I have a Parkzone j-3 brushless cub. I bought the stock lipo battery and charger for it. I got the plane all put together and charged the battery last night. After it was finished charging, I plugged it into the plane and tested rudder, elevator, and throttle to make sure that everything was working (and it was). I put the plane away for the night hoping to get a test run in today. I woke up this morning grabbed my plane and gave it another check. This time it did nothing. Absolutely no power. I had left the battery plugged into the plane last night and had turned everything off, so I figured it shouldn't drain the battery, but I figured I wouldn't count out that possibility. I tried charging the battery. The charger just flashed its green light ("ready to charge" ), but when I hit the start button, it did nothing. I finally notice the fuse on the charger is blown. I buy a new fuse. I try charging again. This time the lights start flashing, alternating red and green ("error" or something to that effect). I can't for the life of me figure out what's wrong. Is the battery messed up? Maybe the charger?

Sounds like your battery is messed up probably a cell reversed polarity. now you know you cannot leave battery plugged in on electrics it will drain battery.

yeah, I guess. I just don't see how a fully charged battery could completely drain overnight. Nothing was on. It's almost like saying that you should take the batteries out of your tv remote because you wouldn't want them draining overnight. There's no activity, no power consumption (I would think).

I leave batteries plugged in all the time in all my planes and cars. Does yours have a true on/off switch? My dad's skyfly just plugs in and has no switch, so it may be off but the system is actually on. Is it a lipo battery, maybe its just a wierd lipo thing. I have left all my nicad and nimh reciever batteries plugged in for long periods of non use and they still have lots of power. I would try to return the battery or something, horizon hobby is supposed to have one of the best customer services.

it doesn't have a true on/off switch. But when I turn off my tx, the light on my rx goes off too, so I kinda figured that the power was shut off. It is a lipo, and I don't know much about lipos, so maybe lipos are susceptible to this type of thing. I don't know. I would try and return it, but I bought it from my lhs, and they don't allow you to return items. A horrible policy IMO.

You have to unplug the battery or install a switch. Simply turning off the transmitter does not turn off the receiver.
It is very likely that you over discharged your lipo.
Lipo batteries can be ruined if any single cell drops below about 2.9 volts or so. Some chargers will not even attempt to charge a lipo if it detects voltage below 3.0 volts per cell.
You can try to bump the voltage up with a very slow charge but there is a chance you ruined your lipo battery and will need a new one.

Just an update on this. As most of you have said, the battery is probably ruined. When I go to charge it, it beeps twice (indicating 2 cell) then it gives me that error. Directly after getting that error I check the fuse in the charger, and the fuse is blown. I'm guessing that, like you guys said, the battery is junk. Any contrary thoughts on the matter would be appreciated.

This is a common problem new comers to electric make. Anytime you have the battery plugged into an ESC there is current flow and will almost certainly completely drain a battery if left on for an extended period. Even if you have one of the ESC's with a switch on it, there is still current drain as all the switch usually does is to not allow the motor driving the prop to be turned on, the ESC is still drawing current. Your LiPo is most probably beyond saving. You might try charging with a constant current charger (like used for NiCad and NiMh) at a low value of about 20 to 50 milliamperes but; if you do, monitor the battery very closely and if you see any puffing of the cells stop. If puffing happens, you might as well pitch the battery. Monitor the voltage of each cell while doing this and as soon as all cells are above 3 volts, your LiPo charger will work for completing the charge. Do balance the cells as they will be severely out of balance at this time. Even if they recover and will charge, they may (most probably will) have diminished capacity.