You can do it, but it is not recommended. Did you try a reset??? You have 4 buttons on the Cobalt II. Hold the 3:00 o'clock and 9:00 o'clock buttons down at the same time for like 15 seconds and see if the computer comes back up. Once it comes back up you can try a recharge. Usually this does it, but it may not depending on how long it sat between charges.
The computer is always on, even when it is in sleep mode. There is no "off" in a sense that the battery is not discharging. Obviously, when the screen is lit and you are diving it uses more power, but it is still using some when not being used. It will consume more power in use if the screen is turned up in brightness and also if you have it set to record data at shorter intervals.
Because the computer is always on, you should be making point to charge it at least once a month. I have two Cobalt II's and the one my son uses has never had an issue. I just got mine back from Atomic last week. It would lose charge faster than it should. It also would never read "100%" fully charged. I could leave it on the charger and it would never get above 98-99%. Then, in one or two days it would be down to like 85%. It should only loose like 1% per day, if that.
I am telling you all this because I don't know how long you left yours sit. The good one I have has gone for 2, almost 3 months and never went into "dead" mode and needed to be reset, as it should. It got low, but it came right up when charged. I believe the one I use had a bad battery from day one. I tried to keep up with the once a month charging, but after about a year it started to show the battery issue. Finally about 2 months ago it went into dead mode within one month. I sent it back to Atomic and they fixed it perfect and at no charge, but it took 6 weeks. Here it is knocking on April's door and if you have any spring dives planned you might want to get yours addressed.
Edit: one other thing to definitely take a look at...the old style charger had an AC adapter that went from 110v to USB the USB cable had a smaller plug that went into the final adapter to the computer. I had trouble with the adapter that plugs into the computer itself. It was broken inside and charged about half the time. I sent that back with it and received a newer style charger. The adapter is the same, but the AC part has a hard wired cable that has the tube style end which plugs into the opposite side of the computer adapter. Not that it matters where or how a cable plugs in, but the charger has been changed to a different configuration. If you have to send your computer back I suggest sending the charger you have with it. Also, they told me the reason it's not recommended to replace the battery yourself is because the computer should be tested for leaks after the battery gets changed. It is tested for leaks at the factory at maximum depth pressure. other then a dive and having it ruined if it does leak I don't know how you would do that.
