Ford calls C-MAX Energi a "hybrid-plus" because it not only operates just like C-MAX Hybrid, but also features an external charge port and larger lithium-ion battery pack so owners can plug in the vehicles to fully charge the battery using either a standard 120-volt outlet or the available 240-volt charge station.

ENGADGET: Ford C-Max Energi pricing: $29,995 after a federal tax credit, available this fall

Focus Electric, Fusion Energi plug-in hybrid and C-MAX Energi plug-in hybrid all have an LED charge port that displays a circle of light directly related to battery life.

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C-MAX Energi operates just like a traditional hybrid with two key differences - owners must plug in the vehicle using its charge port and C-MAX Energi has a larger battery pack.

ENGADGET: Ford's 2013 C-Max Energi rolls out 95 MPGe, arrives later this fall

We can imagine the day when you'll be able to charge the battery on your plug-in hybrid car at night, because your smart meter reminded you that nighttime electricity is cheapest.

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Electric cars can also only go 70 to 100 miles on a single charge before the driver has to plug-in and power up again.

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Because owners plug in to charge the advanced lithium-ion battery in C-MAX Energi, the vehicle is able to achieve increased electric mode and total range.

ENGADGET: Ford's 2013 C-Max Energi rolls out 95 MPGe, arrives later this fall

But in vehicles that run for long periods on electricity only, like plug-in hybrids and all-electrics, engineers need to fully charge batteries to maximize driving range, the drawback to electric vehicles.

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The Tesla Model S is a plug-in electric luxury sedan than can travel between 160 and 300 miles on a single charge, depending on the type of battery chosen.

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