A 2004 Prius does not have a single standard “group size” for its high‑voltage battery. The car uses two different battery systems: a high‑voltage traction pack (NiMH) and a separate 12‑volt auxiliary battery. The 12‑volt battery you’ll see referred to as a “group size” is typically Group 35, though aftermarket or regional variations can occur.
To understand what that means, it helps to distinguish the two systems at work in this model year. The Prius’ main, high‑voltage battery is a proprietary nickel‑metal‑hydride (NiMH) pack designed for the hybrid system, while the 12‑volt battery powers accessories and vehicle electronics when the engine or HV system isn’t running. In everyday terms, “group size” most often applies to the 12‑volt battery, not the traction pack.
Two battery systems in the 2004 Prius
The 2004 Prius relies on a dual-battery setup:
- High‑voltage traction battery: NiMH pack, not classified by a standard automotive “group size.” It delivers the energy needed for hybrid operation and is managed by Toyota’s hybrid control system.
- 12‑volt auxiliary battery: Commonly a Group 35 sealed lead‑acid unit (in most markets), used to run the vehicle’s electronics and to provide a stable start‑up voltage when the HV battery is offline. Variations can occur by market or aftermarket replacements.
In practical terms, the group size question most often refers to the 12‑volt battery. The high‑voltage traction pack is a purpose‑built module stack rather than a conventional, replaceable automotive battery.
How to verify the battery group size on your car
To confirm the exact group size for the 12‑volt battery on your specific vehicle, use these checks:
- Check the battery label or stamping on the top or side of the 12‑V battery for a group size code (commonly Group 35 on many 2004 Priuses).
- Consult the owner's manual or a Toyota service bulletin for your region.
- Ask a Toyota dealer or an experienced hybrid technician to confirm, especially if your car has had aftermarket replacements.
Because the high‑voltage pack is not a standard “group size” component, anyone working on the traction system should rely on professional service and follow safety procedures to avoid electric shock or other hazards.
Replacement considerations and safety
When replacing either battery, note these points to avoid problems:
- 12‑V battery replacements should aim for a Group 35 sealed lead‑acid unit or an equivalent aftermarket unit certified for use in your market; AGM variants are also common for hybrids.
- HV traction battery replacement or service requires specialized equipment and safety protocols; it is not a DIY job for most owners.
- Always verify compatibility with your vehicle’s generation and regional specifications before purchasing a replacement.
For most owners, the key takeaway is that the 12‑volt “group size” for a 2004 Prius is typically Group 35, while the main traction battery is a bespoke NiMH pack that isn’t described by a standard automotive group size.
Summary
The 2004 Prius uses two battery systems: a proprietary high‑voltage NiMH traction pack (not associated with a conventional group size) and a 12‑volt auxiliary battery, which is usually Group 35. When replacing batteries, verify the 12‑V group size from the label, manual, or a knowledgeable technician, and treat the high‑voltage system with appropriate safety precautions.