BMW Promises Fix for Anaemic Acceleration of BMW i3 REx in Range-Extended Mode

When it comes to overtaking, there’s nothing more terrifying than committing to an overtaking manoeuvre on a hill to discover that you don’t have the power you need to successfully and safely accelerate past the vehicle you’re trying to overtake — especially if there’s someone else coming the other way.

Slower on hills: the BMW i3 REx could lose power on steep hills if its battery pack is fully depleted.

Slower on hills: the BMW i3 REx could lose power on steep hills if its battery pack is fully depleted.

Luckily, it’s a situation which only likely happens if you’re someone who enjoys the charms of an underpowered classic car, but as Consumer Reports discovered recently, it’s a problem which also afflicts BMW’s first mass-produced range-extended electric car if you happen to come to a hill after depleting its on-board battery pack completely.

And it’s a problem BMW says it is planning to fix with an upcoming ‘enhancement’ in spring 2015.

Available in both pure electric i3 EV and range-extended i3 REx variants, the BMW i3 is a funky, futuristic, four-seat plug-in car that not only lays claim to being BMW’s first mass-produced electric car but also to being the most efficient plug-in car you can buy today. On paper, that means the BMW i3 EV travels an EPA-approved 81 miles from its 22 kilowatt-hour lithium-ion battery pack, while the slightly heavier i3 REx — complete with its two-cylinder, 650cc gasoline range-extending engine — manages 72 miles per charge.

While less efficient and marginally slower in the 0-60mph dash, the BMW i3 REx’s range-extending engine offers customers an additional 78 miles of range –150 miles in total electric+gasoline– from the car’s tiny 1.9 gallon gasoline tank, meaning it has sold far better than the electric-only i3 REx. Yet due to the way the on-board range-extending engine works, the i3 REx can become extremely anaemic in certain situations when its battery pack is fully depleted, as Consumer Reports discovered when trying to accelerate up a hill on gasoline power alone.

The independent testing organisation, which buys all of the cars it tests direct from dealerships at full list price in order to retain its impartiality, says one of its BMW i3 REx test drivers recently tried to overtake a truck on a hilly two-lane after the car’s battery pack had been depleted. With only the tiny 650cc, 34 horsepower engine providing power, the BMW i3 REx’s normally powerful acceleration gave way to an asthmatic, dangerous overtake.

In most situations, the BMW i3 REx's range extending engine can keep up. But outside of the city there may be problems.

In most situations, the BMW i3 REx’s range extending engine can keep up. But outside of the city on steep, faster roads, there may be problems.

“The i3 began to lose power without warning, subjecting the driver to more exposure in the oncoming lane,” Consumer Reports said. And while the car soon regained its power on coasting after the overtake — which Consumer Reports noted allowed the car time to replenish some of its battery pack — the test organisation said it was “disconcerting to say the least.”

According to the organisation, further testing revealed that prolonged use of the throttle with no speed variation or braking while the battery pack was reaching empty meant that acceleration dramatically suffered, increasing 0-60 mph times from the usual 7.5 seconds experienced in all-electric mode and 9 seconds in range-extending mode to anywhere from 27 to 40 seconds. For those taking notes, the Tata Nano, the world’s cheapest car, manages 0-60 mph in 30 seconds.

The problem isn’t a new one either. Back in November last year, we brought you news of a BMW i3 REx test-drive carried out in the UK by Telegraph motoring correspondent Chris Knapman. Driving on the busy M20 motorway north from Kent into London, Knapman’s loaner BMW i3 ran out of electric power and switched on its tiny gasoline engine as he was trying to drive up a steep section of road at high speed. With no battery power left, the tiny 650cc engine struggled to keep up with the rest of traffic, slowing down to an unbelievable 44 mph on the steepest part of the motorway, some 6mph below the legal minimum speed for motorway traffic.

In Knapman’s case, the circumstances which lead to his unfortunate incident could have been overcome, since the European i3 REx can turn on its gasoline range-extending engine before the battery pack is depleted — as long as it is below 75 percent full — at the discretion of the driver on longer-distance trips, ensuring there’s never any loss of power.

Last year, a British Journalist suffered the same problem on the M20.

Last year, a British Journalist suffered the same problem on the M20.

In the U.S. — where the need to satisfy tough Californian regulations on plug-in hybrids and range-extended EVs mean that the range-extending engine only comes on after the battery pack has dropped below 6 percent full — that’s not an option. And with only six percent of battery charge remaining, it’s all too easy to deplete any remaining battery power if you’re in a particularly hilly area.

While the BMW i3 REx is designed primarily as an suburban and urban vehicle and most i3 REx owners won’t experience this problem, we note that this could cause problems for those who like to make use of the range-extending engine for longer-distance trips, especially if they’re planning on making a longer-distance trip to see family or friends and are unaware of this issue.

With such a small engine on-board, it comes as no surprise that this is an issue for some, but BMW says that it is working on an enhancement in the form of a battery state of charge indicator and an early-warning system to tell drivers when the car is likely to lose power due to low battery state of charge and high power drain. Similar to the mountain mode on the 2014 Chevrolet Volt, BMW says the i3 REx will also introduce a software update which will use the GPS system to automatically trigger a boosting of the battery state of charge when it detects the car is headed for hilly terrain.

Free for all BMW i3 REx owners, the update will be rolled out some time next year.

Until then, if you’re making a trip in a BMW i3 REx and plan on running its battery pack empty, be sure to remember that your car won’t have as much power in gasoline mode as it does when using electric power.


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  • I’d be interested to see what the “fix” is. 650cc engine is less than the 1970’s Daf 33 (850cc) which wasn’t known for its acceleration. How exactly do you fix 650cc? 1200cc maybe?

    • If I had to guess, it’ll involve doing something similar to the mountain mode in the Chevrolet Volt: prevent the battery pack from dropping so low and essentially keeping a larger ‘reserve’ of electric power for use on challenging hills.

      • But wouldn’t they then lose any HOV lane status if the engine can be turned on earlier by the driver by engaging mountain mode? Maybe they don’t care about that status anymore since the Rex had a tough time getting the HOV stickers in the first place.nnnEven with running the engine earlier, the battery still has the potential to run down. I wouldn’t call it a fix, a workaround would be a more appropriate classification. The real problem here is the 650cc engine can’t possibly drive the car adequately on its own, At least the Volt has an engine capable of propelling the car at highway speeds indefinately.

        • Andyj

          Losing HOV lane status was my thought too JP.nIt’s a quandry.nC.A.R.B. needs to have a rethink because under any circumstances the car has more economy than some other HOV capable cars but it will have to be in concert with BMW who really do need to upgrade out this issue.


    Clearly they chose one of their smaller, probably horizontally opposed cylinder, motorcycle engines to power the generator. If they provided an option to have a larger battery or second battery of the same weight as the engine in that space, I suspect they would have a much more successful product.nnI test drove and liked the REx but would prefer and would buy an i3 with a second battery for range extender rather than a REx engine, anemic or not. nnThe big advantage of the engine is the fast refuel time especially since CCS appears to be a figment of the imagination for this country. nnThe i3 REx has the range I need at least three days a week and the reserve for cold weather for the other days.