Mighty mouse batterly levels accurate?

[Nuks]

I've had my mighty mouse for about a month (usually when people say the first batter gets low). It has been at full levels until yesterday, when it went down a bar. Just then it went down another two, and now read "low battery". I know this is about the time that it should start to go, but it just went, all of a sudden. Is this normal, or not?

Thanks.