If you had four batteries, you could use 4 of these chargers at the same time.
I am often in countries that have blown out or the electricity goes off for hours. Sometimes on an Island or other locations, there is only electricity by generator for 1-3 hours in the evening. I need to work during this time period, and to charge all my batteries at the same time.
If the black box in your laptop cord broke, you could also use as a backup system until you purchased a new cord.
This solution is difficult, and this store is a good place to start searching: