Many people think that the bigger the battery, the better for an electric bus. Actually, that's not the case. A battery that's too large will increase weight and consume more power, while a battery that's too small won't last a day. So how do you calculate the appropriate size? Let's take a real bus as an example—the Zhongtong 10.5-meter LCK6108EV4.

This bus comes standard with a Lishen lithium iron phosphate battery, a 538-volt voltage platform, 320 Ah, which translates to a nominal capacity of 172 kWh. The motor is a 150 kW permanent magnet synchronous direct drive. The advertised range is 288 to 320 kilometers, but that's measured using the constant speed method; the actual driving experience is completely different.

What are the operating conditions for a city bus? Frequent starts and stops, full load, air conditioning in summer, and heating in winter. The actual power consumption is approximately between 0.9 and 1.1 kWh per kilometer. Assuming a rate of 1.0 kWh per kWh, a 172 kWh battery pack, with a 70% usable capacity, only yields about 120 kWh. At best, this is sufficient for a daily mileage of 110 to 130 kilometers.

But what if a route needs to cover 200 kilometers per day? 

First, calculate the daily energy consumption: 200 kilometers multiplied by 1.05 kWh per kilometer equals approximately 210 kWh. Working backwards from this, considering the 70% usable capacity and battery degradation, a battery pack of at least 333 kWh is needed. The original 172 kWh is clearly insufficient.

At this point, there are two approaches. One is to increase the battery size, replacing it with a 320 to 350 kWh pack, requiring daily charging. However, this would add nearly two tons of weight, placing greater stress on the axles and suspension, and increasing energy consumption. The other, more pragmatic approach is to maintain the original battery level and quickly charge the vehicle for over an hour during the driver's lunch break, adding about 100 kWh. Many bus companies actually do this.

There's an easily overlooked point here: battery performance isn't just about capacity; the discharge rate is also crucial. This bus's motor has a peak power of 150 kW, and under full load, starting, climbing hills, and with the air conditioning running at full power, the instantaneous power can reach 140 kW. A 172 kWh battery pack is just enough at 1C discharge; 1.5C would provide some headroom. The same applies to charging; the original 75 kW onboard charger is slow, and a 150-240 kW DC charging station is needed for proper recharging.

In short, when equipping buses with batteries, first understand the operating conditions—how many kilometers they run per day, how many shifts, and whether recharging is possible at midday. Once the parameters are in place, applying the formula isn't complicated. But the formula is just the starting point; ultimately, the route, depot, and budget must be considered. In engineering, there's never an optimal solution, only the most suitable one.