for the second time this spring, i have had a dead cranking battery when i got to the boat ramp.  today, i was able to jump my cranking battery off of one of my trolling batteries.  i drove around the lake for about 10 minutes, i thought that should have been sufficient for my alternator to re-charge the cranking battery, but it was no help at all.  when i tried to crank the battery was dead once again, and I had to jump off again.  is something wrong with my alternator?  this is driving me nuts.  by the way, yes i checked to make sure i didn't leave anything on.  it seems like my starter is just sucking the life out of my battery with every attempt to crank.  i am tempted to take it to a professional and have him change the plugs, alternator, and starter at this point.  Soooo frustrating  >

Let me just say if haven't blown it already, you keep that crap up and you most definitly will!!!!

First the arc created by connecting jumper cables can spike a regulator and blow it.

Second, the charging system in a boat ain't as robust as in your vehicle (which most of your newer vehicle altinators can't handle what you're doing either) and you can very easily burn out your stator and or, rectifier/requlator trying to charge a dead battery with them. That's only about a $1,000 repair bill if it gets both.

Take the battery out of the boat, charge it, test it or have it tested. If it's one you've let sit in there all winter, most likely it's junk anyway and go ahead a buy a new one. The new battery is much cheaper than any part of the charging system you're trying to destroy.

To check and see if it's charging, with a known good battery in the boat, run the motor at approx 2,000 rpm, read across the battery with a digital voltmeter and see if you're getting over 13.8 volts and less than 14.6

Oh, and while you're doing this, clean everything with baking soda and a wire brush to get all metal connections nice and bright before putting it back together.  Then get you some of that protective spary and makes every look red and spray hell out off all the connections back there.

I'm not a mechanic and I have no idea what half the stuff you said means.  I know I have a fuse on my main power wire, so I don't see how jumping off of a trolling motor battery is going to tear anything up.  Wouldn't the fuse prevent anything bad from happening?  

There isn't a fuse between the alternator and the battery, now fuses between everything else yes.  Take ur battery out and charge it, or take it to walmart, autozone, oreilys and have them check it and see if it has a bad cell in it.  Another thing is, like way2slow said, start your motor and see if you are charging by putting a dvom on the battery and if ur voltage is between 13-15 volts your good.

I'm not a mechanic and I have no idea what half the stuff you said means. I know I have a fuse on my main power wire, so I don't see how jumping off of a trolling motor battery is going to tear anything up. Wouldn't the fuse prevent anything bad from happening?

Do you think these guys are telling you falsehoods ?

I am a mechanic and I second what way2slow is saying. The only thing I would like to add is that if you are going to use baking soda around your battery DO NOT get any inside of it or you will kill it. Also sometimes taking to a professional is cheaper than throwing parts at it or destroying good parts by mistake.

Bottom line is there is a 90% chance the battery has died and a 10% chance it is a problem with the alternator.

I'll try to make it simple

Take battery out of boat, run something, a tie wrap, piece of wire, anything, through the eyelets of each lead as you disconnect them and cinch it to keep all leads together for each terminal

Charge battery, let it sit 24 hours and take it to a place that sells batteries and can test it for you. Be sure to let it set at least 24 hour or they may get a false reading as a good battery. If by some chance the battery test good, have them clean the terminals with their terminal brush.

Clean all the terminals in boat with a water/baking soda mix and wire brush until they are all clean metal, no green or white residue on anything.

Reinstall battery, after reconnecting all the terminals good and tight, spray them with some Battery Saver/Protectant.

Now you need to put it on a set of muffs, bring the rpm up to about 2,000 and check the voltage across the battery terminals with a DVM. You need at least 13.8 min and should read 14.4 to 14.6 VDC. If you are not sure about doing this step, you should take it to a shop and have it checked. Just understand, this is the start of their bussy season and it may be their a couple of weeks. Also make sure you have about $500 minimum in your piggy bank if it's not charging and don't be surpised if it ain't a whole lot more.

Actually, it's about a 90% chance you battery is bad and more like a 50% chance you have blown the rectifier/requlator with the bad battery.  Not many boat motors run altinators, they run stators mounted under the flywheel, energized by magnets in the flywheel and then recitified to DC by the rectifier and regulated 14 or so volts by the regulator.  With most of the newer motors, the rectifier and regulator are all one unit.  Some of the smaller and older motors didn't run regulators.

So I need to have the battery fully charged before it can be checked accurately?  

I took it to Auto Zone today to have it checked, but I didn't charge it prior to taking it.  They said the battery looked good, it just looked like it needed to be charged.  

I'm not 100% sure if I have an alternator or not.  My motor is a 1999 150 hp Mercury XR6.

Maybe I'm a dumba... for jumping the battery off the trolling battery.  I didn't know it would hurt anything.  Now that I know, I won't do it again.  

No, I don't think anyone is telling me "falsehoods."  But as I said before, the VERY LITTLE BIT of mechanical knowledge I thought I had, made me think I was safe, because there is a fuse about 3 inches off of my positive battery terminal on my main power cable.

I definitely don't want to tear up my boat.  

I guess now I'm wondering, if I have torn up my regulator (whatever that is), what does that mean?  Am I just going to have to start charging my cranking battery after each trip, like I do my trolling batteries?  I'll take a voltmeter with me next time I go fishing to see what it says.

Yes, battery needs to be fully charged to properly check it.

Even if it were fused, which it's not, fuses make no difference in this case.  Fuses only protect against current overloads, not voltage spikes.  

No, your motor does not have an altinator.  If it has a tach and the tach works fine, that narrows it down to a bad battery, possilbly bad/dirty connections, or the regulator.  I haven't bought a Merc regulator in a long time but you can figure close to $200 for the part and $100 - $150 labor to check and replace it.

Junebug, for all the worry you are causing yourself you need to take it to a mechanic. Everyone is giving you good advise; however, it is impossible to diagnos your problem over the net. Too many variables with electrical issues. It will be cheaper in the long run to have it checked out by a professional. Do not just start changing parts to solve the problem.