Prius Battery Icon Q&A

We get a lot of questions from Prius owners about the battery icon located on the display. Watching the colorful bars fill and drain within the battery gauge can cause reassurance or anxiety for some, so it’s important to know what’s happening and why.  While it cannot provide you with precise data, understanding how this dynamic icon works can help give you some insight on the health of your hybrid battery over time. 

*These questions pertain to the 2004-2009 Prius, but can generally apply to other Toyota hybrids with the exception of the icon colors.

Q: Why doesn’t my battery icon stay fully green while I’m driving?

A: In general the car will try to maintain six blue bars on the display. This corresponds to a state of charge (SOC) of around 60%. This “extra space” allows room for energy recovered during regenerative braking or hill decent that otherwise would have nowhere to go and would be wasted as heat. It is rare to see a full eight bars on a healthy battery unless descending a hill or mountain. Even when the battery icon appears to show a fully charged battery, its actually representing a SOC of about 80%. If your battery was allowed to fully charge on a regular basis it would eventually succumb to premature aging.

Q: I’ve noticed lately that my battery icon is rapidly fluctuating from eight green bars down to two red bars and back. Is this normal?

A: No! A rapid fluctuation indicates that hybrid battery failure may be in your near future. Under normal circumstances, you should notice the battery gauge slowly filling up to six blue bars while you drive and back down to a few bars while stopped or idling. If your battery icon is behaving erratically, you may also have noticed that your engine sounds louder and the car seems to be hesitating more often. There may even be warning lights and a loud fan sound from the rear seat area. For more information on what to do when this happens, check out our FAQ.

Q: My battery icon drops down to a couple red/purple bars sometimes. Is this normal?

 A: If the battery is dropping down to the last few bars, it is typically due to sitting at a long stop light or idling with the car on while the weather is hot. This graph will help explain in further deital:

If you’re at a stop and the air conditioner is running, a two to three minute period is typical for the battery to discharge from six bars to two bars.  From the graph you can see six bars corresponds to 55-65% SOC and two bars is 43-47% SOC.  If we take the middle value for each (60% and 45%) that’s only a drop of 15% when your car goes from six bars to two bars.  

A perfect brand new Prius battery has a capacity of 6500 milli-amp hours. Let’s assume that your older Prius may have a battery capacity of about 5500mAh. A 15% drop for a 5500mAh means it dropped only about 800mAh.  The air conditioner and all the car components on a hot day will easily use 15 amps of current from the hybrid battery; 15 amps will consume that 850mAh in only four minutes. 

Q: When I park my car at night it has around six blue bars, but in the morning, it only has a couple red bars. Is my battery failing?

A: Possibly. The battery seems like it may not be holding a charge and should be monitored closely. There can be other conditions (regenerative braking failure or poor engine power) which cause the battery to not charge properly, but its most likely that it will need to be replaced soon. You’ll likely get a slew of warning lights on the dash soon and its imperative that diagnostic codes be checked before the battery is replaced. I wouldn’t advise taking any long road trips in the meantime!

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P0A80-Replace Hybrid Battery

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Short answer: The diagnostic code P0A80 means that the hybrid battery is failing and needs to be replaced. Other codes may accompany P0A80 such as p3011, p3012, p3013 etc. Those codes refer to the individual block of modules (commonly called cells) that are causing the failure. If your hybrid has P0A80 and you need a replacement or some guidance please reach out to us.

Long Nerdy Answer: Toyota, Lexus and Nissan use a similar battery technology, but I will use the Toyota Prius for this example. The Prius traction battery utilizes 28 modules with six cells to each module. The modules are paired off into “blocks” of two.

The P0A80 will be present when the BMS (battery monitoring system) has detected a voltage difference of 20% or more between the battery blocks. Typically when the P0A80 code is present, one of the 28 modules has failed and more will fail soon if the battery isn’t replaced or properly refurbished. Some companies will replace just the failed module and send you on your way, only to experience another failure within a month or so. Just replacing one bad module is a band-aid repair on what will be a continuing headache, costing more time and money than just replacing the entire battery. In this situation, all the cells should be replaced with others that have been properly cycled, tested and have similar metrics.

Why did my battery fail?

Aging NiMH battery cells are subject to develop what is called a “memory effect.” The memory effect can happen if a battery is repeatedly charged before all of its stored energy is depleted. Hybrid vehicles are prone to shallow cycling because they typically stay within a 40-80% state of charge. This shallow cycling will eventually cause the formation of dendrites. Dendrites are tiny crystalline-like structures that grow on the separator plates inside the cells and eventually block the flow of electrons. In addition to the memory effect, an aging battery may also develop rising internal resistance which causes the battery to overheat and abnormal voltage drops to occur under load.

*Notice that the blue line indicates a module that has high internal resistance, but would have passed tests based on other metrics.
*Cell #23 had normal voltage and capacity, but a high internal resistance was evident under load.

In very rare circumstances the P0A80 code can be caused by corrosion on the voltage sensor harness/bus bars or the hybrid battery ECU. We have only seen this occur a few times in the thousands of batteries that we’ve come across. Typically when corrosion is causing a failure, other codes (P0A7F, P0A1F, P0AFA) will be the main DTC.  Fun fact: our refurbished batteries come with nickel plated bus bars to prevent corrosion!

Engine and braking issues:

Your hybrid battery utilizes a combination of engine power and the regenerative braking system to help it charge. If one of the systems fails, the battery will not charge properly. More information can be found here and here.

How do I know if my hybrid battery is failing?

Check out our FAQ!

C1256-Brake Actuator Failure

Hi everyone, we’ve been getting a lot of 2004-2009 Prius models coming into the shop lately with brake actuator failure. The brake actuator is one of the “Big 3” costly replacements in the Prius and I’d argue that it’s the most dangerous repair to ignore. 

The most common scenario we hear from our customers experiencing brake actuator failure is that some or all of the dash warning lights will appear, but no noticeable symptoms occur right away. If the warnings are ignored, a long persistent buzzer sound may come and go. The buzzer is letting you know that you need to stop driving immediately

What is a brake actuator and why is it failing?

The brake actuator/accumulator assembly consists of a high pressure motor, a pressure tank and valves that open and close to direct brake fluid to the wheels as needed.

 As time progresses small leaks in the system will cause the pressure in the tank to drop which results in the motor having to run more often to maintain the pressure. Most of our customers say they’ve had no indication that there was a problem with the brakes until the warning lights came on, but in our experience when the pump can be heard about every 15 seconds or so, it may be close to failing. You can actually hear the pump trying to pressurize while the car is in ready mode (foot off the brake). Here’s what a failure unit sounds like. 

If you already have warning lights on the dash its important to stop driving the car until a complete diagnostic is performed. The most common diagnostic codes for brake actuator failure is C1256 and C1391

When C1256 code is logged the car will enter a fail-safe mode which will disable antilock brakes, electronic braking force distribution and regenerative braking. (The first two are obvious safety concerns) but another area of issue is how this condition affects the hybrid battery. 

Normally, the engine and regenerative braking work together to charge the battery while driving, but when an ABS code is logged, the regen braking is disabled which can lead to an overly discharged hybrid battery. 

This is such a common problem that we often see actuator codes accompanied by battery failure codes. If you have both brake and battery codes, it is in your best interest to address the brakes first. 

If you have this code, your brake actuator will need to be replaced. Unfortunately, this is not an easy DIY repair and it can be quite costly. We no longer recommend using used or remanufactured parts for the replacement due to reliability concerns. 

At our shop we almost always try to offer a used or refurbished part option to our customers, but this is one area where safety overrides savings. Sites like eBay and craigslist are flooded with used actuators of varying conditions, but you’re honestly not going to know if those used parts work until after you’ve completed the lengthy installation. We only recommend purchasing a new OEM part directly from Toyota. 

Toyota formerly had a warranty enhancement for the 2004-2009 Prius brake actuator, but it expired in December of 2017. If you have a 2010-2015 Prius your actuator may be covered under warranty enhancement program ZJB which can be found here.

Our price: OEM part plus labor $1895 for the standard Gen II or Gen III models.

Toyota Price: $2500-$3800

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Common Inverter Problems

In short, the hybrid inverter/converter is responsible for boosting the voltage from the DC hybrid battery from 200v to 500v in order to create the three phase power needed for MG1 (motor generator) and MG2. The inverter also supports voltage lowering conversion from the hybrid battery to the 12v system. It’s a lot more complicated than what can be explained in a few sentences, but its safe to say the inverter/converter system is the brains of all voltage conversion within hybrid vehicles. 

Determining if the inverter has failed can be tricky, but we’ll try to break down the symptoms, diagnostic codes and some money saving strategies. 

2004-2009 Prius (additional information for newer model Prius further down)

I always like to begin with the most common scenarios that our customers experience. In this case its pretty simple- the Prius will turn on, have warning lights on the dash, but will not go into drive or reverse. In this situation, if you were to clear the warning lights via diagnostic code reader or by resetting the 12v battery the car will go into drive until it’s turned off and back on again. That’s one trick some Prius owners have used to limp the car to a mechanic. If driving the car in this condition isn’t something you’re comfortable doing, I recommend towing or calling a mobile mechanic. 

Part of the trouble with diagnosing inverter failure is that the most common diagnostic code to come up (P0AA6 voltage isolation fault) is also the same code for a voltage leak on the hybrid battery, transaxle and AC compressor. Thanks Toyota… 

If you’ve found code P0AA6 and are unsure where this voltage leak is coming from, you have to look for the sub codes (also called detail codes) to be sure. To get the sub codes you’ll need to be using a high quality code reader. Most of the cheaper ones will not find these codes. There will always be sub code 526 present, but that’s not what we are looking for. There should be a second sub code and it will be either 611, 612, 613 or 614. 

611: The voltage leak is in the AC compressor 

612: The leak is coming from the hybrid battery cells, relays or ECU

613: The leak is in the transaxle

614: The leak is in the hybrid inverter/converter

*If you’re able to get code 526, but not the second necessary sub code you may have to repeatedly turn the car on and off and re-check codes. 

Most mechanic shops and even the Toyota dealerships we’ve encountered do not even look at the sub codes. You have to advocate for yourself and ask them to recheck the codes. Dealerships will almost always suggest replacing the hybrid battery or ask permission for unnecessary further diagnostics. I will say that the hybrid battery is usually the culprit when P0AA6 is logged, but when you’re talking about replacements that cost into the thousands of dollars its best to be sure. 

Ok, now on to another scenario…

Inverter failure with code P0A08/P0A09

The Prius has warning lights on the dash and may or may not drive, but diagnostic code P0A08 (not to be confused with hybrid battery code P0A80) and/or P0A09 code are logged. These codes can mean that the hybrid inverter has failed OR the 12v battery is weak/improperly connected. I recommend checking that the connections to the 12v are properly secured and not horribly corroded either. Next, check the health of the 12v using this method. If you replace the 12v and it is still showing weak voltage, this means that the inverter is not properly supplying power to it and must be replaced. 

*Improperly jumping the 12v battery (putting jumper cables on backwards) can damage the inverter and cause P0A08 and P0A09.

Lastly, if your Prius stalls while driving, but letting it cool down allows the car to drive again, the inverter may be overheating. Check for inverter pump turbulence using this video. Code P0A93 suggests that the pump needs to be replaced. Codes P0A94 and P0A78 are much trickier, but typically require full inverter replacement. 

2010 and Up Prius

Toyota has issued a warranty enhancement for certain model 2010-2014 year Prius vehicles. If you’ve experienced stalling or have diagnostic codes P0A94, P324E, P3004 or P0A1A your Prius may be covered under this warranty. Best thing to do is call your local dealership with the codes and provide your VIN number. More information on this warranty enhancement can be found here.