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!