Q:

Recommended discharge current of coin cell

I am thinking of using small coin cells to power white LEDs (20mA @ 3V FWD voltage). When I look at some coin cell specs sheets, I've seen "recommended discharge current" of only a few mA or less (e.g. >1mA or >3mA). If I choose a cell with 1mA recommended discharge current, does that mean I need 20 of them connected in parallel to provide 20mA (assuming cell has 3V rating)? But I have seen cheap LED lights powered by a single coin cell rated at only 1.4-1.6V (perhaps the LED has much lower forward voltage, but current must be much larger than a few mA).
So what exactly does the "recommended discharge current" mean in the coin cell specs, or does it matter at all for simple electronic project like lighting up LEDs?

A:

According to the image your attached, if you want to use 90% of your energy in the cell, you should discharge at 1mA.
If you discharge at greater currents, you risk in reducing efficiency, generating heat and shortening battery life time.
According to your design, you should consider the trade-offs you're willing to get along with.
Also it is important to not discharge too "hard" - see maximum pulse current information.

A:

But I have seen plenty of cheap and small size LED lightning powered by only one coin cell rated at only 1.4-1.6V (perhaps the LED has much lower forward voltage, but current must be much larger than a few mA)

So have I, a 1.4 to 1.6 V coin cell isn't going to be able to light up an LED (unless a boost converter is used), so usually 3 of those 1.5 V cells are used in series or a single CR2032 or 2 of those in series are used for "cheap Chinese LED lights".
Those LED lights don't care about any datasheet! They're "designed" to be used and then thrown away. They rely on the internal series resistance of the coin cells and LED to limit the current. So when the cells are fresh, the LED might get a higher current than 20 mA. But since this is a "short lifespan"product, the manufacturer doesn't care.
Also the coin cells will be stressed and might not be able to deliver their full energy capacity. Again the manufacturer doesn't care as the user is supposed to buy a new one when the batteries are worn out.
You're trying to do a better job by looking at the datasheet and trying to use the cells within their recommended limits. For a good quality product, that's the way yo go!
Just don't assume everyone else does things that way, especially cheap products from China.

