You can do this and many more things with Tasker.
I have nothing to do with the program except being a user of it. There is a
small learning curve with it but there is also a lot of help on the forums.
Just off the top of my head I just did what you wanted. (which won't make sense until you run tasker)
New profile-->
State-->
Battery Level, select the from and to settings for the Battery level you want this to work at.
Click the
+ to add a new action-->select
Alert--
Here is where I am not 100% sure...
You could try
Notify Cancel should work. The built in help says that will cancel all notifications including permanent ones. So if the battery is low that is what you would want.
You would have to ask over at the tasker forums to be 100% sure.
EDIT: No that just turns off notifications that were created by Tasker. I will keep looking for you but at this point I am unsure if it can be done now. It is still a great program.
EDIT EDIT: Okay there is a setting to surpress the battery that may work but I am not going to drain the battery to test it.
I can help you set it up if you want to try, let me know.
Believe me if it is possible at all, this will be the program that could do it, anything possible in the API he basically adds.
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To have an alarm when the battery hits 100% charge:
New profile-->
State-->
Battery Level, select the from and to settings for the Battery level you want this to work at. Set them as 99 to 100.
Click the
+ to add a new action-->select
Alert--the add whatever type of alert you want, popup, flash, sound, vibrate etc...
I LOVE that program, it can replace so many other utilities once you learn it. It costs about $6.30 but worth EVERY penny because it can do so much to customize your phone...
http://www.appbrain.com/app/tasker/net.dinglisch.android.taskerm