Flat battery?
Maybe consider one of the Ctek smart chargers.
For convenience I have one permanently mounted and connected in the frunk, an MXS 3.8
The routine is:
Arrive home, pop the frunk, plug in the power lead, and leave it.
Battery is always fully charged, even if you leave the car weeks without use.
Of course you don't have to mount the unit permanently in the car, because these units have a quick connect plug that connects to the battery without having to move the spare.