“Ronen Kleiman says he noticed immediately that the electric vehicle he leased in 2014 wasn’t living up to what the carmaker was advertising on its website â€” it couldn’t travel the distance BMW said it could on a single battery charge.”
See Electric car driver finds altered BMW ad lowers range after purchaseSigh!!! When I was a teacher it was as plain as the nose on my face. Some people may be grown up but still as innumerate as a primary school student…
In this case, linked at the right, BMW advertised a car as having a range of “up to” 200km “depending on driver behaviour”. That’s clearly a maximum, an extremum of the function of range versus driver behaviour. Behaviour could mean stop and go in urban traffic, or high rates of speed on the highway. In neither case would anyone with any knowledge of number imagine 200km could be achieved. Yet, here’s a guy and a national television network broadcasting a complaint that BMW may have misled consumers. Consumers can mislead themselves if they don’t know the ordinary meaning of words and numbers.
In case anyone does not understand, let me explain two ways to achieve maximum range in an electric vehicle:
- drive at the optimal speed, probably between 20 and 30 km/h, continually, and
- don’t do lots of acceleration and braking, both of which waste electrical and mechanical energy which come from the charge of the battery.
When one accelerates, energy is being put into increasing the kinetic energy of the vehicle, not travelling down the road. An electrical system will have losses of energy due to production of heat or a drop in voltage due to heavy currents. Some level of energy conversion between off and maxed out will give optimal efficiency. Further, at high speeds, the force on the car from pushing air out of the way is far greater than at lower speeds. The power, KW, required varies with the square of the speed because the car meets air according to its speed and requires a force according to its speed to part the air. So, if you get 200 km at 50km/h you might indeed get only 160km at 60 km/h. 200km is not the same as 160 km but the behaviour has changed. Get it? The air-resistance at 60km/h is (60^2)/(53.6^2) = 1.25 times as much as the air-resistance at 53.6 km/h. The energy required to do the work of pushing air out of the way is Force X Distance, 200 X F is the same as 160 X 1.25 X F, the charge of the battery. There are other factors affecting efficiency more at low speed like the rolling resistance of the tires. Keep them well inflated and the energy used to roll along increases very slowly with speed and is a small part of the whole trip. The drive-train will increase wastage as output increases to some extent. The net result is that efficiency in terms of range per charge increases rapidly at first, peaks at a moderate speed and then declines rapidly as speed increases.
Data has been published for Nissan Leaf in terms of miles/KWH or range per charge and it follows this pattern. So, BMW wasn’t trying to cheat anyone, just assuming people could understand words and numbers. The leaf, with a 24KWH battery can get 82 miles on a charge at 60 mph up to 132 miles at 35 mph. Do the maths before assuming false advertising. My Solo EV has been reported to get “up to” 200 km on a charge of 16.1KWH but the maker advertises 160km because they don’t want this bad PR. Also, the battery deteriorates slowly with age/use so the warranty of ~85% capacity after five years still fits. It’s old technology but new to many. They need to grow up/mature/read/understand/experience these things.