alternator or battery

frosh coach

frosh coach

Member
Apr 20, 2011
237
Long Island Sound, New Haven CT
Boat Info
1997 300 DA,
1994 300 DA (SOLD)
1989 230 (SOLD)
Engines
Twin Mercruiser 350 FWC, 300 HP MPI V-drives
1994 300 Twin 5.7 FWC alpha

Yesterday I took the boat out and noticed I lost my starboard tach, synchronizer, and trim tabs control. After scratching my head for a few minutes I noticed on the depth finder the unit was reading 8.6v and also the VHF was beeping displaying low battery. Also the voltage guage for that engine hasn't worked in a while. Long story short, I didn't turn the battery charger on Sunday when i plugged in to shore power.

The batteries are 2-3 years old. During the ride yesterday (60 mins) i would think the battery would of charged, but it didn't. Is there a way to determine if it's the alternator or battery?
Thanks!
 
Disconnect the suspect battery, check and top off water levels, completely charge, test voltage and let it rest for a couple of hours, then retest voltage (noting any change from your test after charging) and then a good load test. A properly operating charging system is highly dependent on a good battery.
 
frosh coach said:
1994 300 Twin 5.7 FWC alpha

Yesterday I took the boat out and noticed I lost my starboard tach, synchronizer, and trim tabs control. After scratching my head for a few minutes I noticed on the depth finder the unit was reading 8.6v and also the VHF was beeping displaying low battery. Also the voltage guage for that engine hasn't worked in a while. Long story short, I didn't turn the battery charger on Sunday when i plugged in to shore power.

The batteries are 2-3 years old. During the ride yesterday (60 mins) i would think the battery would of charged, but it didn't. Is there a way to determine if it's the alternator or battery?
Thanks!
Click to expand...


Disconnect the suspect battery, check and top off water levels, completely charge, test voltage and let it rest for a couple of hours, then retest voltage (noting any change from your test after charging) and then a good load test. A properly operating charging system is highly dependent on a good battery.
 
Ya, with a volt meter. Test your voltage at the battery with key off. Start the engine and rev it up a bit, check voltage at the same spot.
 
Forgetting to put your battery charger on is not the problem - you didn't do anything wrong there. You should easily be able to go weeks without charging the batteries, assuming no accessories are on. Yes, keeping the batteries on a charger all the time (assuming a quality, smart charger) can extend their life, but the battery doesn't sound like your problem - unless it just all of sudden up and died, which is rare.

Since you didn't say you had a problem starting the engine (and I'm sure you would have), that also leads me to thinking the battery is OK. Besides, if the alternator was working, you would have seen much higher than 8V.

As mentioned, though, it's easy enough to check this with a DVM right at the battery (eliminates possible wiring issues to the dash). You can also check right at the alternator as a definitive check.

Down side... that battery may be toast given how far it was discharged. But it's certainly worth a shot to fully charge it and load test it.
 
Lazy Daze said:
Forgetting to put your battery charger on is not the problem - you didn't do anything wrong there. You should easily be able to go weeks without charging the batteries, assuming no accessories are on. Yes, keeping the batteries on a charger all the time (assuming a quality, smart charger) can extend their life, but the battery doesn't sound like your problem - unless it just all of sudden up and died, which is rare.

Since you didn't say you had a problem starting the engine (and I'm sure you would have), that also leads me to thinking the battery is OK. Besides, if the alternator was working, you would have seen much higher than 8V.

As mentioned, though, it's easy enough to check this with a DVM right at the battery (eliminates possible wiring issues to the dash). You can also check right at the alternator as a definitive check.

Down side... that battery may be toast given how far it was discharged. But it's certainly worth a shot to fully charge it and load test it.
Click to expand...

Thanks everyone I’ll check it out tomorrow.

Lazy daze. I had to use the emergency start to start the boat.
 
frosh coach said:
Thanks everyone I’ll check it out tomorrow.

Lazy daze. I had to use the emergency start to start the boat.
Click to expand...
Ah, well that changes things. It does help to have all the infomation :)

What did the battery show for volts before you started and right after? Since it didn't start for you, I'd assume you checked that?

You could have a shot battery... could also have a bum alternator, too. Either way, doing the things mentioned will help you figure it out. But then there's the question of why that battery was so low. Did it just up and die? Or were things left on? Or is there a short somewhere?
 
Lazy Daze said:
Ah, well that changes things. It does help to have all the infomation :)

What did the battery show for volts before you started and right after? Since it didn't start for you, I'd assume you checked that?

You could have a shot battery... could also have a bum alternator, too. Either way, doing the things mentioned will help you figure it out. But then there's the question of why that battery was so low. Did it just up and die? Or were things left on? Or is there a short somewhere?
Click to expand...

I went down to the boat today. First thing I did was turn off the battery charger. I turned the key to the starboard motor (one having issues) and the Voltameter read 12 V. I turn the key the motor fired right up. At 2000 rpm the meter held at 12 and never moved. The port motor reads 14v when running.
 
frosh coach said:
I went down to the boat today. First thing I did was turn off the battery charger. I turned the key to the starboard motor (one having issues) and the Voltameter read 12 V. I turn the key the motor fired right up. At 2000 rpm the meter held at 12 and never moved. The port motor reads 14v when running.
Click to expand...

As long as the wire harness is intact and not damaged, I’d say you need a new alternator.
 
I'd double check at the alternator with a DVM (don't rely on the dash voltmeter), but it sounds like it's kaput. You still should fully charge the battery (12V is not fully charged... the upper 12V range is) and get it load tested. Better to replace now than deal with it out on the water.
 
If those batteries are deep cycle then 3 years is pushing their life span but it does sound like your alternator is gone. You can measure the voltage at the alternator if it is a two wire setup otherwise at the the battery,alternator should be putting out 13 or 14 volts and battery at full charge at 12.6 volts.
 
frosh coach said:
I went down to the boat today. First thing I did was turn off the battery charger. I turned the key to the starboard motor (one having issues) and the Voltameter read 12 V. I turn the key the motor fired right up. At 2000 rpm the meter held at 12 and never moved. The port motor reads 14v when running.
Click to expand...
Alternator is toast.
 
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