*4.1. Battery Size and Need to Charge during the Day*

Depending on charging strategy, the buses will need to have different battery sizes. The size of the battery must be chosen to meet several requirements. The battery must have enough capacity (kWh) to supply the energy needed for the most demanding bus schedule and should also have some margin to handle disturbances, which may sometimes lead to a shortened or a completely missed charging.

Another size-related criterion is that the battery must have enough capacity to be able to deliver the necessary traction power and to handle the charger power. It is not possible to have a very small battery and discharge or charge at very high power. The fact that batteries in buses with end-stop charging must be capable of charging at high power is a reason for why they are assumed to be more expensive per kWh of stored energy. The higher cost is due to the battery cells being more expensive per kWh when they are optimized for high charging power and the battery system needing a more effective cooling system.

Finally, the battery must not wear out to quickly, and have margin so that it can still meet all the requirements for energy and power also when the battery has aged. Typically, the capacity of the battery is reduced by up to 20% when it reaches its end of life, but the maximum discharge and charge power will also be reduced when the battery ages and, as such, this also needs to be included when deciding the battery size for a bus.

For the end-stop-charged buses, the charge power and number of charge cycles will be the critical factors, and therefore it is assumed that a 100-kWh power-optimized battery is required, despite the fact that a trip typically only requires 25 kWh if it is one hour long.

For the buses which use end-stop charging but only off-peak, a 200-kWh power-optimized battery has been assumed. The peak traffic periods are up to about three hour long which requires about 75 kWh of energy.

The battery size could be optimized for each route and timetable, but it is deemed likely that the market will settle on a few battery sizes as this simplifies moving buses between different contracts and the offers possibility for the buses to have a second life if a contract is not renewed. Therefore, it is not likely that buses will be optimized for the route they operate on, rather, there will be a few standard battery sizes which the operator selects from. The optimal sizing of a battery is a very complex task and is not included in this paper.
