Question: What vehicle was introduced in 1998 by Daimler AG as a City car which battery electric variant's field testing started in London in 2007?

Context paragraphs:
State of charge: State of charge (SoC) is the equivalent of a fuel gauge for the battery pack in a battery electric vehicle (BEV), hybrid vehicle (HV), or plug-in hybrid electric vehicle (PHEV).  The units of SoC are percentage points (0% = empty; 100% = full).  An alternate form of the same measure is the depth of discharge (DoD), the inverse of SoC (100% = empty; 0% = full).  SoC is normally used when discussing the current state of a battery in use, while DoD is most often seen when discussing the lifetime of the battery after repeated use.

VLF cable testing: VLF cable testing is a technique for testing of medium and high voltage (MV and HV) cables.  VLF systems are advantageous in that they can be manufactured to be small and lightweight; making them useful – especially for field testing where transport and space can be issues.  Traditionally DC voltage sources have been used for field testing of cables but these sources have been shown to be ineffective at withstand testing for most kinds of damage.  DC diagnostics have also been shown to yield ambiguous results and are generally considered and ineffective for MV and HV power cables.  Because the inherent capacitance of a power cable needs to be charged when energised, system frequency voltage sources are much larger, heavier and more expensive than their lower-frequency alternatives.

Battery balancing: Battery balancing and battery redistribution refer to techniques that maximize the capacity of a battery pack with multiple cells (usually in series) to make all of capacity available for use and increase each cell's longevity.  A battery balancer or battery regulator is a device in a battery pack that performs battery balancing.  Balancers are often found in lithium-ion battery packs for cell phones and laptop computers.  They can also be found in battery electric vehicle battery packs.

Smart Fortwo: The Smart Fortwo is a rear-engine, rear-wheel-drive, 2-seater hatchback Microcar(or City car) manufactured and marketed by the Smart division of Daimler AG, introduced in 1998, now in its third generation.  Marketed in 46 countries worldwide, Fortwo production had surpassed 1.7 million by early 2015.

Electric-vehicle battery: An electric-vehicle battery (EVB) or traction battery is a battery used to power the propulsion of battery electric vehicles (BEVs).  Vehicle batteries are usually a secondary (rechargeable) battery.  Traction batteries are used in forklifts, electric Golf carts, riding floor scrubbers, electric motorcycles, full-size electric cars, trucks, vans, and other electric vehicles.

Smart electric drive: The Smart Fortwo electric drive, or smart ed, is a battery electric vehicle variant of the Smart Fortwo city car, formerly marketed as the Smart Fortwo EV.  Field testing began in London with 100 units in 2007.  The second generation was introduced in 2009 and was available in 18 markets around the world for leasing or through the Car2Go carsharing service in selected cities, with over 2,300 units delivered.  A near production version was unveiled at the September 2011 Frankfurt Motor Show.

Battery electric vehicle: A battery electric vehicle (BEV), battery-only electric vehicle (BOEV), Full Electric Vehicle (FEV) or all-electric vehicle is a type of electric vehicle (EV) that uses chemical energy stored in rechargeable battery packs.  BEVs use electric motors and motor controllers instead of internal combustion engines (ICEs) for propulsion.  They derive all power from battery packs and thus have no internal combustion engine, fuel cell, or fuel tank.  BEVs include motorcycles, bicycles, scooters, skateboards, rail cars, watercraft, forklifts, buses, trucks and cars.

Plug-in electric vehicle: A plug-in electric vehicle (PEV) is any motor vehicle that can be recharged from an external source of electricity, such as wall sockets, and the electricity stored in the rechargeable battery packs drives or contributes to drive the wheels.  PEV is a subset of electric vehicles that includes all-electric or battery electric vehicles (BEVs), plug-in hybrid vehicles (PHEVs), and electric vehicle conversions of hybrid electric vehicles and conventional internal combustion engine vehicles.  In China, plug-in electric vehicles are called new energy vehicles (NEVs).

Daimler AG: Daimler AG (] ) is a German multinational automotive corporation.  Daimler AG is headquartered in Stuttgart, Baden-Württemberg, Germany.  As of 2014, Daimler owns or has shares in a number of car, bus, truck and motorcycle brands including Mercedes-Benz, Mercedes-AMG, Smart Automobile, Detroit Diesel, Freightliner, Western Star, Thomas Built Buses, Setra, BharatBenz, Mitsubishi Fuso, MV Agusta as well as shares in Denza, KAMAZ, Beijing Automotive Group.  The luxury Maybach brand was terminated at the end of 2012, but revived in April 2015 as "Mercedes-Maybach" versions of the Mercedes-Benz S-Class and G-Class.  In 2016 Daimler sold 3.0 million vehicles.  By unit sales, Daimler is the thirteenth-largest car manufacturer and is the largest truck manufacturer in the world.  In addition to automobiles, Daimler manufactures buses and provides financial services through its Daimler Financial Services arm.  The company is a component of the Euro Stoxx 50 stock market index.

Honda EV Plus: The Honda EV Plus was the first battery electric vehicle from a major automaker with non-lead acid batteries.  Roughly 340 EV Plus models were produced and released.  The EV Plus was taken out of production in 1999 when Honda announced the release of its first hybrid electric vehicle, the Honda Insight.  The EV Plus was made to learn about advanced battery use in an electric car and to meet California Air Resources Board requirements for zero-emission vehicles, the same as the new General Motors EV1.  It served as a test bed for the pancake-style motor, electronic control unit, power control unit and the Nickel–metal hydride battery (NiMH) later used in Honda hybrids and developed further in the first Honda FCX Fuel Cell Vehicles made from returned (decommissioned) EV Plus chassis.

Supporting paragraphs: Smart electric drive Smart Fortwo
Answer: Smart Fortwo