Read the following passage carefully and answer the question that follows.

For decades, the islands around Kochi depended on ageing private ferries that ran without fixed timetables and stopped operating at dusk. That changed when the city launched a fleet of battery powered catamarans as a formal part of its public transport network, the first experiment of its kind in India. The electric boats are quieter and far cheaper to run than the diesel launches they replaced: operators report that energy costs per trip are roughly a third of what diesel used to burn, though each new boat still costs considerably more to build. Quick charging equipment installed at the terminals tops up a boat's batteries in ten to fifteen minutes, which is what lets a small fleet keep a frequent, all day schedule. Just as important, the water service shares its smart card with the city's metro trains and feeder buses, so an islander can ride a boat, a train and a bus on a single fare card. Ridership data shows island households now make far more trips to the mainland for work, study and hospital visits than they did before the service began. Transport planners from other coastal states have started visiting Kochi, but officials there caution that the system's real test will be surviving years of monsoon operation without losing its reliability.

As used in the passage, the phrase *tops up* is closest in meaning to which of the following?