So, sitting around ready to go work on the boat, and wanted to post this question.

Would a dead battery have an impact on engine rpm?

So, here is the situation...last ride of the year last fall, so no time to verify if the issue went away. Went to put the boat in and turned the first key....nothing. Crap, left the radio on when stored a few weeks earlier. Battery is dead a a door nail. Flip the switch to join the batteries, and it fires right up. Fire up the other one, and we are good to go for the day.(I know you should not use the alternator to charge...but was not going back to charge the battery). During the day, I tried separating the two batteries, but if there was a large load applied, such as a spike when the silent choice is turned on, the engine would die. Due to this, I left the batterie switch open between the two. When cruising at around 3800 rpm, I noticed that the engine with the dead battery would run 200-300 rpm lower than the other. It took a lot of throttle to get them synched...maybe 20%more.

So, now the year is going to get started, and we will see, but thought someone would know, or at least make something up

I could think that the voltage could be down at the tach...therefore there really was not an issue, the voltage dropped the spark somewhat resulting in the loss, or the engine was hurt in some other way.

Oh...the engines are stock 6.2 ECTs...catalyst engines.

Any ideas? Always curious to hear how things work.

Thanks.

Yes ! And be careful, if that dead battery is shorted internally, you can fry some stuff, alternator, etc,etc.....

And yeh, can cause performance issues even on a carb motor that has very little electricity demands.

If you have running issues when using drive trim, trim tabs, silent choice, or etc, switch to another battery instantly and when you get back, charge and test the 'dead battery.'

Battery is part of the complete electrical circuit. If the battery is shorted or won't hold a charge it stresses the other components.

you would have a warning horn going off for LOW Battery voltage and you can scan the engine to see if that fault code was set back in those hour timeframes of running the engine(s). Also if the PCM can not see at the bare min 9.8 volts it shuts down and nothing happens but a stored fault code. Low voltage does stress systems on the engine for jist of info.

By chance you do have a working warning horn per engine correct on these CAT engines ???

While I did not do any scanning of the engine, I did run the boat this last weekend and the problem is gone. Both engines are very close in rpm at various throttle positions. WOT is also very close.