I was getting that too, when the battery icon would go gray with an exclamation point. When I looked at the battery status it would say unknown. Usually the battery status either says charging or not charging. I don't know what causes this.
I don't know why that happened, but in addition my phone would turn off by itself frequently. I got the battery on my phone replaced and both problems still continued.
In the end I got Sprint to replace my phone and haven't seen the exclamation point battery icon since.
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