Panamax ship is 5000 teu (twenty foot shipping container equivalent)

I think you get about 4 MWh per TEU ( based on my 12V 100Ah battery)

so about 20 GWh

At 170Wh per kg (and ignoring the weight of the containers and any safety considerations), 20GWh of lithium battery would weigh 120,000 tons. This is a lot more than a typical Panamax DWT of 60,000 tons, which also needs to include the ship's fuel, provisions, crew, etc.