How to test a boat motor alternator / battery

OK so I’m not mechanical and am poor with electrical issue. Is there a easy way to check to see how well you boat motors alternator is working. Can you watch the voltage reading on your lowerance and Tell. I’d think the reading should rise once the motor is running and rise even more once its under full power. If that correct – What should the voltage read before starting and how high should it go when at full power. The reason I’m asking is that I’m having a few Issue’s with my Battery/ Graph. My graph seems to work OK after a full charge but as the day goes on it seems to loose the reading more and more. It also sometimes turns off when I start the motor. My thought is the cranking batter is failing ( 3 season Old )It seems to charge and hold a charge. I think I’m going to just replace it but would like to make sure that my Alternator is puting out a charge. I know this has been discussed here several times but what cranking battery would you buy? Oh by the way the everything other than the Trolling motor is powered by the cranking battery. Thanks in advance for all of the replys

My bet is the starting battery is about shot.

The graph shutting off during starting is a classic symptom.

Particularly if you have a motor with a high Cold Cranking Amp requirement (for example an Optimax).

You could pull the battery and take it to Batteries Plus, Fleet Farm,etc. and have it load tested.

Check your manual for the CCA requirement before you purchase a new battery.

Good Luck!

My boat is in the shop right now because it quit producing electricity. I dont know the device names on an outbord. Coils, stators, etc. My pontoon motor did it last year. You definitely can you use your graph to troubleshoot. Turn on graph and observe voltage. Should be 12v. Whatever it is start your outboard. Voltage may dip for a second but should come back at 13 or 14 volts. Rev motor a little to see if that picks it up. If you dont see the voltage increase or actually see it go below 12 then your whole operation is feeding off battery and you will drain it and eventually get stranded or have to use a trolling battery to get home.

I have a 1990 Merc 60 horse. When it’s not running the voltmeter in my depth finder indicates about 12.5 volts. When the outboard is running (and the alternator should be charging the battery) the voltage looks like 14.2 volts. Hope this helps!

Rootski

If the motor is not charging the battery it will go dead. You mention your graph is OK after a full charge. Does this mean you are using a battery charger? If so, why? If your battery is going dead it’s either a bad charging system, a bad battery (will not hold a charge) or a current draw when the boat is not running. If you have a voltmeter built into your sonar and you see a voltage increase after starting the motor the charging system is likely OK. Another simple test is to turn on your boat lights and see if they get brighter after you start the motor (a voltage increase). If the motor cranks after a day on the water but then not the next time out it’s likely a bad battery. If you have the patience you culd disconnect one side of the battery after coming in. Reconnect next time out. If it does not crank it’s the battery, if it does crank it’s a current leak (or something left on like a GPS antenna).

With all the things pulling a charge off the cranking battery ( Motor starting – 150 HP, Graph, Livewell pumps,Ect. ) I like to put a charger on it after each outing – sometime i don’t run the big motor all that much to have it keep the battery fully charged off of it. My guess is the battery is getting bad. I have a master switch that SHOULD kill all power but I guess that something may be drawing current when not in use. I try to make sure I turn all switches to off in addition to the main power switch each time I store it. I even hooked the GPS Ant. to the main power switch because I was told they have their own power source and will draw power even after the unit is turned odd. With 3 years of use and the cost of a new batter I think my best bet is to just get a new one.

From my experience the graph is rather poor at true volt readings, a cheap simple volt meter is best (under $10. At Harbor Freight, I bought one just to leave in the boat)

Sitting with now draw, and full charge you should be in the higher 12v range, a charger/engine running should bring you to the 14.2-14. 4v range.

Couple more things Lots of places test batteries bring them the battery or the whole boat they can test it, not always super accurate inho, but if they say it’s bad its bad, if they say it’s not, it, may still be bad enough to replace. Secondly have you checked your batteries water level? If not I’ll bet you are low and that causes problems too. Generally speaking with good use of have to add twice a summer with the charging in between. a charger/engine running should bring you to the 14.2-14. 4v range.

Couple more things Lots of places test batteries bring them the battery or the whole boat they can test it, not always super accurate imho, but if they say it’s bad its bad, if they say it’s not, it, may still be bad enough to replace. Secondly have you checked your batteries water level? If not I’ll bet you are low and that causes problems too. Generally speaking with good use of have to add twice a summer with the charging in between.

e running should bring you to the 14.2-14. 4v range.

Couple more things Lots of places test batteries bring them the battery or the whole boat they can test it, not always super accurate inho, but if they say it’s bad its bad, if they say it’s not, it, may still be bad enough to replace. Secondly have you checked your batteries water level? If not I’ll bet you are low and that causes problems too. Generally speaking with good use of have to add twice a summer with the charging in between. a charger/engine running should bring you to the 14.2-14. 4v range.

Couple more things Lots of places test batteries bring them the battery or the whole boat they can test it, not always super accurate imho, but if they say it’s bad its bad, if they say it’s not, it, may still be bad enough to replace. Secondly have you checked your batteries water level? If not I’ll bet you are low and that causes problems too. Generally speaking with good use of have to add twice a summer with the charging in between.

Sorry you get a double posted IDO and my phone’s browser aren’t playing nice lately….

Some of the above advice is 1/2 true. Check the voltage a day or two after a full charge and before you go out. 12.5v is only 1/2 charged. It should be more like 12.8+.

If your outboard charges from a stator, it doesn’t put out a lot of amps, especially at idle. I checked mine and the volts go up a couple of tenths at without reving the motor. An alternator should put out a lot more. \

It certainly doesn’t hurt to charge the starting battery. Especially if you are running stuff off of it.

Just picked mine up. It was the voltage regulator. First one that Zuke shop has seen on my model of engine. I have identified this issue with two outboards in the last year with graph alone. I never pulled out the meter.

Thanks for all the rsponse. I had a load test done on the old battery and they said it was bad. Put a new one in last night. It took a few Hr’s at 2 Amps to charge fully up – will be out a few hr’s tonight and will see if the new one helps with my Graph issues – if not I may need a new transducer – I hope not.

What about your voltage with engine on? Have you checked it yet and ruled that out? Both times I had this issue it started with replacing the starting battery and wondering why it was failing.

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What about your voltage with engine on? Have you checked it yet and ruled that out? Both times I had this issue it started with replacing the starting battery and wondering why it was failing.

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The Gage on the Graph did go up to near 14 volts when under heavy power so I think the alternator is putting out a charge – I just think the batter was not accepting it well – Got a new battery in there now – I guess we will find out next time out if it works better. The place where I had the old battery tested said it showed a bad cell.