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Diagnosing hybrid fuel economy complaints

Monday, December 26, 2016 - 09:00
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Is it worth purchasing a hybrid vehicle to save money on fuel? Maybe yes, maybe no; you be the judge. Two reasons many hybrid vehicle owners state for purchasing a hybrid vehicle are better miles per gallon, and they want to do their part in helping make the environment cleaner. With that being said, it would be great if all the hybrid owners properly maintained the vehicle and used the correct tires and parts for the vehicle. Working on hybrid vehicles for years, I have noticed that most of the hybrid vehicle owners do not properly maintain their vehicle and therefore do not achieve the advertised miles per gallon. It’s such a problem that some state emission programs are starting to look into the EPA rating vs. the actual hybrid vehicle miles per gallon. Since every new vehicle is certified by the federal government for emissions and miles per gallon, there is an issue when hybrid vehicles do not achieve the posted numbers. Many of the hybrid vehicles I have worked on do not average anywhere near the suggested EPA ratings. One such example is a 2008 Toyota Prius (Fig. 1) that only averages 30.5 when it should be averaging about 46 mpg. That’s a very large difference from what the vehicle owner is actually getting compared to the EPA rating. Think about how much more pollution the vehicle is emitting now compared to when it was certified.

Figure 1

Hybrids need maintenance, too

We know that all vehicles need routine maintenance and hybrid vehicles are no exception, especially as they get older and the high voltage (HV) battery starts to deteriorate. Vehicle maintenance from a simple oil change to checking tire pressure to a tune up can all affect the performance and mileage of the vehicle. With hybrid vehicles there is also HV battery maintenance. Just like us, a battery needs a little exercise and cannot live forever. It is important to check the status of the HV battery for proper vehicle operation. If we go back to the first hybrid that hit the streets, we would be looking at a Honda Insight that had a 3-cylinder motor. The Insight was dependent on the Integrated Motor Assist (IMA) electric motor that was sandwiched between the engine and the transmission to help do its part in propelling the vehicle. If the HV battery on the Insight was depleted, it became very difficult to drive up steep inclines or maintain a steady speed on a hilly road. Since many of the original Insights are more than 16 years old now, their HV batteries have caused problems such as low power complaints to poor miles per gallon. Just like everything else on the vehicle, the HV battery sometimes needs maintenance. What happens over time to the HV battery is that it becomes unbalanced and the cells begin to lose their capacity. Take notice of the HV battery level when driving a hybrid vehicle that has some miles on it or one that has been sitting around for a while. You will notice that the HV battery pack indicator seems to drain quickly. When this occurs, there will be a noticeable power loss and a drop in fuel economy. Many vehicle owners do not notice these issues since the power loss and the lower fuel mileage numbers are gradual, so they just keep on driving. If the vehicle owner checked the mpg as seen in Fig. 1, where it is 19 mpg less than what the vehicle should be averaging, the owner would most likely bring the vehicle in for a diagnosis.

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Shared electrical problems

Most of the time the HV battery has problems with voltage drop (VD) at the bus bar connections to the stud. The buss bars (Fig. 2) get corroded and start to resemble the Statute of Liberty. In many cases, HV battery maintenance does the trick in getting the HV battery back up to its normal state — that is if there is no battery cell damage. There are certain factors with HV batteries that involve age, discharge and charging cycles. In the case of the HV battery, the life span depends on temperature, vehicle use, discharge and charge cycles, among other factors. If the HV battery cell or module is dead, it cannot be brought back to life. We always examine the causes of the HV battery deterioration, which can be from heat, time or air flow, to name a few.

Figure 2
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