Lithium ion batteries do not like being completely discharged.

From batteryuniversity.com .....
"[FONT=Arial, Helvetica, sans-serif]Similar to a mechanical device that wears out faster with heavy use, so also does the depth of discharge (DoD) determine the cycle count. The smaller the depth of discharge, the longer the battery will last. If at all possible, avoid frequent full discharges and charge more often between uses. If full discharges cannot be avoided, try utilizing a larger battery. Partial discharge on Li-ion is fine; there is no memory and the battery does not need periodic full discharge cycles other than to calibrate the fuel gauge on a smart battery."[/FONT]


My understanding is that the battery does have a charging circuit that does need to be "calibrated" by cycling through charging to full charge then completely discharging the battery a couple of times.

 

There are sooo many misconceptions about todays cell phone batteries. The whole 'cycle through from full to empty a half dozen times to optimize the battery' thing is a carry-over from older style cellphone batteries. That doesn't apply to todays Li-ion batteries. If you want to just for the sake of making yourself feel better about it (like I do), that's fine, but it really doesn't make any difference.

Likewise, when you run your phone down until it shuts off, you haven't completely drained the battery. Your cellphone is designed to shut itself off well before the battery is actually dead in order to protect the battery. So you really don't need to worry about doing that either. However, if you want to throw it on the charger when it reaches 40% then go ahead, it won't hurt it either.

Basically, there's not much you can do to screw your battery up. We're all just used to older style batteries that were finicky as hell.

 

That's just not true. If you cycle the battery down to zero regularly, you will cut down the batteries life drastically. And heat is a big enemy as well. I've researched pretty thoroughly on this.
Google is your friend here.

Again from batteryuniversity.com...They have a lot of good info here.
Simple Guidelines for Charging Lithium-based Batteries

• A portable device should be turned off while charging. This allows the battery to reach the threshold voltage unhindered and reflects the correct saturation current responsible to terminate the charge. A parasitic load confuses the charger.
• Charge at a moderate temperature. Do not charge below freezing.
• Lithium-ion does not need to be fully charged; a partial charge is better.
• Chargers use different methods for

 

And I did say it was the charge circuit that needs to be calibrated, not the battery itself.

 

Not trying to argue, just repeating what I've learned here on the internet. I found this very useful...

Li-ion batteries myth busting...

While it is true that discharging too deeply will open the safety circuit, making the battery appear dead (allowing the battery to fall below 2.5 volts), cell phone companies are aware of this, and therefore incorporate built in safety features into your device to shut it down before you could drain it down below 2.5 volts. If they didn't, and you were to leave it on until it dropped down below the safety threshold, then you wouldn't be able to charge it back up again.

I owned my Droid X for 9 months before upgrading to my G2x, and I had both an extended battery as well as my stock battery, and what I would most often do was run my battery until dead, replace it with the other battery, and place the dead battery in the charger. After 9 months of doing this neither battery appeared to have lost any life as compared to when they were both new.

Therefore I am personally convinced that letting your phone go until it shuts itself off does not discharge too deeply, or I would have two dead batteries on my hands. And I don't. But like I said, if you want to recharge everytime it gets down to 40% knock yourself out. It really doesn't matter either way.

 

I really hate when people argue here just for arguments sake, so I do try to avoid it. I only took issue with your statement "Basically, there's not much you can do to screw your battery up."

And a Lithium Ion battery should, I've read, last two years. If you cut the life in half, you wouldn't know it in your stated nine months.

 

That's true.

I suppose saying there's not much you can do to screw them up is a little strong. What I really meant was that they've built safety features into both the batteries and the phones to help avoid over/under charging them. But knowing how to care for batteries is always a good thing.

I guess if I ever use the same phone for two years I'll find out either way whether or not my beliefs hold true. But I seriously doubt I ever will. But another factor in my care free approach to battery maintenance is that, if my battery were to cut out on me after a year, it wouldn't bother me to dish $30 bucks out for a new one.

So I may not be as judicial in following the rules as others.

 

"I guess if I ever use the same phone for two years I'll find out either way whether or not my beliefs hold true. But I seriously doubt I ever will. But another factor in my care free approach to battery maintenance is that, if my battery were to cut out on me after a year, it wouldn't bother me to dish $30 bucks out for a new one."

True that.