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This comes from my son who has a 23' Cobalt IO with two batteries and is something that surprised me. He was at the boat ramp and had the blowers on for about 20 minutes, radio on and used trim. All this while boat was just iding at around 750RPM. The bottom line is that one of the batteries [8 months old] died before he could get out of the ramp area. he had a mechanic look at it that told him the alternator and charging system checked out fine and told him that "boats" ,for some reason unlike cars, have a control that prevents the alternator charging until the RPM increases to about 1500. Is this guy smoking stuff or is there something about boat engines that requires alternators not to charge until a higher RPM?
May or may not help, but I was told on the three larger outboards I have owned that there is a low-current charge at low rpms's, and it increases once above 2k rpm. Not sure this will apply to an inboard/outboard motor charging system.
I would like to add that even though the battery is only 8 months old, if it went through a prolonged cold spell discharged it may have caused what seems like an early demise.
That's definitely not true. That said, most boats don't have the high output alternators that cars now do, yet the electronic loads have been increasing quite a lot. At low rpm the alternators don't put out very much - maybe 10 amps and that can be used up by the engine and other loads. Your son may also have started out with a partially discharged battery - especially if it was sitting in the yard or marina unused for a while and most especially if he had those two batteries connected together while the engine wasn't running. (No two batteries are *exactly* alike and whichever one has higher voltage will discharge into the lower voltage one.)
(No two batteries are *exactly* alike and whichever one has higher voltage will discharge into the lower voltage one.)
Ken
If two batteries are in parallel and the generator voltage is higher than either of the two, how is the battery with the higher voltage going to discharge into the one with a lower voltage?
If you have either a digital amp meter or digital volt meter, you will see that there is very little charge at idle. The blower, Radio etc may well have been enough to drain a battery (especiallly since many boats have small group 24 batteries--which come in several AMP hours/MCA sizes). You have to get many engines up to 1500 or so before the voltage comes up above 13 volts and is really starting the bulk charging phase. There is no "control", it is just that an alternator has to obtain a certain minimal RPM before it puts out significant power. In the car when the AC is on, the idle is upped to allow better alternator output.
What kind of engines? An IO has sort of a automotive engine with a regular alternator, right? Cars dont die at idle. Suzuki 4 strokes have alternators with voltage regulators. Mine does some charging even at idle. What is the current drain of your blower? A group 24 battery should be somewhere in the range of 40-50 Amp Hours (if charged!!) -- doubt your blower pulls more than 10 amps -- should run for hours....radios, at full power transmitting, should be in the range of 4 amps each max. Time for some measurements. AT idle, with a charged battery, measure the voltage. If above 12.0 or so, charging is going on . Optimally, should hit 14.2. Turn on blower, radios. If drops below 12.5 or so, you are marginal at that point and a nearly dead battery would then go on to die. Had an old two stroke that turned out to have dead diodes in an unregulared alternator setup. Owner THOUGHT it was charging--no charge at all. Voltmeter or ammerter can tell the story.
What kind of engines? An IO has sort of a automotive engine with a regular alternator, right? Cars dont die at idle. For what it is worth, if it is a car-type alternator, the regulator varies the field (electromagnet) current so that the system can generate significant power at lower rpms. Outboard altnerators are different -- I think most just swing magnets (no ability to chnage the permanent magnet's strengh) past coils to generate voltage. Since voltage is proportional to the rate of change of the magnetic field (one of Maxwell's 4 laws) the power output is much more dependent on the rpm. Suzuki 4 strokes have alternators with voltage regulators. Mine does some charging even at idle. So whether or not an automotive versus an outboard type of alternator is important.....if automotive, would suspect the voltage regulator....
What is the current drain of your blower? A group 24 battery should be somewhere in the range of 40-50 Amp Hours (if charged!!) -- doubt your blower pulls more than 10 amps -- should run for hours....radios, at full power transmitting, should be in the range of 4 amps each max. Time for some measurements. AT idle, with a charged battery, measure the voltage. If above 12.0 or so, charging is going on . Optimally, should hit 14.2. Turn on blower, radios. If drops below 12.5 or so, you are marginal at that point and a nearly dead battery would then go on to die. Had an old two stroke that turned out to have dead diodes in an unregulared alternator setup. Owner THOUGHT it was charging--no charge at all. Voltmeter or ammerter can tell the story.
(No two batteries are *exactly* alike and whichever one has higher voltage will discharge into the lower voltage one.)
Ken
If two batteries are in parallel and the generator voltage is higher than either of the two, how is the battery with the higher voltage going to discharge into the one with a lower voltage?
As I said - when the engine ISN'T running this will happen. When the engine is running both batteries should charge normally.
Hello folks,
I am the son who had the problem with the dead battery. Thank you all for your input however, I am still puzzled by this experience.
Here are the facts:
Boat is a 2002 Cobalt 262 with a 375 hp volvo 8.1GI i/o motor which has two batteries both less than a year old. One is a cranking and the other is a deep cycle. They have not been left in cold storage over the winter.
I Started the boat up in the water at the marina after blower being on for twenty minutes. Battery switch was moved from off to number 1 battery before start. Engine starts on first attempt and idles at 750 rpm. Radio is turned on. After wife and crew are loaded on boat after about 5-10 minutes, we motor out of marina with rpms never going above 1000. I begin to trim the outdrive to the full down position and the power-including engine cutout for a second every time I click the trim button. This tells me the engine is running off the battery. With the engine off, I switch to the deep cycle battery and start the engine. No further problems. After the day is done, I have the marina check the alternator, and load test both batteries. No problems noted. Marina said alternator was putting out 66.4 amps. Alternator when connected to system kept charging batteries between 12.15 volts and 12.18 volts at idle.
I have not had any further problems since then. Dealer told me that I need to go above approximately 1500 rpm due to alternator "relay switch" after start and can then back the idle down. Otherwise, engine, blower, radio, trim, etc. are all running on battery which will deplete quickly without alternator charging. I told him I had never heard of this but would ask around. Thanks again and great forum. (long time reader, first time poster).
JCBIII - 8/16/2007 10:30 AM Dealer told me that I need to go above approximately 1500 rpm due to alternator "relay switch" after start and can then back the idle down. Otherwise, engine, blower, radio, trim, etc. are all running on battery which will deplete quickly without alternator charging. I told him I had never heard of this but would ask around. Thanks again and great forum. (long time reader, first time poster).
This is the most ridiculous thing I have ever heard. Obviously, he does not know how an alternator works and this is how he looks at it in his own mind which is fine.
Now to your issue. When you say everything cuts out for a second - does the engine quit or do you just hear a slight hesitation when you hit the trim switch?
If its a slight hesitation, its probably due to a slightly weak battery or the cables or connections coming from it. You see, at low rpm, the engine's alternator does not put out very much. When you hit the trim switch, you're switching on about 10-30 amps of additional load plus since its an electric motor, there will be a much greater start-up current surge. The battery has to make up the difference between what the alternator is putting out and what the trim motor is asking for. If the battery - OR ANY PART between the battery and the motor - is weak, you'll get a major drop in voltage all through the system because the alternator just can't put out that kind of power at low rpm.
Since it does not do this with the deep cycle battery in the circuit, look for the problem to be *anything* from the positive or negative posts of the starting battery to the point where things become common between the two batteries - it could even be the battery switch. And even though the battery was load tested, I would still try that first. The easiest thing to do there might be to swap batteries and see if the problem follows the battery.
Great input. Yes, the engine would totally quit if I held the trim switch for more than a second.
I'll look for the "weakest link". The terminals are all in great shape. I'll check the perko battery swicth for any loose connections. Nothing looks out of place. I will try swapping the batteries if the problem does persist.
One thing I do now after this incident is start the engine and let it run at about 1500 rpm while at the dock. What do you think???
"Alternator when connected to system kept charging batteries between 12.15 volts and 12.18 volts at idle. " At 12.15 volts the alternator is NOT charging. A lead acid battery at 12.24 volts is 50% discharged (or had a large draw). You have to get well above the battery voltage to begine charging. You want the voltage to be up at least 13 volts for charging. Charging should max out in the 14.4 volt range.
It is possiable that the batteries have been run down at some point and damaged. I never let a battery get below 12.2 volts (resting state). If you frequently discharge them to 12.0 volts, it will damage the battery.
Do you have a voltage sensitive relay in the house battery charging circuit? That has to have voltage over 12.7 volts to connect the battery to the charging source.
I would have load tests done on the battery. If these test OK, then find someone with an ammeter which will measure the draw from the battery (it will probably be more than the 10 amps you can test with most digital volt meters (a shunt will work to test the draw--and that is what I have on my boats to test the amp draw continously). Do you have an amplifier on the radio? This can significantly increase the current draw. I would also look for other current drawing items you are not aware of.
Trim motors can also have high draw--but if the battery voltage is already low, the current draw will be even more. (E=IR). There has to be more "diagnostic work--I would not run the blower more than 5 minutes, and be sure that the voltage is up while the boat is idleing. The analogue volt meters are not accurate enough; put in a digital volt meter or buy one of the small cigarette lighter digital voltage indicators which read to .01 volts.
This is the Dad again who apoligizes for cutting 3' off his sons boat! I wanted to insert one comment on something I found out quite by accident with my Caddy when I was having some problems that seemed like they should be battery related. We did a load test on the battery and it checked out fine. I won't bore you with the rest of the story but in the end it was the battery after all. I was informed by a battery manufacturer that in some cases an old battery will test OK in a load test but tends to lose the charge quickly. In other words a load test does not "always" give a 100% accurate indication of battery condition.
"Alternator when connected to system kept charging batteries between 12.15 volts and 12.18 volts at idle. " At 12.15 volts the alternator is NOT charging. A lead acid battery at 12.24 volts is 50% discharged (or had a large draw). You have to get well above the battery voltage to begine charging. You want the voltage to be up at least 13 volts for charging. Charging should max out in the 14.4 volt range.
This is an excellent point which I missed. Your batteries are NOT being charged unless you have somewhat OVER 13V at the battery terminals. Since the boat seems fine with the deep cycle battery switched in, I wonder if this could simply be a partially shorted starting battery. This sort of thing does happen. If the battery is internally shorted, it certainly could hold the voltage down even when a good alternator is trying to charge it.
I think you're on the track. Post back what you find.
alternator charging and RPM 
