All the tender does is keep the battery at full charge. When a modern car sits idle, there is still battery draw. The tender will "top off" the battery as it slowly loses charge. We're talking milli-amps. Nothing wrong with keeping it plugged in while you're away.I'll be going away on vacation and plan to charge it prior, but I am not sure I feel comfortable leaving it charged the entire time. It's a brand new battery so I believe it will suffice.