How to Replace the 12V Battery in a Toyota Prius

My Prius started acting funny. Sometimes when I would turn the car off after getting to my destination, it would beep like crazy when the keys and I would get out of the car. Even if I restarted it and turned it off again, it still wouldn't turn off properly. This would result in the 12v dying and needing a jump to get going again. Other times (like not overnight) it would just give me a weird Parking "P" error and tell me to park on flat ground. And frequently, I would get the dreaded red ! in a triangle. The first time you see that is not a good feeling. I went for about a month or so of gambling and 2 jump starts, I finally decided to test out the 12v to see how much power it has. Check out this thread to see how to get to the diagnostic screen without using your headlights. Mine was in the 9's which is no bueno.

I purchased an Optima from Amazon and a 12v battery charger. I wasn't sure if I would need to charge the battery or not, I did end up needing to charge it overnight, so it was a good investment.

What you'll need:

• flat head screwdriver
• a socket wrench
• 7/16" or 11mm socket
• 1/2" or 13mm socket
• an extender for your socket wrench (I used a 10" because that's all I had, it was way too long to feel comfortable. Half that would be fine. Maybe even less than that). 

1. Remove the trunk cover, hidden compartment, and corner cover over the battery. So you see:
   
2. Remove the red cap. This is the hardest part of the process. I was able to pry the top so that it kind of bent it a bit, then pried up one of the side ones and worked it off.
   
3. Remove the bolts shown below and swing the black box out
   
4. Remove the negative line bolted to the car
   
5. Remove the black box cluster from the top of the battery. Remove the black cap at the back of the cluster. Then loosen the clamp from around the positive battery lug (you don't need to unplug any wires). You can use a flat head to pry it just a bit, but be very careful of the lug. I was able to loosen mine, then slide the screwdriver underneath it and pry it off/up slowly, to remove the entire clamp/black cluster together. carefully set it out of the way being mindful of the wires
   
6. Remove the black bracket holding the battery down
   
7. Remove the tricky screw/bolt that holds the vent housing onto the side of the car and remove the vent housing off to the side
   
8. Remove the hose attached to the side battery, be careful as you'll reuse this (see location in pic 1)
9. Take out the battery
10. Remove the clamp from the negative lug of the old battery and place it on the new battery. 
11. Place your new battery in the hole and reverse the instructions

Side Notes

When I removed my battery, there was water in the battery well area. I was able to find a small crack near where the hatchback door hinges. As soon as I patch that up, it should fix the problem. A google search revealed that other people have the same crack in the same exact place.


My fuel gauge, even though full, only showed one bar under the E and just kept blinking. I drove around for a bit and it fixed itself, but you can see how to reset your fuel gauge if you're not so lucky. This is for 2G Prius.

Conclusion

I hope this guide helps any of you looking to replace your battery. I'm not sure if it's placebo effect, but my car seems less sluggish and so far, it seems like my gas mileage has improved!!

*disclaimer, I didn't take photos during this process so I have swiped some photos from other sources. But I did photoshop them for better instruction.