2001 ram diesel. Starter is slow, but the two batteries are good

*BOTH BATTERIES* need to be replaced - Must be same group size and same amperage! Basically buy two of the same batteries - high quality batteries!

Proper battery testing should reveal the batteries are indeed failing - that is load testing plus specific gravity testing using a battery hydrometer .. One failing battery will also damage the other - that's why they both need to be replaced as same time.