2003 HD dual battery needs boost

replaced one battery, was completley dead. Now intermittent no start. Boost and it runs fine until it dies. Alt is charging as well. Is there a battery isolator on these, if so where is it ? Thanks dan.

Always replace both batteries if one dies.

Check all cables, clean them good and make sure the connections are tight. I just talked with someone with intermitten battery problems and he bought a new starter even and all it was was a loose wire. Check all grounds for corrosion.

I had dual battery set up on my 1992 Chevy and replace only one battery and never had a problem doing that. But do get the other checked out as it may be going also.