My 2002 Prius was having the same problem. I got 3 years out of my last battery and just replaced mine because I was having the same issue. I replaced the battery with a Optima D51r. I had to retro fit the cable ends and take out the battery box to make it fit but it is well worth it if you have a car like mine you plan on keeping for a while. Seems like the car really depends on a good battery to run all the electronics. Here's a link
12 Volt Toyota Prius Battery for 2001-2003 with installation kit. Hope this helps.
12 Volt Toyota Prius Battery for 2001-2003 with installation kit. Hope this helps.