Is it safe for me to work on or replace my hybrid battery myself?

The hybrid drive battery in a second generation Prius (04’-09’) has a nominal DC output voltage of 202 Volts. This is more than enough to be deadly if you don’t know how to handle it. DC electricity is especially dangerous because of its tendency to cause a victim to freeze up and continue to be shocked. A good recommendation would be that if any of the warnings above don’t make complete sense to you, then you probably don’t have the proper amount of electrical knowledge to safely work with your battery. We highly recommend that if you do choose to replace your battery on your own that you invest in a repair guide, research everything before you do it, and follow all of the safety precautions while working with your hybrid battery. If you are completely unsure about how to replace a hybrid battery, it is recommended to allow a professional to perform the installation. There are some regular maintenance items (cleaning the battery cooling fan) that you can and should perform to prolong the life of your battery. If clients do hire us for installation, we encourage them to be present when we install their replacement battery so that we can show them how to safely do this.

How long should my original battery last?

There are a lot of variables that affect how long your original battery should last but from the research and repairs that we have performed this is what we have learned. Under “normal conditions” you should be able to get between 150,000 and 180,000 miles out of the original battery before it has to be replaced.

The most common reason why hybrid batteries tend to fail is because of heat. Batteries (or anything that conduct or store electricity) do not like to get hot. With that being said, you should never block the intake to your batteries cooling fan. If you like to let your German Sheppard that sheds ride in your back seat don’t be surprised if you start having problems with your battery sooner than normal. There is no filter for the cooling fan so all of that hair just gets stuck in there and restricts air flow. Don’t worry though, you don’t have ban your furry friends from the car. You may just want to consider having your cooling fan inspected and cleaned before it causes problems down the road.

Hybrid cars like to be driven regularly! Ok, it’s actually the battery that doesn’t like to sit for a long time. Your Prius has a Nickel Metal Hydride battery in it. These batteries naturally self-discharge and there isn’t anything that can be done stop it. You shouldn’t worry about a week or two but if you regularly leave your car sitting for a month or more it will most likely affect the life of the battery. For more information on self-discharge and battery imbalance see Why did my battery fail?

What are the symptoms of a failed hybrid battery?

The most common visual indication that your hybrid battery has failed is the notorious red triangle on your cars dash. It’s the same one that comes on if you open your door with the car in drive.

Other indications include the hybrid system trouble warning (top left on multi-function display, red box w/ a car w/ an exclamation point through it), several yellow lights including check engine lights, VSC, ABS, ect.

You may hear your gas engine (ICE) revving higher than normal. This is because your cars computer senses something wrong with the hybrid electric system so it is transferring all of the load the gas engine. This also results in low power, poor acceleration, and bad gas mileage.

You may hear the battery cooling fan running on high (back seat on the passenger side is the intake for the fan). Your car’s computer is trying to protect the battery from overheating and causing further damage.

If you take your car to a local auto parts store (like AutoZone or O’Reilly Auto Parts) they can read the trouble code or codes that are causing the check engine light to come on. The most common code to get when your battery fails is PA080 (Replace Hybrid Battery Pack), it’s not uncommon to get a weak battery block code which are P3011 thru P3024 depending on which part of the battery failed. Any other code that starts with PA0 is most likely pointing to a bad battery.

Signs that your battery may be getting ready to fail

If your battery had a good charge when you turned it off and after sitting over night it shows empty. This failure to hold a charge is normally followed by rapid charging and discharging on the energy monitor while driving. (the reason for this behavior is discussed later, see Why did my battery fail?)

Any poor power or mpg performance with greater than 150,000 miles on your car should be considered a warning that your battery is not performing properly.

Why did my battery fail?

Ultimately what has caused or will cause your battery to stop working will be one failed cell in 1 of the 28 modules in the pack. Here is how it works, one NiMH (nickel-metal hydride) cell has a voltage of 1.2 Volts. Figure 1 is an example of one module from a Prius battery. It is sealed unit that contains 6 (they are numbered) battery cells connected in series. When batteries are connected in series the capacity remains the same but their voltages add together. So when you add them up (1.2V x 6) you get a 7.2 Volt 6.5 Ah (Amp-Hour) module. Note: 6.5Ah is the amount of power that the module could hold when it was new.

Your battery is designed with 28 of these modules connected in series. Figure 2 shows a top view of a Prius battery with the safety cover removed. When you add all 28 modules together (7.2V x 28) you get a 201.6 Volt 6.5 Ah battery pack. (this is when it can be dangerous!)

As the hybrid system in your Prius charges and discharges the battery it (the battery) generates heat, a lot of heat! Toyota designed a very effective cooling system to remove this extra heat. A cooling fan draws in cabin air from an intake that is positioned on the passenger side of the car next to the back seat. If you have a Prius you should be very aware of it because it is important to keep that intake free from any blockages. That cabin air is fed thru some ducting and into the top of the battery casing. The air then flows thru the designed spaces between the modules and then out of the bottom of the battery case thru another piece of ducting that carries the heat out of the car. See Figure 3.

As hard as the engineers tried to design this system to provide even cooling to the entire battery, it just didn’t happen. Because of the way the modules are stacked together the sections on the end naturally stay cooler than those in the middle. All batteries depend on chemical reactions to work properly. Every battery has an optimal temperature for those reactions to take place. If you heat or cool a battery too far outside of its designed temperature band the chemical reactions slow down. The cells in center of the battery spent their life in harsher conditions than those on the outside so they are weaker and have lost more of their original capacity than the rest. Figure 4 shows a graphical representation of a batteries typical module capacity when we first test it.

This is the capacity imbalance that will result in the eventual failure of a hybrid battery pack. The computer system in a hybrid vehicle monitors the voltage in each group of modules as they charge and discharge. The inner modules will indicate full and empty very quickly because their capacity is so low, while the outer modules are actually doing very little. This heavy loading on the weakest modules is what eventually gets them to where they have almost no cycling range left in them. (This is where the rapid charging and discharging symptom comes from, it’s not really the whole battery that’s fully charging and discharging it’s just the weakest modules). Once that happens all it takes is parking the car over night with a low charge on the battery and the lowest cell in one of the weak modules will self-discharge enough in a short amount of time and cause what is called polarity reversal. It ruins that cell permanently and because the cell is in a sealed unit it renders the module bad. So if you remember a module is six 1.2V cells for a total of 7.2 Volts. It is now five 1.2V cells resulting in a 6 Volt module. Toyota designed the computer to prevent battery use if the voltage difference between the lowest and highest modules in the pack exceeded 1.2 Volts, in other words the car knows that a cell has failed and it will not work and has probably triggered a PA080 code and some low block voltage trouble code. The cooling fan may run on high to ensure that the battery gets maximum cooling while it is this malfunctioning state.

What are my repair options if my battery fails?

The first option is to take your car to a Toyota dealership and have them diagnose and replace your battery. This gets you a new battery installed and normally only includes a 12 month warranty. The battery should last as long as the one that came in your car new, so that means you should be good out to the 350K-400K mile mark. The cost for this repair is normally in the $3,500 range.

Have Minnesota Hybrid Batteries diagnose and replace your battery. You will be getting a fully remanufactured battery that has been reconditioned and restored to 100% of its original capacity. You will recover all of the lost fuel mileage that was being caused by your old battery being weak. Our batteries come with a full 12 month replacement guarantee and will last at least 100K miles under normal conditions. That means you will be good out to 250K-300K miles depending on how many miles your car had when you got your new battery. The cost for this repair is $850 which includes all taxes and fees and same day mobile installation at your location. For more information about pricing, visit our pricing list page.

Find a used low mileage battery at a salvage yard and install it yourself or have it installed at a repair shop. Salvage yards know how much it costs to have a battery replaced so they get a premium for their used batteries. Expect to pay somewhere between $900 and $1600 for a used battery from one of these places. They most likely won’t be honest about the year or the mileage of the vehicle that the battery came out of. They won’t let you see the vehicle that it came out of and they will not give you a written guarantee that the battery will work (all electronics sales are final, look for it, it’s normally a sign on the wall behind the counter). We have sourced many batteries from salvage yards and only about 20% of them would have actually worked if they had been installed. This all goes back to not letting batteries sit, when a car is wrecked it can be several months before the battery is put up for sale, (wrecker service, insurance companies, vehicle auctions, salvage yard, disassembly, inventorying, advertising) all of this takes a long time. If you can find a shop to install your “maybe good” battery it will cost you another $300 and they will not guarantee it because it wasn’t their battery. So you paid $1200 for a battery with a lot of unknowns and no guarantee.