Some plug-in hybrid batteries degrade much more slowly than others
- ADAC study of 28,500 PHEVs shows big differences between brands in battery aging rate.
- Mercedes packages age slowly; Mitsubishi degrades faster, even with low mileage.
- Before buying a used PHEV, it is recommended to always check the condition of the battery.
As in a fully electric vehicle, the battery in a plug-in hybrid (PHEV) loses its capacity to hold a charge over time. A new study shows which PHEV batteries stay healthy the longest – and which degrade faster.
This matters a lot because EV batteries are expensive. If the PHEV kit is faulty, it may be uneconomical to repair and may even cause the vehicle to be written off. You’d be having a very bad day if you bought a used PHEV only to find out that its battery is toast and you have to pay over $10,000 for a new one.
Germany’s ADAC, its largest auto club, analyzed battery health data from around 28,000 PHEVs. It found that while driving in electric mode and charging frequently will accelerate the loss of capacity in all vehicles, there is a huge difference between how models from different brands perform over time.
Mercedes-Benz PHEVs performed best. According to the analysis, they lose capacity very gradually over time. However, whether they were driven a lot or a little in electric mode had little impact on battery health. Overall, Mercedes PHEVs retained an average of almost 90% battery capacity over 200,000 kilometers.
(You can view charts detailing how each automaker’s PHEVs fared here.)
BMWs also show a gradual, predictable degradation curve, albeit steeper. And more depended on driving habits. At 200,000 kilometers, vehicles driven infrequently in electric mode retained more than 90% capacity, while vehicles driven predominantly in electric mode still had approximately 77% health. Cars driving in electric mode had an average of just over 85% capacity left. ADAC notes that the BMW data had many outliers.
Volvo and Volkswagen PHEVs see “normal” degradation, even though these vehicles run the greater part on pure electric power. Average drivers still had over 85% battery capacity at 200,000 km in both cases, and the data does not show significant drops.
However, the same cannot be said for Mitsubishi and Ford. Ford plug-in hybrids experienced steep degradation early in their lives, with heavy electric mode users witnessing a 10% drop in capacity before 25,000km. (Lack of data makes it difficult to draw conclusions about Fords with higher mileage.
Mitsubishis fared the worst.
“The data from Mitsubishi is particularly noteworthy: Many vehicles in the study show significant battery aging even after relatively low mileage, which then stabilizes somewhat,” ADAC said.
Even vehicles driven infrequently in EV mode lost more than 20% of battery capacity at 100,000 km and more than 25% at 200,000 km. Mitsubishi PHEVs that were driven in EV mode most often lost over 30% of their battery capacity within 200,000 km.
The battery is discharged from the PHEV.
Photo: ADAC
ADAC recommends that you always check the battery condition before purchasing a used PHEV. It indicates what values it considers good depending on the kilometers driven. At 31,000 miles (50,000 km) the battery should still have at least 92% of its original capacity, 88% at 100,000 km, 84% at 150,000 km and 80% at 124,000,000 km.
As with EVs, to maximize battery life, keep it between 20% and 80%. Frequent draining or full charging to 100% will accelerate degradation. In the few PHEVs that also support DC fast charging, using this feature can also potentially cause faster capacity loss.
Keeping a PHEV parked outdoors in extreme temperatures is also bad for the battery in the long term. The capacity of the battery can also be affected by leaving it parked and not in use for long periods of time, when the battery is completely discharged and then remains empty.
ADAC also says that if you plan to drive your PHEV mostly in electric mode, getting an all-electric vehicle is a better option. We’ll add that if you can’t commit to regularly charging a PHEV, just get a non-plug-in hybrid, which is better on gas and better for the planet than an internal combustion car. If you drive a PHEV and never plug it in, then it’s just a waste of fuel; it can burn more gas than the same car with the same engine but without electrification.
It’s worth noting that because PHEVs are still relatively new, some may still have an active battery warranty that should be transferable to the new owner if you buy used. For example, Toyota offers a 10-year or 150,000-mile warranty on its Prime models, while BMW, Hyundai and Ford offer 8-year or 100,000-mile warranties.