Pompey were happy to let the 34-year-old regain full match fitness in a spell at the Valley as the Addicks target a promotion push from the Championship.
Primus told the Pompey website: "It's good. I have a connection to Charlton and I'm going to be close to my family.
"It's a great opportunity for me to play at a high standard, with a club who have ambitions and are moving forward.
"It's a chance for me to get match-fit and to play regular football, which I need to do.
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