Tesla Roadster Sport 2.5

by admin on 25 November 2010

Motor

The heart of the Roadster is its three-phase electric induction motor (induction means it uses copper rotor windings rather than permanent magnets).  It weighs approximately 32 kg (70 lb) can spin up to 14,000 rpm, delivers 288 bhp and 273 lbft of torque (295 lbft in the Sport which has a higher density hand-wound stator).  Maximum torque is produced from 0 rpm up to around 6000 rpm and it is typically 90% efficient (80% at peak power).  The motor is mated to a permanent single speed gearbox (no clutch required) which also includes the differential.

The table below is not supposed to be a specification-for-specification comparison between the Roadster motor and an internal combustion engine (ICE) but rather to point out the key differences.  An example of an engine with similar horsepower and torque performance as the Tesla Roadster Motor would be the 2006 BMW N54 as used in the 335i (3.0L twin-turbo straight-6 306 hp / 295 lbft), we have used some of its specs for this comparison (it's worth noting that the N54 has considerably more low-end torque than most).

  Tesla Roadster Motor Quality Petrol ICE
Weight * 32 kg (70 lb) 195 kg (430 lb)
Efficiency 80 - 90% 30%
Max torque band 0 - 6000 rpm 1500 - 5500 rpm
Complexity Simple Complex
Cooling Air Water
Maintenance None Oil changes, maintenance
Drive Direct single-speed Gearbox & clutch/tc
Maximum revs 14,000 rpm 7,000 rpm
Energy density Low (Lithium-Ion) Very High (Petrol)

* Excluding gearbox

Battery ('Energy Storage System' / ESS)

  • The Roadster uses 6,831 "18650 form-factor" Lithium-Ion batteries (first image above) in its ESS (Energy Storage System):
    • First a 'brick' is made up of 69 "18650 form-factory" batteries connected in parallel (3.7 V, approx. 152 Ah capacity)
    • Next a sheet is made up of 9 bricks connected in series (33.3 V / approx. 152 Ah capacity)
    • Finally the pack is made up of 11 sheets wired in series (366.3 V / approx 152 Ah capacity)
  • Each 18650 battery measures 18.6 mm in diameter, is 65.2 mm long and weighs 44 g
  • The 18650 battery cells deliver their optimum power when warm (over 20 C / 68 F)
  • The ESS is liquid cooled using a 50/50 mix of water and glycol (anti-freeze) (or warmed if needed)
  • Each battery cell has two fuses and each sheet has a main fuse
  • The energy density of the ESS is approximately 124 Wh/kg
  • The expected life of the ESS is seven years
  • To achieve the same 152 Ah capacity with typical car lead-acid batteries you would need 30 batteries weighing 900 kg

Range

The Roadster has the highest capacity battery of any commercially available plug-in electric car.  At 56 kWh it has almost twice the capacity of that found in the Nissan Leaf, and this results in much better range.  In normal driving conditions you should expect to see over 200 miles (320 km) from a charge, naturally actual range depends on several factors; driving style, traffic condition, range mode (Max range, Standard, Performance), ambient temperature (extremes of hot and cold will draw more power from the HVAC system which maintains the battery temperature) and accessory usage (air conditioning, headlights etc.).

As you can see from the graph above (data supplied by Tesla) the amount of power required (in Wh/mile) increases as the various 'loss components' increase in relation to speed, chief among these is wind resistance (aerodynamic loss), other losses include tires (mostly constant), ancillary (mostly constant) and drivetrain (proportional to speed).  If we then invert this data we can see that at a steady 20 mph in optimal conditions we should be able to achieve almost 400 miles on a fully charged battery and at a constant 60 mph we should get just over 200 miles range.  Click here to see a detailed break-down of the energy loss distribution (again, data supplied by Tesla, more info here).

As you drive the Roadster's computer gives you two different range figures; the "ideal range" which you could achieve driving as moderately as possible and the "estimated range" based on an analysis of the previous 40 miles (65 km) of driving.  You can also have the car display a live graph of power usage (kW) on the vehicle information touch-screen.  When driving on the highway I found myself actually trying to keep my kW usage as low as possible while maintaining my speed (which as I'm sure my friends would attest is very much unlike me), but it becomes almost a game to keep the graph trace as low as possible (perhaps this was Tesla's intention).  As I only had the Roadster for a brief time I wasn't able to do a full range test (I may add that to this review at a later date), however considering the different types of driving I subjected the car to the range readout always seemed accurate and even slightly conservative.  It's worth noting that there are also different charging modes which can affect range (see below).

Charging

Tesla offer a range of different charging options for the Roadster, depending on whether you have access to high power electrical outlets or you're on the road with just a normal consumer socket.  The table below (shamelessly copied from the Roadster owners manual) gives some insight into the different charging times and it's clear that you really need a 240V outlet to charge-from-flat overnight (although in reality it's unlikely most people will drain the entire battery everyday).  To maintain the maximum lifetime of the battery pack the Roadster never uses more than 90% of its capacity (although this isn't indicated) nor does it allow the battery to get below a 10% SOC (state of charge).

There are three different chargers available:

High Power wall Connector is a special wall-mounted unit ($1,950 excluding installation) which can provide up to 70A/240V (16.8 kW) Universal Mobile Connector can provide up to 40A/240V (9.6 kW) from a 240V outlet (or up to 15A from a 120V outlet), comes with a NEMA 14-50 connector, nine others available Spare Mobile Connector can provide up to 15A/120V (1.8 kW) from a 120V outlet

Charging matrix

Charge Specification (amps/voltage) Charge TIme in Standard mode Charge Time in Range mode
12A/120V 48 hours 54 hours
15A/120V 37 hours 41.5 hours
10A/240V 23.5 hours 26.5 hours
13A/240V 18 hours 20 hours
16A/240V 14.5 hours 16 hours
24A/240V 10 hours 11 hours
30A/240V 8 hours 9 hours
32A/240V 7.5 hours 8 hours
40A/240V 6.2 hours 7 hours
48A/240V 5.3 hours 6 hours
56A/240V 4.7 hours 5 hours
64A/240V 4.2 hours 4.7 hours
70A/240V 4 hours 4.5 hours

There are also four different charging options:

  • Storage: keeps the batteries at a medium charge to optimize their life if the car isn't going to be driven for some time)
  • Standard: charges the battery to a 90% state-of-charge
  • Range: charges the battery to its highest possible state-of-charge and also limits the vehicle power to 50% to optimize range *
  • Performance: charges the battery to deliver maximum power at achieve the fastest 0-60 times *

* Revert to Standard automatically after 72 hours or if the charging door is opened after the car has moved

For lots more on charging and range from an owners perspective see this excellent article on Tom Saxton's blog.