Oh, sure, I've seen several as well.
One right in a dum@ss service manager's face when he hooked a plugged in 150 amp quickcharger to the battery he was hunched over the top of. Good thing there was a garden hose nearby or he wouldn't have made it out of there with any skin on his face.

I am talking about how it affects the ALTERNATOR. Foul didn't blow up a battery - he smoked an alternator. An alternator is designed to charge at full rate amps for a pretty good length of time without overheating, and his baked the paint right off it.

I had one brandy new Optima blue top and one several season old 1000 cca battery I got when I worked at Boat US (don't remember the brand at the moment). But, with the switch on BOTH that day, I got about 12.5 or so at WOT, and less than 12 at idle. Switched to the good battery and got about 13.5 or so at idle, switched to the old one and got about 11.5 or so at idle. Figured it was just down, but it never got better the whole time (I was only out about 20 minutes prop testing some loaners). Shame on me for leaving it on BOTH I guess.

I run with my batteries on both also.

I blew up a die hard in my face when I was jumping a car when I was about 17 years old. It certainly was a die hard, it started the car for about a month after that, and it was an old one out of my mom's car when I got it.

Here is what happens with a dead short. China Syndrome.

Joe,

I've seen posts melt off of healthy batteries, if the cable connection was bad. Happened to a new set of 32 volt Rolls batteries on a friend's boat. One cable was left loose. Several days of starting those big detroits and then no juice. Went down expecting a breaker reset and found a puddle of lead and a bunch of smoke. Could have been scary.

I've got pix of the royal McGyver fix I out on it to get it from Marathon to Lauderdale where they had a rpelacement waiting for us. I'll dig it up for you guys...

The battery on the right was exactly one week old. I'm still not sure what caused it, but I think it was a bad rebuilt starter I bought. I saw the battery post glowing red like a burner on an electric stove. It was damn tough to reach down and take that cable off.

Foul,
Getting back to your original post , since you didn't try it first , the "new" alternator could have been bad. That's the reason the somewhat low volt reading on the dash. As you ran the boat that day , you slowly discharged the batteries till it quit and of course at that point wouldn't restart.
Also since the alternator was so hot , it too , could have had a short.
Best bet??? Change the batteries and the alternator. By now the batteries probably aren't much good and you likely fried the alternator. Any decent auto store should be able to test the alternator if you want to try to save the price of a new one.

OK cuda , just what does cardisoma guanhumi mean???
NO biminis

In a later post you said that the optima showed 13+ volts. The old batt showed 11.5, and both together showed 12.5

This indicates a lame cell on the old batt. Did you check the cells with a floaty thing (a battery tester that checks specific gravity of the electrolyte)? That will tell you a lot.

I know the new battery is good.... I loaned it for a few days to the neighbor to get her truck to the auto repair shop. It worked fine for her. As for the old one, on the garage floor later that same day, it showed 9 volts on my tester. It's toast.

As for the alternator, saying it was "new" is kind of a misleading statement. I put it the last week of April or so, the motor quit running on the 22nd of June. Between those two days, nothing out of the ordinary happened.

Do any of you guys install a "fusible link" between the battery and the alternator? Cars come with them standard... just in case the alternator shorts , the wiring won't melt down. I installed them on my boat... One suggestion.... if you have two batteries and one engine and charging system... run the boat with the switch on "both"... then when anchored switch to "one". Save battery "two" for starting the motor. On twin engine boats.. run the battery switches with one switch on "one" and the other switch on "two". Then switch one to "off" when anchored.