This is my first post, and by no means do I even pretend to be an electrical engineer!
I've been following the range extender option with some interest, especially as BMW states it's only intended for "Emergency" or "limited" use.
From what I can see reports from some BMW representatives that you can drive indefinitely at motorway speeds by constantly refueling might be false.
The figures just don't stack up. Of course final judgement should be reserved to official long term or economy run tests!
So let's start.
BMW i3 technical specs:http://www.bmw.com/com/en/newvehicles/i ... _data.html
Electric range 190km (118 miles)
Electric range (mean customer value) 130-160km (80-99 miles)
Max. total range (most efficient mode) 190km (118 miles)
Capacity of lithium-ion battery in kWh 18.8
Fast charging, e.g. at DC fast-charging station: DC; 125 A; 50 kW (80 %) Under 30 min.
Charge time of high-voltage battery in h at 16 A (80%); 6.4 KW 6-8 hours
The REX engine has a max output of 34hp
As far as I understand this does not mean that it will supply 34hp/25KW to the battery, due to efficiency losses.
Take a highly efficient 30hp commercial generator as an example:http://www.eg-electric.com/catalog/item ... 426153.htm
After energy losses it's only able to supply 15KW of continuous electrical power.
At best I would expect the REX to only supply 17KW to charge the battery.
That's not taking any other efficiency losses into consideration during actual charging, so it's a very optimistic figure.
Here's energy usage examples of the Nissan Leaf and Mini-E at motorway speeds:http://www.mynissanleaf.com/viewtopic.php?f=31&t=13265
The LEAF uses less power at 45 mph than the Volt (9.2 kW vs 10.1 kW). At 70 mph they are basically the same, but the Volt edges out the LEAF (22.8 kW vs 22.7 kW). The Volt is clearly more aerodynamic.
From the battery (from the wall):
45mph = 4.85mi/kWh (3.94mi/kWh)
60mph = 3.70mi/kWh (3.04mi/kWh)
70mph = 2.92mi/kWh (2.48mi/kWh)
Constant Speed Range @65mph
Range: 104.15 miles
Energy Used: 29.344 kWh
Efficiency: 281.7 Wh-DC/mile
Specific Energy: 112.9 Wh/kg
Charging Energy: 35.40 AC kWh
So by my calculations that would give the Mini-E 18.3 kW power usage over 1 hour @65mph
In summary, cruising an i3 at 70mph you will require around 20kW per hour.
The BMW i3 battery only has 18.8 KW capacity.
You cannot indefinitely drive at 70-75mph motorway speeds, as the REX will only provide a maximum of 17KW in an hour.
At some stage your battery will be completely depleted, even if you continue to refuel.
Being able to engage REX at 80% charge will go some distance to alleviate it, but at some stage you're simply going to run out of energy at the battery.
So guess we'll have to wait and see whether one is restricted on power during REX operation.
I really like the i3 and will still go for a test drive as soon as it's available.
But at the very best it only looks to be suitable as a very expensive 2nd car for families, or for someone who only ever drives in and around city boundaries.
I've got a sneaky suspicion that driving from London to Edinburgh (400 miles) is just not something that's easily within reach for the i3, even with the REX option.
(And let's not start the whole flying/train debate - I do use it when it makes sense).
I hope that battery capacity improves at least two-folds in the next couple of years, or that the price of these cars are halved.
Just some more info, a Nissan Leaf only does 80 miles at 62mph:http://insideevs.com/nissan-leaf-side-b ... 2-vs-2013/
That's with a 24KW battery capacity (roughly 21KW usable)
Another more worrying post, with someone only managing 53 miles driving a Leaf at almost motorway speeds:http://www.mynissanleaf.com/viewtopic.php?f=31&t=14790
Tesla Model S 60KWh at 74mph seems to average roughly 320 wh/mile, or
Thus you can expect 180 miles at most out of a 60KwH battery pack.http://www.greencarreports.com/news/108 ... 60-kwh-car