How to keep rats out of your car’s engine compartment

Rats invaded our car’s engine compartment a few years ago. Luckily, we found them before they did any damage to the engine itself, although they did remove quite a bit of the insulation under the hood (whereas a neighbor had her engine effectively destroyed by a bad infestation). After some research, I picked up an under-hood, battery-operated rodent repeller along with peppermint rodent repellent pellets and a few burlap sacks. Using Lithium AA batteries, the ultrasonic / strobe combo runs for over half a year before batteries needs to be replaced. I set a recurring event in my calendar to remind me to replace the batteries.