Mercedes-Benz S500 Plug-in Hybrid review

The Mercedes-Benz S500 Plug-in Hybrid is the first of ten new petrol- and diesel-electric powered models confirmed for launch by the German car maker before the end of 2017.

Planned for Australian delivery by the end of 2015 at a price officials suggest will be close to the existing $309,530 (plus on-roads) for the regular S500, it succeeds the earlier diesel-electric S300 BlueTEC Hybrid and petrol-electric S400 Hybrid but adds one vital new element, namely the ability of its battery to be charged via either standard household mains electricity or a high voltage electrical system – rather than simply by recuperated means or on the go via the petrol engine.

This allows the S500 Plug-in Hybrid to make use of a much larger lithium ion battery than its less advanced predecessors, which in turn provides its electric motor with far greater energy reserves and, crucially, a more than ten-fold increase in range when running in electric mode.

On a feathered throttle on flat roads and in optimal weather conditions, it is claimed to be able to travel up to 33km before its battery is depleted and the petrol engine is automatically fired up.   

At the heart of the plush luxury sedan is a newly developed plug-in hybrid system described as boasting modular properties that will allow it to be tailored for use with varying outputs, combustion engines and electric motors and in a wide range of upcoming alternative drive Mercedes-Benz models.

In the S500 Plug-in Hybrid, the new system employs a turbocharged 3.0-litre V6 direct injection petrol engine that develops 245kW at 5250rpm and 480Nm of torque between 1600 and 4000rpm. The electric motor, meanwhile, is set within the forward section of the gearbox, and generates 85kW along with 340Nm.

Together, the petrol engine and electric motor provide a combined system output of 325kW and 650Nm. This gives the S500 Plug-in Hybrid greater reserves than its Stuttgart home town rival Porsche's Panamera S E-Hybrid, whose supercharged 3.0-litre petrol engine and electric motor deliver a combined 306kW and 590Nm.

The combined output is channeled through a specially-adapted version of Mercedes-Benz's in-house produced seven-speed automatic gearbox to the rear wheels in any one of the four different driving modes: Hybrid, E-mode, E-save and Charge. Additionally, the gearbox offers three separate modes: E (Efficiency), S (Sport) and E (Economy).

As an incentive to prospective buyers, Mercedes-Benz is offering a six year/100,000km performance guarantee for the S500 Plug-in Hybrid's 8.7kWh battery, which can be charged in a claimed two hours on an optional 400 volt wallbox or four hours on a regular 240 volt system.

More than anything else, though, it is the battery which adds to the weight of the new car. At 2140kg, it tips the scales 145kg above the more traditional turbocharged 4.7-litre V8-powered S500 in long wheelbase guise.

Whether gliding around city streets or cruising out on the open road, the S500 Plug-in Hybrid is a superbly serene and relaxing way to travel.

The advanced driveline operates with Swiss watch-like precision and whisper quiet properties in all modes. The transition between pure electric running and the engagement of the petrol engine as it switches into hybrid state is virtually imperceptible.

The combination of petrol power and electric propulsion also provide the plush leather lined luxury sedan with a broad and entertaining range of performance qualities. In electric mode it provides pleasingly urgent step off and flexible properties at speeds up to a limited 140km/h.

There are also sufficient reserves on offer in petrol-electric hybrid mode to haul it along with an impressive turn of pace and compelling high speed vigour when the conditions permit.  

Despite the weight burden brought on by the battery and all its various ancillaries, Mercedes-Benz says the S500 Plug-in Hybrid is capable of accelerating from 0 to 100km/h in 5.2 seconds and reaching a top speed limited to 250km/h. By comparison, the 45kg lighter Porsche Panamera S E-Hybrid offers respective figures of 5.5sec and 270km/h.

Official combined cycle economy on the European test cycle is put at a remarkable 2.8L/100km – a figure that corresponds to average CO2 emissions of just 65g/km.

Mercedes-Benz is realistic enough to acknowledge few motorists, if anyone, will be able to replicate this in everyday driving conditions. Although with a fully charged battery and resolutely light throttle loads you can manage to extract some amazing economy figures out of it, even over extended distances.

I got close to the prescribed consumption figure with an average of 3.2L/100km/h over a 77km route, although 22 km of it was achieved on electric power alone and it was all at a fairly moderate speed over relatively flat terrain. For the record, the Porsche Panamera S E-Hybrid is claimed to deliver 3.1L/100km and 71g/km.

It may offer a bewildering choice of driving modes but it is not a difficult car to operate. You can jump in and drive off as in any of Mercedes-Benz's more traditional petrol or diesel flagship sedan models without giving it so much as a second thought. But to really get the best out of the S500 Plug-in Hybrid you need to become familiar with its four main driving modes – a process which takes time and persistence.

Hybrid combines the petrol engine and electric. E-mode offers pure electric running until the battery charge is depleted. E-save allows you to preserve the energy within the battery by favouring the petrol engine until another mode is chosen and allowing the stored energy to be used for pure electric running later on, when entering congested city centres, for example. Charge employs the petrol engine to top up the battery on the run.

At one point during our first drive of the S500 Plug-in Hybrid's digital instrument display indicated the battery charge had dipped below 25 per cent, suggesting its electric range was limited to under 10km. When I placed it into Charge mode, the energy store was replenished to 85 per cent within 25 minutes, restoring its electric range to almost 30km.

Happily, the added weight hasn't affected the ride. Riding on standard 18-inch wheels shod with low rolling resistance tyres – 245/50 at the front and the 275/45 at the rear – the new Mercedes-Benz provides the same cosseting qualities as other S-Class models

A neat feature is the so-called haptic accelerator. It operates in combination with the stereo camera mounted within the windscreen, providing an impulse within the throttle to signal you are too close to the car ahead and can recuperate energy by lifting off – at which the S-Class automatically switches into a freewheeling state with the petrol engine and electric motor disengaged from the gearbox.

As we noted on the earlier S300 BlueTEC Hybrid and S400 Hybrid, the action of the brake pedal is compromised by the need to recuperate energy during braking, lacking for uniform firmness throughout its travel.     

Unlike the battery of the S300 BlueTec and S400 Hybrid which reside underneath the bonnet, the battery used by the S500 Plug-in Hybrid is mounted within the forward section of the boot. This robs 95 litres of luggage capacity, reducing it to a total of 395 litres. Practicality takes a hit, then.

Apart from these criticisms, the S500 Plug-in Hybrid is a very convincing proposition, offering a truly impressive electric range and stout combined petrol-electric performance together with outstanding refinement and the sort of economy you might expect from a compact hatchback rather than a fully loaded luxury sedan.

It is limousine you can drive with a clear conscience – albeit one that is burdened with greater weight than petrol and diesel propelled siblings, limited on luggage space, requires a charging infrastructure to fully realise its potential and, with that eye watering price tag, only ever likely to sell in limited numbers in Australia.

2015 Mercedes-Benz S500 Plug-In Hybrid specifications

Price: TBA

Engine: 3.0-litre twin-turbo V6 petrol/electric

Power: 325kW

Torque: 650Nm

Transmission: 7-spd automatic, RWD

Consumption: 2.8L/100km