The technology relates to a scalable power system. A scalable power system includes battery modules connected in parallel, a network of diodes, a battery management system that may be localized or centralized, a localized battery management system configured to control an individual battery module, a centralized battery management system configured to control all of the battery modules in the scalable power system. In a system with localized battery control, a parallelizing circuit board may connect battery modules to a system bus in parallel. The battery modules can be charged and discharged by the system bus. A method of adding a battery module to a scalable power system includes discharging a highest-charged battery module, then discharging a next highest-charged battery module until there are no remaining charged battery modules, then adding an additional battery module in parallel, and charging the connected battery modules in parallel.