The above is more or less accurate but a tad simplistic.
Line Maintenance is customarily covered by a contract that specifies exactly what service level is provided. The contract will incorporate a rate for 'additionals' such as wheel/brake unit changes, defect rectification etc. but the aircraft will have manpower allocated for the duration of the visit. Materials are typically charged 'per use'. Thus the 20 minute battery change that you mention covers only changing the battery. All the other time involved may be absorbed in the overall contract. The part should be already available at the stand (but isn't always there!) when the aircraft arrives and is then replaced by the transit crew already allocated to the aircraft for the duration of the transit.
In the Hangar, maintenance is costed on a per job rate. There is generally a fixed price for the complete routine check, with defect rectification, Service Bulletins and other additional work (such as battery changes and so on) charged on actual time consumed. Thus the charge for an additional job covers all the steps involved in getting the job done. Several people will be involved in any one hour of labour - storekeepers, planners and mechanics - not necessarily the one man who wields the tools. The charge will then cover the total manhours expended on the task.
When estimating the time that will be needed to accomplish a Service Bulletin the planners will assume a fixed factor of 1.5 or even 2 times the standard time given by the manufacturer in the bulletin. The SB standard time assumes all access already open with stands in position; tools, materials and documentation pre-positioned ready at the work location and all manpower briefed and familiar with the task. The planning estimate is not used for invoicing: The bill will be for the actual manhours consumed, including all the fetching, carrying and reading.