Toyota Prius 12V Auxiliary Battery: What Fits the 2008 and 2005 Models
The short answer
A 2008 Toyota Prius and a 2005 Prius both belong to the second-generation Prius, sold for the 2004 through 2009 model years. Within that generation, the 12V auxiliary battery arrangement is shared, so a replacement that correctly fits a 2005 should also fit a 2008, and the reverse. The factory battery is an absorbent glass mat (AGM) type, a sealed design in which the electrolyte is held in fiberglass mats, and it sits in the rear cargo area on the right-hand side rather than under the hood.
The condition that matters most is the vent arrangement. Because the battery lives inside the passenger compartment, the original has a vent port that connects to a hose routed outside the cabin. A replacement should be a sealed AGM battery with a compatible vent fitting so that hose can be reattached. A conventional flooded battery, or a unit without a vent connection, is not a suitable substitute for this location.
What to match when choosing a replacement
Battery type: sealed AGM, suited to installation inside the cabin. Vent fitting: a port that accepts the existing vent hose. Case size and terminal layout: the original is commonly identified by the Japanese industry size code S46B24R, where the final letter indicates terminal position; confirm this against the label on the battery currently in the car. Terminal style: the factory unit uses small post terminals typical of that size class, so the existing cable clamps should fit without adapters. Hold-down: the replacement must seat in the factory tray and accept the original clamp.
Capacity, cold-cranking amps and reserve-capacity figures were not supplied for this guide. Compare those ratings using the label on the existing battery or the owner's manual rather than relying on a general figure.
Why this battery behaves differently from a conventional car battery
In the Prius, the 12V battery does not crank the gasoline engine; the high-voltage hybrid battery and motor-generators handle that. The 12V battery powers the computers, lights and relays needed to boot the hybrid system. When it is weak, the usual result is that the car will not reach the READY state, rather than a slow-cranking engine.
For a jump start on the second-generation car, Toyota provides a positive jump terminal inside the fuse box under the hood, so the battery in the rear does not need to be reached directly. Follow the procedure in the owner's manual for connection order and ground point.
Differences this guide could not resolve
Market: the information here reflects the second-generation Prius as generally sold; vehicles built for other markets may use a different battery specification, so check the label and manual for your specific car. Prices and availability: no pricing or stock information was supplied. Specific brand fitment: no tested replacement models were supplied; confirm vent compatibility and dimensions with the seller before purchase. Prior modifications: if a previous owner fitted a non-original battery or altered the vent setup, the parts in the car may not match the factory arrangement.
Next step
Open the right-side cargo panel, read the size code and type printed on the current battery, and confirm that it is the AGM unit with a vent hose attached. Use those details, together with the 2004 to 2009 Prius fitment listing, to select a sealed AGM replacement with a matching vent port, case size and terminal orientation.