Service Batter indicator

The message pops up because your battery health has fallen into the "replace" range. That doesn't mean anything is wrong with it, it just isn't able to hold a charge as well as it used to. It's normal for the health of your battery to fluctuate some day to day, and yours seems to be fluctuating into the "replace" range sometimes. If it still holds its charge enough for you, just ignore the message. If it gets to a point where you're not getting enough usage time out of it, then replace it.

 

The heat generated by using your Mac normally won't have any measurable effect on your battery. iStat Pro will give you accurate information about your battery health and cycles. The "Service Battery" indicator pops up when there is a problem with your battery or when your battery health (maximum capacity) drops below 75%. As sweetbrat indicated, your battery is still useful. This should answer most, if not all, of your battery questions:

Apple Notebook Battery FAQ​