Had something weird happen today on the lake. I put the boat in and it started up just fine. We fished for a few hours making several stops (engine off). While we were at one spot, I looked over at my fishfinder/GPS and it was starting to fade out until it turned completely off. I tried to turn it back on and it would immediately turn off. Suspecting a battery problem, I tried the trim and it was very weak until it also quit. Needless to say, the engine would even turn over.

So, we switched the engine cables over to one of my trolling motor batt's. It started fine and we fished a few more spots. The first strange thing I noticed is that the fishfinder also started working immediately.

When i got home, I switched the engine cables back over to the cranking battery and the engine turned over just fine!

I'm at a loss here on how to explain what happened. Any ideas???

Bad connection? Are your. CAble ends corroded, nuts tight

No corrosion. I thought about the bad connectin thing too. But if there was a bad connection or a loose nut, then I wouldn't have had any power at all. At least thats my thinking. It was almost like the battery drained itself within a 20 minute stop and then revived on the way home.

By the way, its a brand new battery.

Defective post on the new battery?

One of your battery cables is probably loose...giving power sometimes, and nothing at other times. You can get a replacement clamp at a parts store.

So you moved the engine cable but left the accessory wire on the starter battery? Take the battery and have it tested. A voltmeter would tell you pretty quick by monitoring the voltage at the posts while under load.

Correct, I only removed the thicker engine cables from the cranking battery and moved them to one of my trolling motor batt's. So, that left me with the following:

Trolling Batt #1 - trolling motor wire, 3-bank charger wire and engine cable
Trolling Batt #2 - trolling motor wire and 3-bank charger wire
Cranking Batt - probably 3-4 other wires/connectors from various onboard systems. Honestly, i haven't traced all of them to see where they go. But, i know for sure that one is for the radio and one is the 3-bank charger.

So, I'm not sure whether or not the fishfinder wiring is tied in with the engine cables.

As for a loose connection, the nut was on there tight enough to require a pair of pliers to get the nut off. I'm not saying that it definetly isn't the problem, but i highly doubt it.

Sounds like loose cables to me.

I would get the cranking battery tested. If it tests good then it was likely a loose connection.

Had the same issue today!

I had the 3-bank charger plugged in over night. When I put the boat in the water, it started up just fine. I tied it to the dock and turned the engine off while I went and pulled the trailer out of the water. When I came back, the battery had all died. As I did before, I put the cranking cables on one of my trolling motor batteries and it started just fine.

The difference this time was that when I got home and switched the cables back, the cranking battery was still dead.

So here is my question - what should be attached to the main cranking battery? Cranking cables and on board charger cable only? I'm wondering if I accidentally added a connection to my cranking battery when I moved things around during a battery replacement.

I wake up this morning and the "Go Fish" light is lit up on my on-board charger telling me that all 3 batteries should be good. So I go turn on the trolling motor and the thing will barely turn.

At this point, I'm all out of guesses.

Its time to stop guessing and get some on-site help other than a forum board as you may have more serious issues now than just a boat motor not cranking.

When you swap a trolling motor battery to temporarily replace a cranking battery, you need to 1st disconnect all cables (red and black) to each battery, THEN switch the batteries...moving all of the power and ground cables.... the motor power, console power, bilge pumps and any/all the associated grounds.

Stop moving only the motor starter + lead and not moving its associated ground and the console power and other circuits.

The series wired trolling motor batteries should always have only the trolling motor, another trolling motor battery and their associated on-board charger leads connected.

If your setup did not already have the trolling motor ground connected with a heavy gauge cable to your cranking battery ground, you may have already damaged the trolling motor/US2 circuit by getting powers and grounds crossed.

Thankfully you did not have a battery explode or a fire occur already.

first thing is go get the battery load tested, you may have a bad cell, this would let it charge but not hold a full charge. chargers would read it as good to go.
take a hydrometer to each cell if you can, wal mart sells em for about 3 $ if ya need one.

with all hooked up normal plug in your charger, it should read over 13 volts on all 3 banks while charging (use a voltmeter), if its reading good unplug it then change one bank from trolling motor to start battery and plug in charger again, recheck output.

Lou - as a EE, I certainly appreciate your input. But, I'd like to try to get this fixed without having to drop a few hundred bucks at a marine repair shop.

I just put a volt meter on all three batt's. here is what I got:
Cranking - 13 volts without charger on, 15 volts with charger on
Trolling #1 - 13 volts without charger on, 15 volts with charger on
Trolling #2 - just over 6 volts both with and w/o charger

Don't know if it matters or not, but Trolling 2 is the batt that has the in-line switch connected to it (kinda looks like a small breaker). I checked this switch for continuity and it is ok.

I just recently bought a INtEX trolling motor 40lbs transom electric motor from rubberboats.com (they were of zero help to my problem) I have purchased three different batteries now over the last week. The problem Im having is the "battery meter" on the motor will light up full charge then go down to half within 10 seconds. The first two batteries were both marine batteries (cold crank) with over 80 amps per hour. I just bought a brand new marine "deep cycle" with 750amps per hour thinking this would solve my issue. It is doing the same exact thing soon as I hook it up it shows battery strength at half charge. Don't know what to do. The rubberboat website and the INTEX manufacturer say the batteries are the problem (but three batteries now). Just trying to have some fun with the kids and its becoming a headache.....any help??.....
Thank You!