Alternator/Battery

[ronjon1944]

being my first year at boating with a merc i/o 3.0, I charge the battery before I go out(13+ volts). after the eng. warms up the volts drop to 12.2- 12.4. I thought that my alternator was going bad. A friend who has had many boats said unlike autos the alternator is only going to put out 12 volts and the eng. will start at 10+ volts. Not wanting to get stuck on the lake I did not use my fishfinder or radio.

 

[WillyBWright]

Re: Alternator/Battery

I'm afraid your friend is mistaken. It should charge in the 14v to 15v range and a healthy battery should maintain between 12v and 13v. Sounds like your battery is good and the alternator isn't. Probably bad diodes. That happens from loose or corroded battery connections, or from accidentally connecting the battery backwards (even for a split second). They can be rebuilt anyplace automotive alternators are. But some shy away from marine stuff, so call around. Do NOT be tempted to use a car alternator or your boat could go BOOM.

 

[KaGee]

Re: Alternator/Battery

Ron,<br /><br />I would not jump to any conclusions based on the readings from your dash voltmeter. You need to go and get you an inexpensive Digital Volt/Ohm meter available at Radio Shack, Lowes, or Sears.<br />Connect the meter to your BATTERY and then check the readings. If an abnormality is found, you can have both items easily checked at Auto Zone or similar place.<br /><br />Also as a matter of maintenance, you should clean all of your main connect points i.e. battery posts, ground points on the block, 12 volt connection on the starter.<br /><br />I am a firm believer in the multi-battery system. I would add another battery with a switch. That way you always have plenty of juice for electronics and a starter backup. You can alternate which battey you use which keeps them charged, or you can go all out and buy a battery isolater (expensive!) to do the job for you.

 

[Don S]

Re: Alternator/Battery

A battery with only 10V in it will not even turn over an engine, let alone start one. Testing like KaGee mentioned (at the battery with a DVM) is the only way to find for sure the voltage going to the battery. Also check at the alternator B+ terminal (Big red wire) to be sure there isn't a connection between the alternator and the battery that is bad.<br />Did you check the alternator belt? If it's loose, it won't spin the alternator all the time and you will get low voltage.

 

[tommays]

Re: Alternator/Battery

just so you no a brand new battery will hold a 12.8 volt charge or very close as the battery ages and gets to were it will only hold a 12.5 volt charge its pretty tired and will crank a motor very slow because it at that point it delivers way less cranking amps<br /><br />tommays

 

[ronjon1944]

Re: Alternator/Battery

The battery holds a good charge,have checked it after 24hrs with no drop in voltage. belt is tight, every connection is clean and tight. eng. starts fine with no drag (slow). What willybwright said about the alternator should be charging at 14+ volts is where the problem is, and mine is only doing 12 volts.<br />I don't know where my friend came up with the 12 volts is all the marine alternator will put out.

 

[Boatist]

Re: Alternator/Battery

Ronjon<br />I agree your friend is very wrong. The batteries are no different than a auto battery. They may be a more rugged but the same otherwise. A 12 volt lead acid battery at 70 degrees should be 12.6 volts. 2.1 volts per cell times 6 cells or 12.6 volts. When you start your engine your battery voltage should jump to 13.8 to 14.8 volts in a few minutes the same as a car. Some newer type alternators will charge to 14.8 volts for a short time then as things heat up drop the voltage down to 14.2 volts. Anytime your battery drops below 12.6 volts with the engine running the battery is discharging not charging. Most 3.0L I/O engines have at least a 35 amp alternator and many as high as 65 amps or more.<br /><br />On boats I think the number one killer of alternators is battery switches. Many battery switches if turned with the motor running will kill your alternator. The best type switch has a alternator field disconect circuit to protect the alternator if wired correctly. Next best battery switch in call a make before break. It connect the second before breaking from the first battery and as long as the switch is good and you do not turn thru the off setting it will not hurt your alternator.<br /><br />As Willy said a loose or corroded battery cable can blow output diodes, the regulator or the Diode triode inside the alternator. When the connection is broken to the battery the output voltage will go so high in the fraction of a second it will blow the output diodes the same as on a car.<br /><br />If you have two batteries I like a battery isolator to charge both batteries and protect the alternator. With the isolator connect you would have to disconect both batteries to blow the alternator. They do not have to be expensive. If the marine dealer is too high try a RV shop or automotive store. Key in getting a battery isolator is to get one big enough for your alternator. Alternator rated at 35 amps get a 70 amp isolator. alternator rated at 100 amps get a 200 amp isolator.<br /><br />Check you voltage at the battery with the engine running. If less than 13.8 volts with motor running at a high idle then pull the alternator and take it in and have it tested. You can use most automotive parts in side the alternator. The diodes, voltage regualtor, but the case of the marine alternator has special screens to prevent sparks inside the alternator from getting out of the alternator where it could cause a explosion and fire.<br /><br />Good Luck

 

[Dunaruna]

Re: Alternator/Battery

Originally posted by ronjon1944:<br /> The battery holds a good charge,have checked it after 24hrs with no drop in voltage. belt is tight, every connection is clean and tight.

Its already been mentioned but worth repeating - a tight and clean connection does not alway mean a good contact especially with earth connections - remove, scrap/wire brush and reconnect. Its also worth checking each connection with an OHM meter, quick and simple.<br /><br />Aldo

 

[ronjon1944]

Re: Alternator/Battery

Thanks Everyone... as always you have been a great help.

 

[rodbolt]

Re: Alternator/Battery

what!!!!!<br /> automotive parts<br /> where is capt bligh and a cat o nine tails <br />but the advice so far seems good. a few minutes with a DVM is worth more than parts changing.

 

[KaGee]

Re: Alternator/Battery

Awwwww....! Boatists said..."Automotive Parts"....snick!<br />

 

[collawash]

Re: Alternator/Battery

I just dealt with a friend's boat with the same powerplant. May I offer an inexpensive advice? Check all the wiring, look at the insulation, ring terminals, butt comnnectors, etc. Remove everything!!! Use a Dremel tool with a wire brush and clean every single post, connector, ring terminal, alternator terminal, battery post and then reassemble. Run your tests again and you might be pleasantly surprised! He now charges at a consistent 13v and his single battery is always at 12.5v. An hour of time, $2.00 bottle of liquid electrical tape and a flashlight and patience saved my friend's wallet. <br /><br />Ditto on the 2nd battery option and always carry a multi-tester on board. Good luck.

 

[KaGee]

Re: Alternator/Battery

WOW! only $2. for liquid tape? It's $5-$6 around here!

 

[collawash]

Re: Alternator/Battery

My wife finds this stuff at the $1 store. I have more liquid and regular vinyl electrical tape than I will ever use!