Why aren’t we all driving EVs then?
Historically, mainly because of range and performance. The only electric vehicle most people will have come across is the milk float. This job is an ideal one for ‘traditional’ electric drive as the range required is relatively low, the speed required is also low but the load requirement is quite high. This suits electric drive perfectly - hence the popularity of the electric milk float - in urban areas at least. Other successful examples of electric drive vehicles are:- forklift trucks, golf carts and campus/airport service vehicles.
The main problem when you start thinking about using electric drive for conventional cars is the fact that the drag on the vehicle (which the power of the motor or engine has to overcome to accelerate or maintain speed) is proportional to 3 main variables:-
1/ the frontal area of the vehicle (i.e. the cross sectional area of the vehicle at its widest and highest point - and don’t forget the wheels and mirrors,
2/ the square of the speed of the vehicle,
3/ the weight or mass of the vehicle.
Less important is its shape but the coefficient of drag ‘Cd’ is a function of the slipperiness and the frontal area... Anyway, the faster the car goes the more the drag increases - exponentially so. In fact the amount of engine power required to maintain 45MPH doubles in order to maintain 60MPH.
With the above in mind, the problem then is POWER. The motor generally isn’t an issue. Either AC or DC motors can be used – both are relatively cheap and easy to use and quite efficient – anywhere from 70-90%+. The best ICE’s get is maybe 25% (a gas turbine or ‘jet’ engine is around 30%). No, the main problem is batteries and getting enough energy density into them. Lead-acid (Pb-A) batteries have ruled the roost for ever – indeed one oft-forgotten fact is that at the dawn of the motor carriage age the roads were dominated by electric cars – ICE ones were considered dirty, noisy, smelly, dangerous and difficult to drive. It was only the invention of the electric starter that started to make ICE cars more appealing especially (dare I say it ) to ladies – otherwise all those wealthy types (the only ones who could afford a car in the first place) would have to get hot, sweaty and dirty hand-cranking the engine. Oh, the irony.
Recently (in the last 20 years or so) advances in battery technology have created many alternatives to Pb-A, eg NiMH (nickel metal hydride) and NiCd (nickel cadmium) but the best currently available (for the money) is Lithium and specifically – in my view, anyway – lithium iron phosphate or LiFePO4. This type of battery has general all round advantages, particularly in terms of safety. E.g. it won’t burn or explode when punctured or crushed, has a good high current charge/discharge capability (‘3C’ or 3 times its rated capacity continuously, 10C for short bursts) and high cycle life of up 2000 cycles based on 80% depth of discharge (DOD) per cycle – one ‘cycle’ being one full discharge and recharge.
It is the cycle life of LiFePO4 cells that make them so attractive as the best Pb-A cells only give around one tenth the cycle life of LiFePO4 and only then if they are molly coddled, discharged to 50% DOD max and kept at 15 to 40 degrees C. This makes them less expensive in the long run. They also have the benefit of one quarter of the weight of the same Pb-A capacity. This means much better acceleration and hill climbing speed for a given battery pack capacity. On top of that they only use half the volume of Pb-A.