As you say, probably knackered. They only last so long. 8 yrs not bad going. You could try charging, then run all the way down to absolute flat, I mean absolute flat, so it will not give any power whatsoever, then recharge again, and do same again. If that does not improve them, Knackered.
But you do not just throw away, recycle, battery drop point or local tip, will have a point for batteries, harmful to the enviroment.