Sept. 2015

My Toyota dealer quoted me a price of $287.27 to replace the 12 V battery in 2009 my Prius.
In order to save some money I bought a group size battery 51 for around $100, plus about $30 for new positive terminal, a standard negative ground cable and two battery vent kits. The battery fits perfectly except for the height, I had to put in a quarter inch wooden shim underneath the holddown strap in order to hold it tight.

How to open the hatch when battery is dead. Fold-down backseats and remove floorboard to access tool tray, remove small cover, top center rear. See picture below. There is a small silver lever that has to be pushed to the left at the same time the hatch is pushed open, it is very heavy, best to have someone on the outside helping.

The positive terminal is replace with an AutoZone LYNX 06070 Marine battery terminal, there are two in the package, use the one with the smaller diameter stud, cost me $3.29 + tax. This stud has to be shortened by about a quarter of an inch if I remember correctly. Also the width has to be filed down slightly to fit into the Prius wiring hardware, see picture below. These two things are probably the most difficult of the whole changeover.
The negative cable was replaced with a red AutoCraft Battery cable, Top Post Terminal, 4 Gauge, 10", cost $4.49. Because I could not find a 10 inch cable that was black.

The vent kits did not fit, but after reading the battery manufacturers recommendation, I am not going to worry:
"Can VRLA batteries be installed in sealed battery boxes?
NO! Never install any type of battery in a completely sealed
container. Although most of the normal gasses (oxygen and hydrogen)
produced in a VRLA battery will be recombined as described
above, and not escape, oxygen and hydrogen will escape from the
battery in an overcharge condition (as is typical of any type battery).
For safety’s sake, these potentially explosive gasses must be allowed
to vent to the atmosphere and must never be trapped in a sealed
battery box or tightly enclosed space."

There are plenty of videos on YouTube on how to remove and replace the battery. Like this one:
https://www.youtube.com/watch?v=6fsX4ODT5OA



Note: Replacement battery for the Camry Hybrid is more difficult, I could not find a good fit. However, the group size 51 I used in my Prius should work even though it's a little bit smaller. Regarding temperature sensor. It is no problem, just gently pry the sensor off the original battery with a knife and re-install it onto the new one. It's simple attached with something like double sided sticky tape.

Disclaimer: Please note that the information discussed in my writing above is for informational purposes only. Everything you try or do is at your own risk.