LG

The battery in my LX-400 phone will not stay charged. Even when fully charged, turned on but not used it will be completely drained and turn off in 8-10 hours. The phone has been replaced under warranty. I have used 2 batteries, the original and a new replacement. I have tried 2 different chargers. None of this has worked. The original phone worked fine for 10 months before developing this problem. The replacement phone has never held a charge longer than 10 hours.

I'm sorry for the problems you are seeing. How long to "Fully Charge" these batteries? Do you have any applications or Browser that might be running on the phone?
If you have fully charged the batteries after they finally give out (over night) and they are still showing short Battery life Then unfortunately I would point to the phone as the Culprit. The batteries shouldn't suffer from the "Battery memory" problem that plagued the old Ni-Cad batteries, but they still need that full charge... But it sounds like you are doing that so the phone is the only thing I can think of, other than the possible issue I just mentioned.
If you are still under warranty I would contact Customer Support (800) 793-8896

I just had the phone replaced under warranty last week (5/13/2010) and the new one is discharging the same way the original one did. I charge overnight and do not run any applications or use the web.

How new are the Batteries? I believe you mentioned that one had just been replaced... I'm wondering if the old phone damaged them. The only other thing I can think of for rapid battery Drain would be poor cell signal. I mean that if your phone struggles to get a signal your battery life can be significantly less. I generally go a weekend with out charging my phone, but a few weeks back something must have happened to the local Cell tower and my phone struggled all night to connect and drained my battery by morning. Not trying to blaim a cell tower but this is something else to look into... We have tree pieces to the puzzle Battery, Charger, or Phone. Most likely it's one of those. If you have replaced the phone and tried different chargers, then the Battery is the next thing to suspect... If it's not something with the service...

Thanks, I have one new battery and the original. The Sprint service center checked them both and said they were fine. I do have weak signal in some parts of the house but the puzzeling thing is that all was well for the first 10 months after I got the phone and all of a sudden this problem arose. From the number of bars on the display it doesn't appear that the signal has changed, either better or worse.

That's a strange one... If it's not the Batteries or the NEW Phone, or the Chargers... Then I'm stumped. I'm wondering if Sprint can tell you whether anything, as far as the system goes, has changed in the last few months...