In my experience, I don't see any desktop warning until the battery is very low indeed, such that it might be too late for me to do anything about it. Probably

In my experience, I don't see any desktop warning until the battery is very low indeed, such that it might be too late for me to do anything about it. Probably depends on exactly when it runs out, and if you happen to be actively using the computer at just the right time. There may be some way to make the current battery levels display more obvious than the default; I've not looked into it much.