DIY 18650 battery packs

I'm thinking of getting one of these any suggestions?

How do I figure the mAh in an 18650?

Thanks again for the help.

Discharge it at 0.2c down to 3volt to find out the renaming mah in old 18650 cells , you can get a hobby charger for quite cheap.

I just ordered and empty 18650 lcd battery bank box for only £3.30 on ebay, It`s a low quality product, It takes 8 x 18650 all in parallel, my questions is it safe, I have 4x 2200mah and 4x 2400mah 8x in total but these are old cells so they are more like 1800-2000mah.

So are these mixed capacity's safe to use in parallel.

John.

If they are new batteries isn't there a formula for finding the mah?

If they're new batteries, the capacity should be disclosed wherever you bought them, and it's normally printed on the cell wrapper as well.

So 3,400 mah is the total capacity?

I thought 3.7 volt batteries had more capacity than 1.5 volt.

Oh, I think I see now. You seem to be thinking about total energy, not mAh capacity. 3.7V cells do contain more energy per mAh than 1.5V cells.

The way to calculate energy is to multiply the voltage and the mAh capacity together. That gives you a figure in mWh (milliwatt-hours).

A couple of examples to compare:

Panasonic Eneloop Pro AA NiMH

1.2V x 2500mAh = 3000mWh or 3Wh.

Samsung INR18650-25R 18650 Li-Ion

3.6V x 2500mAh = 9000mWh or 9Wh.

Both of those cells have the same capacity in mAh, but the Li-Ion cell contains three times the energy, and it's the energy that determines how long your light or powerbank will run for.

Nine times out of ten, you don't need to worry about energy, though, because you'll normally be comparing like with like. For example, you'll only ever be comparing different 18650 cells for your 18650 battery pack. That means you can use the mAh capacity as a convenient shorthand for total energy, because all the cells you're comparing will have the same voltage anyway.

Thanks.

I'm just trying to figure out which contains more energy a 10,000 mah portable power bank or a DIY 4X186350 power bank.

An 10000mah power bank holds 37.00wh of power, and 4x18650 3400mah hold 50.32wh of power. but depending on conversion efficiency 10-30% of that will be lost as heat, see links below.

Don't buy power bank before you read this

https://milliamps-watts.appspot.com/

John.