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1998 D3C III Hystat not charging

I have  1998 D3C III Hystat LGP that is not charging the batteries??  I have replaced both batteries and alternate..  Serial # 5GS00269..  Hired CAT service truck and they told me to replace the batteries which I did.  I have to recharge the batteries manually after 18 - 20 hrs of operational time...  What am I missing as checking for faults??  Fuse, Relay??  need help on resolvng this concern so I dont destroy new batteries and or have to replace the battery cables..