Charging boat batteries overnight?

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Dworwood said:
I did a quick search and found a 50 alternator for that but I would check with Mercury to know for sure.
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Dumb question, but if my motor has an alternator on it. Wouldn't that already be charging the starting battery when it's running? Or does it not work like that?
 
Paul_G said:
Dumb question, but if my motor has an alternator on it. Wouldn't that already be charging the starting battery when it's running? Or does it not work like that?
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You are correct. The DC alternator uses the excess power produced by your motors alternator to charge your electric trolling motor battery bank.
 
Paul_G said:
Dumb question, but if my motor has an alternator on it. Wouldn't that already be charging the starting battery when it's running? Or does it not work like that?
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Not a dumb question, but you try to isolate your starting battery so it always has enough power to get your engine started and that is all your big motor is charging at the present time, probably, If it was tied to your trolling motor batteries and you ran them dead you would be in trouble as that would run your main battery dead as well without an isolation device. This charger isolates the different battery banks and charges them in order of importance, meaning the starting/main battery first and once that is full it switches to your other bank of batteries. Normally my starting battery doesn't get too low so most of the time it is charging my TM batteries when I am running the big motor. I believe the minn kota requires at least a 30 amp alternator for the charger to work so if your motor has a 50 amp alternator it would work on yours too. I actually have a back up starter battery and a switch for either battery 1, battery 2, or both together so I have three banks that I charge off the big motor. I hope that all makes sense and helps!
 
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Assuming my motor does have an alternator I think it may be broken? Yesterday I tested the voltage of the starting motor battery before and while the motor was running. The voltage actually started to go down while the motor was running. Not up as if it was charging. Thoughts?
 
With your on board chargers, you do need to pay attention to you starting batteries. Both of my boats have duel batteries for the boats electronics, radio and such. I also have a 24 volt trolling motor, with the boat having an on board charger. So, it charges what ever batteries need it the most. If I have it right. I have found if you Flip the switch to only one battery for the boat systems. It keeps that one completely charged. The trolling motor batteries are deep cycle group size 34. The on board charger extends the trolling time on the batteries, but on a four day trip they are getting really low. So, don’t just depend on your alternator to keep them charged. I bought a cheap Westinghouse, 2000 watt generator with a noco 30 amp charger specifically for my trolling motor. There are times where I keep it on the boat depending where I am going. That set up still takes four hours to get up to a full charge form a half charge.

I wish my boat was big enough that I wouldn’t look like a complete bozo by putting a wind generator on it.
 
Paul_G said:
Assuming my motor does have an alternator I think it may be broken? Yesterday I tested the voltage of the starting motor battery before and while the motor was running. The voltage actually started to go down while the motor was running. Not up as if it was charging. Thoughts?
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You may want to have a mechanic check it. What size motor do you have? It could be one of several things that's not working. And depending on what kind you have you may have to up the RPMs to see a voltage increase.
 
Dworwood said:
You may want to have a mechanic check it. What size motor do you have? It could be one of several things that's not working. And depending on what kind you have you may have to up the RPMs to see a voltage increase.
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It's a 200. I called Mercury this morning. I have a stator setup with a voltage regulator on my motor. I'll run the motor at a higher rpm as they said the same thing. That at idle it wouldn't be putting out more than 12.5v.
If the regulator is bad I've found one locally in stock so that's good.
 
Paul_G said:
Assuming my motor does have an alternator I think it may be broken? Yesterday I tested the voltage of the starting motor battery before and while the motor was running. The voltage actually started to go down while the motor was running. Not up as if it was charging. Thoughts?
Click to expand...
I am pretty sure all motors with electric start have an alternator, otherwise your starting batteries would only be charged when connected to an AC charger. On my boat, the voltage reads just above 12V when I am idling, but anything above 2K RPM it goes up to 13+ volts.

I have a 90 HP 2 stroke 2003 Mercury outboard, which I believe only has a 30 amp alternator. I put the MK onboard charger on and it definitely keeps the trolling motor batteries topped off. I am not sure how long I would have to run around with the main motor to charge the trolling motor batteries if they were mostly drained, but the onboard charger seems to prevent this with intermittent use.
It is pretty simple to install, the only thing that requires a bit of work is if your trolling motor batteries are up in the front of the boat, you will need to run additional wiring back to the charger (the leads that come on the onboard charger are not that long).
 
Well, $450 dollars later my motor is charging my starter battery again.

Turns out my motor has two voltage regulators I had to replace. Yay for me. But I'm officially ready for Powell now.
Also bringing a couple 100' cords to plug in the boat at the hotel if they let me. :D
 
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