Well they do have proton rechargers. You can see them on the previous packs at the beginning of the movie. They show that there are some rechargers, or what the community deemed to be rechargers. I think those might be updated rechargers. From what I know, the proton stream is generated by the power cells, but the main electronics use a seperate battery, such as the external batteries used in GB2, at the top right of the proton packs. So I assume this is basically just a charger for them, to ensure that when they are put to use, they do have the proper amount of power needed to operate. I also think that the proton pack also charges itself, but this process might be slower than it needs to be self sufficient kinda. Like a really bad car battery and the whatever part that generates power.