2000 S500 - key won't turn/battery is dead

Over the last week or so, the car would struggle a little more and more every time I
turned it on. Finally the other day, I tried to get into it and it wouldn't even unlock with
the remote. I used the blade key and got in and tried to turn it in and the key turned,
dashboard lit up very briefly, then it died. Now, I can't even get the key to turn
(assuming the infrared isn't even turning on to see the key). I just tried to jump it with
my friends car and it still wouldn't let me turn the key. This battery is no more than a
year old, as the same thing happened a year ago and we just replaced the battery. Is
there such a thing as a battery being too dead to be able to jump it? Do I need to go
buy a battery charger and just allow the battery to charge over night and try again
tomorrow? If not that, can I go buy a new battery, use cables to charge the battery as
it sits in the car, then just return the battery? Thanks guys.