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Battery observations of the 2010 Prius and changes in design between the models

May 25, 2009

I have now seen a production 2010 Prius thanks to Leo Carlos at Hollywood Toyota. I got to look at the batteries. There are some interesting changes. The high voltage traction battery is in a metal case about the same size of the 2004 to 2009 but the batteries take up less space and the cooling fan is the right side of the metal case. This could mean that the individual battery packs are now in metal cases instead of plastic and thus take up less room. See http://www.peve.jp/e/hevkinzoku.html

If the individual cells are in metal cases the cooling would be significantly improved.

The 12 volt battery was in the same place and is the same size and model.

The 2000 to 2003 (Classic) Prius had a .035 amp drain when parked. Toyota had admitted with a service bulletin that the battery was too small. Yet the battery was the same size for the 2004 to 2009 Prius without smart key. Why? The drain when parked went from .035 amps to .011 amps. For cars with smart key the drain increased to .15 amps. cars so the battery size was increased and the 2004 to 2009 Prius comes with a switch to turn smart key off. The switch is on the lower part of the dash near the drivers knee. For the 2010 model smart key turns off automatically if the car has not been used for over five days (reference owners manual). The will drastically deeply discharging the battery.

I turned off the car and locked all the doors and left the overhead light switched on. After two minutes it did not dim so sadly it does not appear that Toyota has added anything to prevent accidental discharging of the 12 volt battery. However, this was hardly a conclusive test.

The stock battery does not appear to be a deep discharge battery as a few incidents of leaving the lights on overnight kills it. I have a Optima Yellow top (deep discharge) battery in my 2001 Prius. I have left a light on in the car at least twice overnight. The car started fine and I would not even have know the light was on except the fixture was hot. The battery is now over 5 years old and still running strong. (For more information on obtaining an Optima battery go to the bottom of the page.)

Another 2010 engineering change is that the 12 volt battery is significantly easier to replace by changing two things:.

  1. The black box labeled “Supply assy brake control power” is no longer in front of the battery.
  2. The duct tube for air used to cool the high voltage battery no longer runs through the compartment but ends so that all the air will flow through the 12 volt battery compartment. The outside vent was moved back along the outside wall so it next to the center of the battery. The battery compartment also appears smaller possibly the wheel wheel is higher in the 2010. The wheelbase has not changed and is still listed at 106.3 inches. (Wheelbase - The distance from the center of the front to the rear axle.)

    All the air used to cool and heat the main battery now flows through the 12 volt battery compartment. This means that the air heated by the high voltage battery pack is heating up the battery. Yes but it is heated passenger compartment air so assuming heating and cooling are being used in the passenger compartment it should be only somewhat hotter than 70 degrees. A good temperature for a lead acid battery. Here is why:

    If the battery is too cold it will last longer but it's output is lower and it requires a higher voltage to charge it. Hot and it has a shorter life. Since the battery is not in the front it is not exposed to engine heat. This explains why some Prius owners get many years of life from the stock battery. It also means the Optima should last significantly longer than 10 years since an Optimas in engine compartments where they are exposed to lots of heat and have to crank the engine often last 10 years. (In Prius the traction battery (high volt) battery cranks the engine.)


The computers and everything that is run on 12 volts already have to deal with 11.? to 12 volts at start up (depending on the state of your battery) and 13.8. Enough voltage range for all the computers in the car to handle. Of course there could have a separate electronic control unit for the 12 volt battery and then a voltage limiting circuit to keep a higher winter charge voltage from getting to the on the 12 volt bus but why? This is a simpler solution but it has the liability that if you never go for a long drive in winter your battery is never charged while warm and never gets fully charged.

In summer assuming you use the AC you battery is cooled. This promotes battery life. Again a good solution provided you use your AC when it is hot. To give you some idea the normal life of a normal 12 volt car battery in a warmed engine compartment in Arizona is supposed to be only about two years.
12 volt battery compartment in 2004 to 2009 Prius

A 2004 Prius - The large square duct in the left of the picture is the for the hot air from the high voltage battery and it exhaust out the side of the car and into the battery compartment. The yellow is the top of an optima battery. Neither the vent tube or the box to the right in front of the battery is there in the 2010!
An Optima battery in a 2010 Prius!
An optima battery in a 2010!

In the 2001 to 2009 Prius heated and air conditioned air also hit the 12 volt battery compartment after being heated by the HV battery as there is a huge hole in the duct in the middle of the 12 volt battery compartment. In the 2010 it is just more so because there is no duct and all the air flows though the 12 volt battery compartment and the outside duct is further to the back of the car right next to the battery. On the 2000 to 2003 the battery was on the left side and the 12 volt battery and there was also a hole in the duct. I always wondered why it was moved from the left to the right. Also when they moved the battery from the left (driver)side to to right side negative electrical terminals reversed to keep the negative to the back of the car. It now seems obvious that this is done to keep the positive from being shorted to the frame of the car during a rear end collision. The negative is already connected to the frame of the car so it is can not be shorted it all.

When you need a new 12 volt battery for the 2001 to 2003 Prius we have one which is considerably cheaper than what most dealers charge and it has 35% more capacity that the original GS battery.

For the 2004 to 2010+ we also have an optima battery. This 12 volt battery has lead plates that are tightly wrapped into a spiral for vastly superior vibration resistance (longer life). Because of the tight wrap the lead does not have to be significantly alloyed for strength and is 99.99% lead. This results in lower voltage drop as the battery discharges and further contributes to the longer life of this battery! It may cost a little more but because the battery will last longer in the end you will pay less and waste less time maintaining your Prius! It comes with a three year full replacement warranty.

original 2001 to 2003 Prius 12 volt battery The original battery in the 2001 to 2003 Prius.

The 2010 has a new gas engine that has grown is 1.8 litters instead of 1.5 litters. The spark plugs are behind the head and will be a little harder to replace. With the 1.5 litter engine they were in front of the head and were very easy to replace.

All of the hybrid displays showing your MPG and your average MPG for every 5 minutes of your trip and now displayed with the Miles per Hour right above the steering wheel. You can change the display right from a switch on the steering wheel. The basic design of displays is the same except there are more displays to choose from and the computer now remembers and display your 5 minute averages for your last trip!


Optima battery and installation kit for Prius 2001 to 2003
Optima battery and installation kit for Prius 2001 to 2003

List price: US$201.59
Sale price including free shipping to the US US$186.00
 Toyota Prius 12 Volt Auxiliary Battery with install kit for 2004 + including Plug in
Toyota Prius 12 Volt Auxiliary Battery with install kit for 2004 + including Plug in

We recommend the direct replacement not this product.

List price: US$201.59
Sale price: US$186.00
Installation Kit for Optima battery in Prius
Installation Kit for Optima battery in Prius

For Years 2001-2003, 2004-2009 & 2010+

US$20.00


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