Increasing battery life of smart lock

Hi,

I have a smart lock that runs of 4 double A batteries in series (I think its series). I want to increase the length of life of the smart lock as it only last 5 days! I was thinking of running a car battery to power pack and remove the double A batteries. My impression is that 4 double A's is 6v. Therefore if i get a 12v car battery and use a cigarette lighter that converts the 12v into 5v then i can then run the 5v usb cigarette lighter into the 6v battery pack that has the batteries removed. I would think the smart lock could urn of 5v since it so close to 6v (also when the batteries start to die it will end up providing 5v anyway so it should work).

Is this logic correct? Would this work or would i break the smart lock. Is amps an issue here? Thanks