Plug-in your hybrid
After four years of driving it, I traded in my Volvo XC90 Recharge plug-in hybrid-electric vehicle (PHEV), for the fully electric equivalent, Volvo EX90.
Here are some thoughts I’ve had about the PHEV driving experience.
Drive train types
Plug-in hybrids exist on a spectrum of electrification:
In my mind, the points on the spectrum are:
- Internal combustion
- a small, high-amperage automotive battery is used to start the internal combustion engine once per trip
- Auto-stop
- a slightly larger battery allows the engine to stop and start more frequently, avoiding idling even in city traffic
- Mild hybrid
- auto-stop, plus:
- a small electric motor helps with acceleration, while the engine also provides power
- Full hybrid
- a slightly larger electric motor is used entirely for initial acceleration
- sometimes, up to a mile of EV driving is possible
- usually recharged via running the motor in reverse (“regenerative braking”)
- Plug-in hybrid
- full hybrid, plus:
- an onboard charger allows the batteries to be charged externally
- the electric motor can be used fully for many miles of 100% EV driving
- the engine stays off whenever possible (going downhill, decelerating), unless…
- the battery is empty
- you command high acceleration
- manual “Battery Hold” or other combustion modes
- automatic AWD mode, due to a loss of traction
- manual “Charge” mode to burn gasoline to charge the battery
- Full EV
- delete the engine, belts, fluids, radiator, and exhaust
- big battery
- DC Fast Charging, for road trips or when you can’t charge at home or work
- you unlock the “where do I charge, and for how long” game, instead!
There are more distinctions (parallel vs. series vs. split hybrid; electrification of accessories; regenerative braking vs. alternator), but I think the “amount of electrification” is the big axis.
Fuel cost
Let’s use Car and Driver’s 2022 Volvo XC90 Recharge numbers for this example.
It has an 18 gallon gasoline tank, and a 14.9 kWh battery pack.
The engine gets 25 mpg city and 27 mpg highway (450+ miles of range!).
The electric motor has a 36 mile range, or 36÷14.9 ≈ 2.4 mi./kWh.
Right now, on my residential electric rate plan, home grid power is a flat 11¢/kWh.
Gasoline is $2.45/gal.
So the cost per mile of electric is 11¢/kWh ÷ 2.4 mi./kWh ≈ 4.6¢/mi.
The cost per mile of gasoline on the highway is $2.45/gal ÷ 27 mi./gal ≈ 9.1¢/mi.
Aside: In the city, it’s slightly worse, at $2.45/gal ÷ 25 mi./gal ≈ 9.8¢/mi.
This means that you should avoid spending more than 9.8¢/mi * 2.4mi/kWh ≈ 24¢/kWh to recharge this car.
Fuel cost optimization minigame
At either end of the spectrum, with an electric vehicle or an internal combustion vehicle, the car only has one fuel source.
With most types of hybrids, there isn’t a fuel source decision to make. On some, like the Toyota Prius, there is an “EV Mode” button that lets you fully drain the battery if you know you’re going to only drive a few thousand feet (re-parking the car, perhaps?)
With a plug-in hybrid, you unlock a dull minigame called “which fuel should I use right now?”
To be clear, you don’t have to play this game. If you don’t, you will spend (at most) 5.2¢ extra per mile. But, if you drive a plugin hybrid, I assume you care about optimizing your cost of driving.
From the costs we calculated above, we can infer two rules to the minigame:
- Prefer electric to gasoline: it’s significantly cheaper.
- If you must burn gasoline, prefer to burn it at coasting speeds rather than in stop-and-go traffic.
The default “Hybrid” drive mode in this car prefers electric driving, so as long as you have a trip less than 36 miles, you win!
The minigame only becomes interesting—in a dull way—when you are doing a lot of driving in one trip. The optimal strategy is to switch to “battery hold” mode whenever the car is going to cruise for a while, unless you have enough battery to complete the remainder of the trip. If charging isn’t available at your final destination, remember that “the trip” includes the return journey.
(I learned that later models of this car, when using the built-in Google Maps integration, will automatically play this minigame for you!)
When to use Charge mode?
This is a double energy conversion, so it’s going to be twice as inefficient: you run your gasoline engine to move the car, and then re-convert some of that kinetic energy to recharge the battery, to be converted back into mechanical energy again later. You should just use the gasoline to move the car directly.
Exceptions:
- If there’s an emergency, like a snowstorm, and you can park your car outside, it’s a convenient way to recharge what amounts to a gigantic portable battery.
- I’ve heard rumors that Volvo added this functionality to make the car follow “zero emission zone” rules: drivers would be ZEZ-compliant by making the car a ZEV, and one way to do that is to run it in “Charge” mode on the highway on the drive into the city. (Still cheaper to charge off of the grid, and just use “Battery Hold”, but if you forgot…)
Charging speed
A U.S. standard power outlet is 15A @ 120V AC = 1.8kW max. power.
A long-duration load like EVSE is limited to 80% of this capacity, or 1.44kW.
This means the minimum charge time on a standard outlet is 14.9 kWh ÷ 1.44kW = 10.4 hr. That’s a charge speed of 1.44kWh/hr. * 2.4 mi./kWh ≈ 3.4 m.p.h.
The onboard charger is capable of charging at up to 16A, which multiplied by the maximum input voltage of 240V AC is a maximum charge rate of 3.84kW.
Charge time then drops to 14.9 kWh ÷ 3.84kW = 3.88 hr. In terms of speed, that’s 3.84kWh/hr. * 2.4 mi./kWh ≈ 9.2 m.p.h.
Aside: In practice, I observed more like 15hr and 5.5hr.
For comparison, gasoline pumps in the US are limited to 10 gal./min.
Refuel time is 18 gal ÷ 10 gal./min. = 1.8 min. 10gal./min. * 25 mi./gal. * 60 min./hr. = 15000 m.p.h.
Not mentioned: emissions
Tailpipe emissions vs. power grid cleanliness are a complicated topic, as is the fact that PHEVs are generally heavier and have more embodied material in them, so I won’t try to calculate anything about these.
Total cost of ownership
The MSRP of the 2022 XC90 Recharge is $65,895 - $73,695. The MSRP of the 2022 XC90 is $51,995 - $66,195.
The “low end” trim of the XC90 Recharge is about equivalent to the mid-to-high end trim of the non-EV version, making the difference in price something like $5000.
The average American drives about 14,000 miles per year. If you can convert 100% of your driving to electric, you save (9.1¢ - 4.6¢) = 4.5¢/mi.
Multiplied by 14,000 miles, that’s $630/yr saved. So you have to own the PHEV for about $5000 / $630/yr. ≈ 8 years, best case, to see an ROI, strictly on fuel cost savings.
In practice, there are also significant maintenance costs associated with internal combustion engines, making the calculation lean a little more toward the PHEV. Regenerative braking means the brake pads are worn less. EV mode means less wear on the engine, belts, and transmission. So, it’s complicated, but I believe the annual maintenance costs can be slightly lower for a PHEV that spends most of its time in battery mode.
The resale value of each car also trends differently, but I couldn’t begin to guess how much.
Summary
I can’t say I made a positive ROI with this car, but it was my first electric vehicle of any type, so I am glad to have been able to gain practical experience with this new technology.