SKS fob battery dead, now what?

In the mean time use the key to open the door and insert the fob to start the car.

 

Funny things keys once you have not had to touch one for a while.

 

Which brings up a very important maintenance point: lubricate the lock cylinder on the driver door.

In the average Prius, the metal emergency key never gets used. At least around here, you drive the car in conditions ranging from -40 C in winter, to +38 C in summer. Salt is used in winter to deice the roads

Around a year ago, I noticed my Fob was getting weak. Drove to the mall, got a replacement battery, and everything was fine again. However, back at my house, I decided to try the emergency key

My lock cylinder was so stiff/corroded it barely turned. As I was parked in my comfortable heated garage, no problem. Rummaged around for appropriate penetrating lubricant (Please, no snickers, I'm being serious here), and gently worked the cylinder until it was free again

I'm now in the habit of lubricating the lock cylinder every 3-4 months, and using the emergency key to ensure it works smoothly.

 

Hobbit has a neat writeup on servicing the mechanical lock on the drivers door:

Prius door lock