[Monkey Wrench]

I guess there are more scientific tests but it sounds pretty normal to me.
also agree withthe others , the initial drop after charging, it might be ok.

wont hurt to follow that ground and make sure its got a good connection to the body and the engine. that ground cable is important for every circuit in the car, they all use that circuit because they all must lead back to battery ground. its easy to ignore and important.
also the alternator probably has a ( blue? ) ground cable to engine. if that is bad it can cause charging issues.

to clean cable ends and battery posts, try a jar with soem baking soda and water itll bubble and clean nicely.

I also like to soak a little dish rag in it, let it soak up the baking soda and water, then let it dry, lay it neatly under the battry, now any acid spill gets instantly neutralized by reacting with the baking soda, rather than rust damaging the battery compartment or making holes in your pants when you next pull the battery out.

do not get baking soda INSIDE the battry but you can use a brush and clean the outside case with it.
lots of cars have rusty battrer boxes so I do this, it helps a lot to preent it.

I know if Im woring on and around a car Il frequenrly pull the cables off or one cable since im not driving it, it is very esy then to get used to the habit of the batery posts being not tightened up properly, Just pushed on,

simple stuff, but loose or dirty ones often cause issues. it is easy to forget to tighten them properly after working on the car.