Join Date: Feb 2011
Location: San Francisco Bay area
Thanked 55 Times in 55 Posts
Rep Power: 5
Re: Enging/starter battery pretends to be good but....
Just wanted to mention my experience last weekend - the engine started great on bank 1, but not on bank 2.
I check the bank 2 batteries, they measured fine. But still the starter went grrrrr grrrrr.
So I selected bank 2, and switched on every light in the boat, so I was pulling about 20 amps. I then started measuring voltage drops. Luckily, I started at the battery end.
I had a drop of 0.5V between the battery post, and the the cable clamped to it. Also feeling the cable clamp showed it to be getting distinctly warm, just at 20A draw.
So a drop of 0.5V at 20A would be a drop of 5V at 200A starting current. So that's why the engine was not starting. BTW, at that kind of resistance and voltage drop, something in your system should be getting VERY hot.
A cleanup of all the battery connections and application of some dielectric grease and it starts up perfectly.
BTW, a good habit when working on 12V systems especially around batteries is to take your watch off, especially if it has a metal strap.
Bristol 31.1, San Francisco Bay
Last edited by MarkSF; 09-05-2012 at 02:29 PM.