes)

plug in values

10 Ohm may be too low for your battery. I recommend using a light bulb as your discharge resistor. Try to find something closer to 20 Ohm maybe. (you can connect multiple light bulbs in series to get the resistance you want.)

Eh. Get yourself a proper charger first. If you just want the basics and ability to charge multiple (say 4) cells at once, something like Nitecore Digicharger D4 or Xtar VC4 or XP4. These will do NiMH's as well. Prices hover somewhere around like 20-30 bucks.

If you want to find out the capacity, you'll want an analyzing charger. For example the Opus BT-C3100 and so on. These have constant current discharge abilities and also offer a "refresh" feature which quite literally just means that the charge does a few discharge-charge cycles on the batteries. These of course are more expensive (idk, 40-60 USD or more) but if you're handling a lot of cells at once it's quite nice to be able to cycle them.

And if you're planning to use these in some battery bank or some other application with multiple batteries either in series or parallel, it's quite handy (and important) to be able to match similar cells.

There's plenty of other models out there, google is your friend.

Making the circuit to discharge a battery at 10-20ohm seems simple enough, but those calculations are what really confuse me. My knowledge is still quite basically. I know how electricity flows and a few different things, but I have no clue how to, for example, convert 12v to 5v using inductors, resistors, etc. I don't know the math for it.

So for what you said to do above, I have no clue how to do the math for that either. I can defiantly retrieve the required data and put it into an excel spreadsheet like you have done.