Weak starter cranking often related to battery wear,cable connection/grounding and most often brush carbon of starter.
If the initial problem is due to battery,changing to new or bigger battery will help.how to check.go to electrician and check for cranking voltage(how low the voltage drop).if too low this might indicate battery problem,may need to recharge.
Bigger battery + normal alternator=full recharge slow,high load on alt.but battery life longer.
Normal battery + higher amp altenator=full recharge fast,alt last longer.but faster recharge leadto faster bat wear.
My personal experience change battery,change alt,extra grounding +relay to starter relay,still weak cranking.at last change new starter brush carbon.rm50 only.very strong cranking.
weak cranking and need cranking for a long time only start, sometimes jerk at the starter