Battens really can't be too long. Many sails have full battens. That can however be too stiff.
The battons are probably too stiff, but,
yes they can be too long. A full batten should be long enough to pretension shape like what they do to concrete by tensioning cables before it sets. Release the tension and the concrete compresses on itself. The batten should want to make a shape, giving in under normal loads to reverse itself.
A batten that only goes part of the way is flawed, in that it doesnt want to take any shape other than flat. As soon as there is a difference in rigidity the surface fails at that weak point. This is where the flexibility parameter comes into play.
The batton is too long if it does not contribute to a curved surface like a compressed full length batten. A partial batten should just be long enough to keep the leach from flapping, but at a point that is not anywhere near the max chord.
I have a mainsail with both a full and 2 partials. I fully compress that upper full one so it has a shape even when there is no wind.
The two partial lower battens go in pretty far for my taste, but i keep the outhaul and vang tight almost always. Otherwise, the mainsail folds at the end of the batten. They are very old granny pants sails.
In model sail making, i found this out on day one of the first design with roach. My full length batten ruined the sails as they were just taped on=flat batten=flat sails. The partial battens need to just go past the "red line".