Efficient Batteries, Better BMS Needed For Future e2Ws

Technology Efficient Batteries Better BMS Future e2Ws
Efficient Batteries, Better BMS Needed For Future e2Ws

Between 2013 to 2016, the global market for electric scooters and motorcycles grew at an average CAGR of around 3.3 %, going from $ 7,800 mn to $ 8,600 mn in that period. Due to the numerous challenges – the weight disadvantage that current batteries have, limited range, inadequate charging infrastructure and slow buyer acceptance – growth has indeed been slow. However, globally, government authorities are now beginning to push hard on promoting electric mobility, which is expected to increase the pace of high-quality battery-powered two-wheelers in the near future. Analysts expect the global market for electric two-wheelers to reach the $ 10,000 million mark by 2021. Here, we take a look at the state of existing e-2W tech and possible future directions.

CURRENT SCENARIO

Of the approximately 17 mn PTWs (powered-two-wheelers) sold in India every year, only around 400,000 units are battery-powered – a share of less than 3 %. A large percentage of these are low-powered scooters that are often of dubious quality and questionable longevity. Now, however, given the Indian government’s renewed thrust on e-mobility, the big OEMs – everyone from Hero MotoCorp (which has also made a strategic investment in electric two-wheeler start-up, Ather Energy), Honda, Bajaj, TVS, Mahindra, Yamaha and others are gearing up to produce electric scooters and motorcycles, which should be comparable to IC-engined bikes in terms of power, performance, handling, range, reliability and longevity.

Interestingly enough, electric bikes aren’t an all-new phenomenon. Electric motors are, inherently, much simpler than internal combustion engines and have fewer moving parts. Indeed, in some form or the other, e-bikes have been around for the last 100 years.

With electric motors developing maximum torque from close to zero rpm, electric bikes offer impressive low-rpm acceleration, but challenges come in the form of the range vs speed trade-off. Electric batteries are simply not as efficient as, say, petrol when it comes to storing energy, which is why current electric bikes and scooters are better suited to relatively short-distance travel, at moderate speeds. Yes, current battery-powered electric production bikes can attain very high speeds, but at the expense of riding range, which is not acceptable to many riders. Also, the charging time – anywhere between 2-4 hours for a full charge – is not practical for most riders who’re used to filling up their bike’s petrol tank in a minute or less.

CHALLENGES IN BATTERY DEVELOPMENT

Various types of electric batteries have been used for electric automotive applications. These include lead-acid, nickel-metal hydride, lithium iron phosphate, lead sodium silicate, lithium titanium oxide, and lithium-ion batteries, each with its own set of pros and cons.

Most players in the electric battery development and production space admit that technology has been moving at a relatively slow pace and manufacturers haven’t been able to crack the energy density vs size/weight equation. Consumers want battery packs that are reasonably compact and lightweight, yet sufficiently energy-dense so as to allow speeds of up to 180-200 kph and a range of 150-200 km or more. With existing technology, that’s a tall order. With bikes that have lithium-ion batteries, the batteries represent around 40 % of the bike’s purchase price. And that purchase price is, in the case of some high-end e-bikes, more than 50-100 % higher than comparable IC-engined machines. Yes, some OEMs are offering what they call ‘modular power packs,’ which are essentially quickly swappable battery packs, but that only seems to be an interim solution.

More than two-wheelers, there seems to be more EV-related development in the four-wheelers space, so can bikes benefit from battery tech borrowed from cars? Most current electric cars use lithium-ion batteries. Tesla, for example, which uses batteries supplied by Panasonic, uses small-format cells that are energy dense, but can be prone to overheating. As a workaround, Tesla uses advanced cooling and battery management systems to keep things working efficiently.

Most other OEMs, on the other hand, use battery packs with large-format cells, which are not as energy dense but are also not as prone to overheating. Over the next few years, such batteries are expected to become more efficient by up to 75-80 % (with a significant increase in their capacity to store energy), and with the development of new types of cathodes, prices are also expected to come down and, at the same time, with new charging methods, charging times will be reduced by 80 % or more. The latter will, however, be dependent on the setting up of a network of specialised charging points, since home-based electric outlets are usually not capable of running 50-120 kW fast chargers that some OEMs are current providing. AC to DC conversion is also required, since batteries can only store DC. For this, bikes either need to have an on-board AC to DC converter (if an AC charging point is used) or the converter needs to be integrated into the charger itself (if a DC charging station is used).

In the future, fast charging DC stations will be the preferred option, since these would negate the need to carry charging electronics, including the diode array, cooling hardware, transformer and wiring looms etc., on the bike itself, thereby saving weight and complexity. Regardless of the charging station type, though, batteries are currently the biggest roadblock in EV adoption and industry analysts say that big, dramatic improvements are still needed in battery efficiency if electric two-wheelers are to go mainstream.

ELECTRIC MOTOR COMPLEXITIES

On electric bikes, while the battery pack produces the ‘juice’ that powers the vehicle, it’s actually the rotary electric motor that takes up that juice and converts it to torque that provides forward motion. Compared to IC engines, electric motors can be much simpler, with fewer parts. There’s the stator, which is the non-rotating part and the rotor or armature, which is the only moving part. When direct current passes through the stator and the rotor coils, their respective poles are magnetised and by using a system of switches, that’s what is used to make the armature rotate. The basic operating principle remains the same, whether the electric motor is being used to run a washing machine or a motorcycle.

While DC motors have been used in applications that require power output for only brief periods of time (electric bike drag racing, for example), these utilise carbon brushes and commutators that are prone to rapid wear. Also, DC motors usually do not have the energy efficiency that’s required for longer riding range on an electric motorcycle. So, yes, AC motors are much more suitable for automotive applications. There are AC electric motors of various types that are currently in use, including the AC single-phase motor (used in many household electric devices) and the AC three-phase induction motor. The latter can deliver up to 180 % as much power as a single-phase motor and is hence usually the preferred option for powering something as big and heavy as a motorcycle.

Large, liquid-cooled AC electric motors can operate at more than 90 % efficiency, which really helps because then almost none of the power produced by the battery pack is wasted. Reducing power losses via optimised design for internal bearings and motor winding is important (every little bit helps when trying to maximise an electric bike’s range), as is keeping the motor sufficiently cool. While larger motors can act as their own ‘heat sink’ and air-cooled motors have been successfully used on electric bikes, the addition of liquid-cooling adds the potential for improved power and torque delivery, since such motors are able to rev at higher speeds for longer, more sustained periods of time.

In addition to liquid cooling, electric two-wheeler manufacturers also need to use sensors and an ECU-controlled heat management system, which keeps track of the motor’s temperature and regulates it in a way that prevents damage to wiring and other internal components. For this, variable speed fans are sometimes used, which switch on automatically when the sensors detect excessive heat when the electric motor is running.

On electric bikes, the motor also performs an additional task in addition to providing forward motion. And that is, regenerative braking, by which some of the bike’s kinetic energy that’s produced during braking gets stored in the battery for reuse. Electric bikes can have multiple settings or modes for regen braking and in the more aggressive modes, just the act of getting off the throttle provides ‘braking’ via the electric motor, which briefly turns into a ‘generator,’ with its AC output being converted to DC and being sent to the battery pack for storage. This adds to the vehicle’s riding range – perhaps not by much, but every little bit helps on an EV.

High-end electric motorcycles already offer performance that’s comparable to 600 cc supersports machines

THE NEED TO COLLABORATE

Unlike cars, where petrol-electric hybrids have acted as a stepping stone to the development of full EVs, bike manufacturers seem to have skipped the step entirely and are looking at making a more straightforward move to pure electric bikes. This is probably the right step too, given the much smaller IC engines used on most motorcycles (as compared to cars) and a completely different set of requirements pertaining to weight, power delivery, economy and range.

What doesn’t change, though, is the reliance on specialised charging infrastructure. While car manufacturers have come together and are working on common standards for batteries, electric motors and even charging specifications, there seems to be no such co-development happening in the two-wheelers space, which may hamper the pace of progress.

Ideally, OEMs need to collaborate on electric motorcycle platforms, and figure out some basics in terms of batteries, motors, controller software and charging infrastructure, which then all e-bike manufacturers can work towards. If these standards for e-bikes, in the context of charging station requirements and interoperability between multiple standards, can be shared with those being used by car manufacturers, things would be easier for all.

Acquiring sufficient funding for a charging infrastructure is also likely to be a challenge in each region, since the sheer scale of the network required might make it prohibitively expensive for governments to have a go at it on their own. Hence, a PPP business model could be required to make it work. Last, but certainly not the least, local power grids would also need to be upgraded and be made more robust, in order to cope with the charging demands of hundreds of thousands of two-wheelers, with the figure ultimately going up to millions over the next 2-3 decades.

FUTURE DIRECTIONS

Electric motorcycles already offer phenomenal performance. Take, for example, the Italian-built Energica Ego, which is powered by an oil-cooled, permanent magnet AC motor, fed by an 11.7 kWh lithium-ion battery pack. With 107 kW of power, and 195 Nm of torque available from zero to 4,700 rpm, the Ego can accelerate from zero to 100 kph in three seconds and hits an impressive top speed of 240 kph. However, the batteries take 3.5 hours for a 100 % charge, or 30 minutes for zero to 85 % charge using Energica’s proprietary fast charging technology. Then again, prices for the bike start at 25,400 euro ($ 30,200 or Rs 19.64 lakh) and above, which is where things fall flat for most potential customers.

Despite current challenges, with continuing fast-paced evolution of battery technology and better optimised electric motors, and governments’ push on the adoption of electric vehicles globally, it seems to be a foregone conclusion that electric bikes will replace IC-engined ones in the foreseeable future.

TEXT: Sameer Kumar