is it the starter motor

only started yesterday when starting engine turns slowly but scream battery slows then finally starts checked battery reads 12 volts and with engine running 15 also connected it to other car with engine running does same is the starter motor going or something else checked earth straps ok there any help would be great.

your engine might need the heater plugs but if not it is most probably the starter. does it still turn over slow when the engine is warm?

Thanks for reply just got under took starter motor off the end of the casing is cracked where the 2 long bolts hold it together one of the bolts must have been overtightened took it apart sprayed loads of wd and put back together but didn't tighten it too much so the casing stayed nearly closed started great quick repair till i get new starter motor