Hi, just got a boat that came with an old, run-down 12v battery. I tried
charging it fast and my charger said "fault." Then I charged it slow and it
apparently charged ok. I topped off the water inside. It's 70 Ah, Series
24. I also have a new battery (105 Ah, Series 27) but I'd hate to trash the
old one in case it's still useful.

Can I wire them in parallel to add the currents and have essentially one
monster 175 Ah battery? Or will the old one somehow screw up the new one?
If I can, which should be on the 'outside' and which on the 'inside' of the
circuit, or does it matter? I'd rather not have to switch between them, and
I'd rather not throw out the old unless I have to.

Thanks!