It's not how big your battery pack is, nor what you do with it that counts. Certain cars can lengthen your total driving range.
The rated electric driving range is simply the range available per charge. The more interesting figure is the total driving range in a day. This is how far you can drive accounting for recharging sessions throughout the day. Put another way, by how much can the range be extended by available recharging or refueling methods.
Total Driving Range: Let's try to define this a little better. One definition is "The theoretical maximum distance drivable in 24 hours with optimal driving and recharging." But an extremely small number of people do anything like this. Instead, 12 hours driving per day is typical for road trips.
Is this theoretical maximum driving range useful for you in your life? Probably not, but it's a way to compare vehicles. You need to know which electric car handles your long trips, your weekend trips to the mountains, or to Grandma's house, or whatever it is you need.
"Extended Range EV" is the phrase GM invented for the Chevy Volt. The Volt is billed as being more than an EV, because it has "extended range". Meaning, the gasoline engine extends the vehicle range.
There's a semantic thing to quibble about. The Volt is an electric car in the sense that it can be plugged in to recharge the battery pack. For a long time the quip was "it’s not electric if you can’t plug it in", and by that measure the Volt is an EV. But there is also a well defined phrase, "plug-in hybrid", describing a hybrid vehicle (two power sources working together to drive the car) which can be plugged in to recharge the battery pack. The Volt is a Plug-in Hybrid EV (PHEV), and it even says so on the EPA sticker. To the extent the "Extended Range Electric Vehicle" has any meaning, the Volt is also an EREV.
Let's not get lost in this semantic thing. The Volt, and other PHEV's, are fine cars and the gasoline engine is a pragmatic solution. The gasoline engine range extender means it's not as "pure" an electric vehicle as one that only runs on battery power. But, it also demonstrates the value of quickly replenishing the on-board energy storage for longer total driving range.
There's two general ways to extend total driving range with an electric car
Some say that stopping to charge does not qualify as a "range extender" system. That a range extender system must add range while the car is driving. That's another semantic quibble that could detour us into a verbal swamp. Isn't the "Total Driving Range" the measurement we need to focus on?
Portable towable battery packs: This is a design idea some have floated, to have a battery pack on a small trailer towed behind a car. One could easily rent the trailer for long trips, connecting it to a special charging port that allows the car to drive while the addon pack is plugged in. Battery trailer exchange stations could be located at stops along highways, similar to the propane tank kiosks seen at gasoline stations today.
Fuel Cell: Fuel cell vehicles do have an all electric drive train, and a small battery pack to hold electricity. Hence, it’s not that much of a stretch of terminology to call the fuel cell a range extender. There’s even some work to design fuel cell vehicles with larger battery packs where the fuel cell is used as a portable range supplement.
Gasoline Engine range extender: These vehicles are what we normally use the phrase “range extender” to describe. The Chevy Volt, Fisker Karma, Via Motors VTRUX line all have explicitly used the phrase “extended range electric vehicle.” BMW does not use this phrase to describe the BMW i3 REX, instead REX is an acronym Range EXtension.
Solar panels on the roof: Many time this is asked, why don't "they" put solar panels on the roof? The problem with that is there's simply not enough space on a car roof to carry enough solar panels to generate enough electricity to make any significant impact. See The number of solar panels required to power an electric car for a deeper discussion.
DC Fast Charging: CHAdeMO or Combo Charging System or Supercharger can all quickly add range to an electric car. DC Fast Charging adds between 150-300 miles of range per hour of charging. That means about an hour of recharging after 4-5 hours of driving. See When will electric cars charge faster than it takes to fill-up an equivalent gasoline car?
Battery swapping range extender: This concept is way older than Better Place and Tesla Motors, both of whom have implemented modern roboticized battery swapping systems. A manual battery swap system was used for electric taxicabs in New York City around 1900. With a well designed station the battery exchange can add range at a rate similar to refilling a gasoline tank. However, modern battery swapping schemes have all failed to gain traction in the marketplace. See Wouldn't battery swapping be preferable over waiting to recharge?