Right-sizing of the bus battery is the way forward

Electric bus (e-bus) is in the news lately as the State Transport Undertakings (STUs) are procuring or announcing plans to procure e-buses for their intra-city fleets. In Episode 1 “Charging infra for buses to be planned according to route and not on per bus basis”, we highlighted the importance of planning charging infrastructure for an intra-city e-bus fleet according to the route and why a straitjacket thumb-rule approach (e.g. one slow charger per e-bus and one fast charger for every 10 e-buses) proposed in certain guideline documents may not be effective. In this article, we deal with the heart of an e-bus i.e. battery. 

Range anxiety is a widely recognised challenge in the adoption of EVs on the Indian roads. Imagine an e-bus with its passengers getting stranded in the middle of an arterial road during the rush hour. Chaos is an appropriate word to describe the situation. One shouldn’t be surprised to see a big headline on this next day on the front page of the daily. Just to clarify here, this apprehension is not from any stretch of imagination. It is recently reported that two e-buses launched by the Kerala State Road Transport Corporation (KSRTC) ran out of charge mid-way. Such bad publicity is the last thing the EV industry would wish for. Then, how to avoid such a situation! 

A common solution often advocated to address range anxiety is to adopt a large size battery for the e-bus. It seems quite straightforward and easy to implement. Phase-II of the FAME[1] scheme also supports this idea; it links the subsidy amount to the battery size of the vehicle. This is a welcome move but calls for some caution. Is the adoption of “large-size” battery the remedy to the problem? One should bear in mind that a battery constitutes 50-60% of the cost of an e-bus. Hence, with the adoption of large capacity battery, the cost of an e-bus would also go up which would consequently make the e-bus procurement more expensive (the cost is already quite significant) and TCO[2] of the bus operation less economically attractive. This would really dent the prospect of shifting to electric format from the current ICE- bus fleets. Not to mention, oversizing of batteries also adds to the weight of an e-bus which is already burdened by the heavy drivetrain and the chassis.

In this puzzle, are we missing the piece related to efficiency? Improvement in the energy efficiency of the drivetrain of an e-bus (translated to an increase in km/ kWh) and higher specific energy (kWh/ kg) of the battery can go a long way to overcome the range anxiety issue. Unfortunately, the existing policy prescriptions haven’t given much stress to this aspect. 

AEEE’s recent study on optimal battery capacity for an intra-city bus fleet points out that three fundamental things need to be considered while deciding the battery size for an e-bus.

1.     Route length - The primary factor to determine the bus battery capacity serving a route should be the concerned route length. AEEE research has studied the longest and shortest bus routes of the Tier-I and Tier-II Indian cities and found that the root-mean-square (RMS)[3] of the intra-city bus route lengths is about 33 km (in a single trip). Hence, to find a commensurate battery capacity should be the goal.

2.     Usable energy of battery – To know what battery capacity would be appropriate for the e-bus, one has to examine the assured range offered by the battery capacity. This warrants an understanding of the performance characteristics of a typical battery. To start with, one should bear in mind that the rated battery capacity, which is the total amount of energy that the battery contains, in practice does not correspond to the total energy available for usage; the primary reason being the limit up to which a battery can discharge i.e. the maximum depth of discharge (DoD). This is to maintain the health of the battery. Going by the thumb rule for lithium iron phosphate (LiFePO4) batteries (commonly called LFP batteries), the maximum DoD is considered to be 70% which implies that 30% of a battery’s rated energy capacity is non-usable. It is worthwhile to mention here that this 70% of the rated battery capacity is displayed as 100% on the display panels of the EVs and is called the displayed battery capacity.

Of the available 70%, it is recommended that the battery of an e-bus should never be discharged below 15% to avoid any possibility of getting stranded. This implies that in order to avoid range anxiety, at least 15% minimum energy (charge) should be reserved in the battery when an e-bus arrives at a charging station. Therefore, the usable energy in a battery with a rated capacity of x kWh is actually (70 - (70 * 15%))%*x kWh i.e. 59.5%*x kWh.

3.     Assured driving range – It is important to understand how to translate usable energy of a battery to the expected driving range of an e-bus. AEEE’s assessment of the mileage data of the existing e-bus models manufactured by the major OEMs in India indicates that per km energy use ranges between 2 kWh and 0.83 kWh; the difference is primarily due to auxiliary energy consumption by the HVAC system. However, there are several design and operational factors which too can influence the actual driving range of an e-bus, such as the efficiency of drivetrain, the kerb weight, the average and maximum gradient of the route, the climatic condition, etc. 

So, what would be the most suitable battery capacity for an intra-city public e-bus fleet in Tier-I and Tier-II cities in India? We have to wait a bit more to know the result. This finding and many more are available in the forthcoming AEEE report -- “Charging India’s Bus Transport”, an outcome of extensive research and deliberations carried out over the past 6 months.