Toyota Prius Gen 2 Battery

As a battery design goes, the Toyota Prius Gen 2 Battery is rather inspirational, if just because of the number made. However, the modules are very neat in construction, it is air-cooled with waste cabin air and has been tested with a 159,000 miles on the clock with a SoH of >82%. This is an impressive, but simple design.

Toyota Prius Battery

This pack was used in the 2004 Toyota Prius. Second generation of the hybrid battery design and using NiMh cells.

This is a HEV pack and so quite a small total energy of 1.3kWh and using a smaller 40% SoC window. This small window is so that the pack will be able to last the lifetime of the car with high charge and discharge rates from braking and acceleration.


  • total energy = 1310Wh = 1.31kWh
  • usable energy ~520Wh
    • Usable SoC window 40% (40 to 80%)
  • peak discharge power 21.2 kW10s and 30.0 kW1s at 50%
  • continuous power [W]
  • nominal voltage [V] = 201.6V
  • capacity [Ah] = 6.5Ah
  • voltage range [V] = 180V to 270V in use
  • weight [kg] = 37.2kg (82lb)
    • module = 1.04kg
  • volume pack = 66.9 litres (based on dimensions)
    • module = 0.57 litres
  • pack dimensions [m] = 863.6 (34″) x 406.4 (16″) x 190.5 (7.5″) mm
    • module = 19.6mm x 106mm x 275mm
  • number of cells [#] = 168
    • 168s
    • 1p
  • charge time [minutes]
  • cell format = prismatic
  • modules
    • 6 cells in series in each module
    • 28 modules in each pack
    • each module contains a charge controller and a relay
corroded busbars on a Toyota Prius

A regular repair on these battery packs is to strip out the bus bars and replace the connecting plates and nuts on each battery module. Corrosion can be caused because the air used to cool the battery comes from the cabin that can be hot, cold, moist etc. When rebuilding always use a drop of Stabilant 22A on the nut/stud.

Andy Latham, Salvage Wire

Image supplied and copyright of Salvage Wire.

  • cooling system = forced air cooling
    • cool air taken from cabin, the cabin air can have moisture and hence some bus bar corrosion is seen in service
    • temperature is measured using 3 thermistors, and the air intake temperature with a 4th thermistor
    • dimples on module case to create air pathways
cooling options

Battery Cooling Options

There are many battery cooling options, which is better or best depends on the cell selection and application. There is no right and wrong. However, letโ€™s look at them and at a first attempt list of advantages and disadvantages.

  • cell make and model
    • Panasonic
  • pack cost

Key Pack Metrics:

  • Gravimetric energy density, pack = 37.2Wh/kg
    • Module = 45Wh/kg
  • Volumetric energy density, pack = 19.6Wh/litre
    • Module = 82Wh/litre
  • Gravimetric power density, pack = 570W/kg10s
  • Volumetric power density, pack = 317W/litre10s
  • Estimated cost $650 to $700 => ~$500/kWh
    • This appears high, but remember that a small battery pack has a lot of overheads in the control system, case, cooling, HV connections etc. Plus this is a high C-rate hybrid pack and so the cells are designed to last a lot of cycles.
  • Cell to Pack mass ratio
    • module to pack mass ratio = 78.3%
  • Cell to Pack volume ratio

Also, these specifications and metrics can be cross-correlated to check their legitimacy.

Toyota Prius battery pack
Toyota Prius battery pack with the lid off.
image supplied by and copyright Ralph Hosier Engineering Ltd

Other key features:

  • Safety
    • Service disconnect that breaks the HV circuit and has a switch that also signals to the BMS when it is installed. The service disconnect has a 125A fuse built into the plug, the fuse is part of the plug and not replaceable on it’s own.
  • BMS design
    • The battery SoC set point is 60% – designed so that the battery can absorb the energy from braking events and deliver this energy back to the driveline for acceleration.
    • Power in and out limited down to zero based on SoC and temperature
    • Voltage measurements are made across 12 cells (2 modules), hence 14 voltage measurements in total as seen in the screenshot below. This means that cell voltage limits and balancing can only be applied at this group level. This does bring into question how individual cell voltage safety limits can be adhered to and the impact on lifetime and safety.
image of voltage measurement channels from a Toyota Prius battery pack
Voltage measurement channels from a Toyota Prius Gen 2 battery pack (image courtesy of Aqib Idrees)
  • BMS broadcasts
    • Voltage of the pack overall
    • Current (positive for discharge, negative for charge)
    • Minimum & maximum temperature
    • Maximum current it’s able to provide
    • Maximum current it’s able to accept
    • State Of Charge (SOC) – estimated using coulomb counting and then updated based on OCV measurements when the vehicle is off.
    • Any fault codes (DTCs)
  • HV Distribution
    • Contactors: Panasonic 80A, 12VDC, SPST NO, Stud terminals
  • HV and LV Connections
  • Cooling Connections
  • Structural / non-structural pack
  • Case material
  • Sealing strategy
  • Venting strategy
    • modules vent gases directly outside the vehicle through a vent hose connected to each NiMH battery module.
  • Durability โ€“ % of pack capacity available after 10 years
    • warranted for 10 years or 150,000 miles (in states with California emissions laws) or 8 years or 100,000 miles in all other states
    • US DoE tested 159,000 mile 2004 Prius in 2008 and battery pack had >82% of original capacity
  • Availability
  • Recycling
  • Shipping

The specifications and data on the Toyota Prius Gen 2 2004 battery pack have been gleaned from a number of sources. There are contradictions in the data published online, however, we have tried to resolve those issues. Please do point out any errors or better sources of data as we would like to improve on these for the benefit of all battery designers.

Also, if you have images of the Prius battery that we can use to illustrate more of the design features then do let us know (editor). Or in fact if you have data on other packs that we can share on our benchmarking pages then do let us know.


This is the first of our Battery Pack Benchmarks and hence we would like to understand if this is useful and how we could improve it. Also, if you have material that could be added to improve it or we should link to.

6 thoughts on “Toyota Prius Gen 2 Battery”

  1. “it is air-cooled with waste cabin air and has been tested with a 159,000 miles on the clock with a SoH of >82%”

    This is all nice and well in chilly Europe ๐Ÿ™‚

    In warm and humid Malaysia, I wish they had ducted the cold air directly from the cabin evaporator/blower to the battery pack. The battery in my Gen-3 Prius had to be replaced at 60,000 miles. Malaysian-delivered car, registered in 2011. Cooling the pack with just cabin air, especially for a car that has been parked under the sun at 35 deg C ambient, just doesn’t cut it.

  2. We had a Nissan Leaf too (24kWh) when we lived in the UK. Bought it new in 2015. It lost its first SoH “bar” on the instrument cluster in 2021 at just over 60,000miles, which I think indicates ~85% SoH. During the pandemic lockdowns, it was effectively a large “battery-on-wheels” plugged into our V2G charger, allowing us to use off-peak electricity during the day.

    What a great all-round car it was; shame we couldn’t ship it (and the V2G unit) to Malaysia. But as you rightly pointed out, it would have struggled with the heat and humidity here.

  3. Vent fails, excess pressure causes the negative post seal to fail. Electrolyte leaks out the negative post corroding the bus bars, sense wires and main negative cable. Add condensing humidity and the electrolyte travels to the ECU via the sense wires and begins to corrode the pins. Eventually shorting them out, no fuses on the sense wires, and causing a fire. great battery…

Leave a Comment