Outboard motor overcharging.

This problem comes up over and over again in the newsgroups and boating boards
and NO ONE has answered it...
A lot of older outboards came with un-regulated charging systems and I have
I read about people reporting that their voltage had risen to 16+ volts. Last
year after over TEN years of correct voltage I bought a new battery and mine
started doing the same thing. I verified this with the boats volt meter gauge,
a Fluke digital VOM and a Simpson 260 analog VOM. There is 16 volts there.
Everything in the charging system is within specs. I have even replaced a lot
of the system and its still 16+ volts. After not getting an answer from anyone
(including the manufacturer) as to what can cause this I just let it go to see
what would happen. Its been running 2 seasons like this so far and nothing bad
or what should be expected has happened.
1. Nothing has burned out or been hurt. (Fish finder, lights, guages, GPS)
2. It has not caused the battery to boil off water, I have not added one drop.
The battery is FINE.
Some people have reported that turning on accesories will drop the voltage and
this is correct. It worked on mine for a while but it will eventually charge
the battery and runs around 15 volts. Leaving the lights on for a while to
discharge the battery will do the same thing. Its amazing how this 5 amp
system can charge so fast.
After many trys I finally got to actually speak with an engineer at the
manufacturer. He told me they were always aware of this high voltage and their
guages (OMC) were specifically designed to handle the higher voltages without
harm. I asked him why they would even make a system that could even produce 16
volts in the first place and he said "it could do 45 volts". WHAT!!!!!!
I asked why they were not concerned about the battery gassing and causing an
explosion. His reply was that their un-regulated charging systems produce a
very low amperage. Mine is around 5 amps. He said in a simple un-regulated
charging system there is a stator (generates AC) and a rectifier (changes AC to
DC) and the battery acts as the regulator. He could not tell me why mine has
run all these years at 13.5 - 14.5 volts and suddenly started running at 16.
He did give me these tips for ANYONE with an un-regulated charging system:
1. NEVER use a maintenance free battery.
2. NEVER use any of the new AGM batteries.
3. NEVER use one of the new spiral wound batteries.
3. ONLY use a battery with caps so you can check the water level on a regular
I saw this motor run at the correct voltage for years and I refuse to just let
it keep doing this because it can. Being somewhat of an electrician I need a
I checked every wire and ground in the boat, they were fine. A loose ground
could cause a rise in voltage. I tested every plug and ground on the motor. I
disconnected everything from the boat except for the motor to eliminate wiring
problems and it STILL runs at 16 volts. I threw some money at it and bought a
new rectifier which didnt help.
Now I am down to the battery. The factory manual calls for a battery with "a
minimum of 360 CCA". I bought a battery with 1000 CCA and it went to 16 volts
instantly. I borrowed a battery with 600 CCA and got 16 volts after it charged
up. I bought another small battery with 420 CCA and have not had the chance to
test it yet. If this does not work I am going to try a battery combiner and
let it charge my deep cycle trolling battery too. If I give it a load, its
fine (about 14 volts). I could rig some kind of regulator to it but it never
had one before and shouldnt need it now.
On an interesting note a weak battery will let it run within normal voltage
range because it will never charge up fully. Problem with that is you cant buy
a weak battery.
I will test the new small battery (420 CCA) to see if that works, it stayed
under 14.5 volts on the muffs at around 1000 rpm's for about 10 minutes which
is a record. :) Since the smaller battery will discharge more during starting
it might give the charging system something to do but being smaller it might
charge up faster.... hmm.
If anyone out there has any logical suggestions I will try them and I would
like to hear from you. There has to be an answer to this. Could something
cause the VOM's to read incorrectly?
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Sounds like there is no voltage regulation, and this is what I would expect a system like that to do. the +16 volt on a normal lead acid battery will provide a constant overcharge to it shortening the life. (cheap design of motor voltage output)
Fixes (well... what is needed is a voltage regulator, but.... )
1. add load - this should lower the voltage, around 14.5 is a good max - - but the load is on all the time. (cheap+poor shunt regulation)
2. add series resistance to generator/alternator output - this may not be possible depending on how your motor wiring is, as the battery supplies current to the starter motor. The restiance would be a power diode or two in series, Motorola has some that are 30 amps. That would drop your voltage down 16 - 1.4 to 14.5. A resistor won't work as well in letting the battery get charged up. (this would be on or two series diodes in the line out of the generator/alternator wire that then attaches to the battery)
Check VOM on a good AA battery should be 1.4 to 1.5 volts .............
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Thanks for the reply but all you did was re-type what I wrote.
to it shortening the life.
It been running like this almost 3 years now. No damage to anything.
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Not so, the additional diodes in series with charger output, these are not the "rectifier diodes", but something you would have to add in by splice into to inline wiring. They drop the voltage down. Else your stuck with what you have. If it hasn't caused a real problem, forget it.
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What hes saying is that there is a 0.6 volt drop across a diode, which will bring your voltage back to the levels you want.
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Yes I understand that and I thank you for the responce but that is not what I am looking for here. I know I could regulate it but I want to know why I have to after all this time.
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If it is working for the last three years with the same battery and is OK now, no reason to fix it. If it goes through a few batteries, then add the diodes if you can (could be tough mounting them internal)
If one was designing boat motor electronics module for charging a battery, with your measurements, one would have to change the design to be compatible with most lead acid batteries. This is verified by the warning the CSR gave you, they know they have a problem.
Is a matter of saving (battery) cost, not really safety.
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Have you changed any accessories? Since you say the output of the charger is only about 5 amperes, perhaps you were loading it keeping the voltage down. A new accessory that uses less current might result in higher voltages. Seems to me that just turning on a lamp or two would make a big difference.
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I never changed any accesories. Turning on the lights will bring the voltage down. Here is what is on the system: 2 LED lights 2 12 volt bulbs for bow and stern 3 small lights for the guages 1 Fish finder 1 horn 6 small LED's in toggle switches. 1 Bilge pump
Thats it. It worked perfect for years. Then as many other owners have reported, I switched the battery and it started over charging. Someone out there has to know the answer to this.
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It seems pretty obvious that your old battery was taking the excess current.
It appears that the old battery must have been below full charge all the time.
There are a couple good sites about batteries:
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William Darden at the last site could probably answer your question.
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Ok.. the car battery makers have been futzing around with the basic designs for that last few years. At first to please the car makers who wanted a lighter, cheaper battery that would still start a V8, they came out with the 'chloride' battery. It was about half the weight and still would start a car. But they found out later it couldn't take a constant applied voltage, Fleet delivery vehicles that ran the engines for 8 hours or more a day were eating the chloride batteries up and they wouldn't hold a charge.
I worked for a business/police band two-way radio maker at the time and because our radios were draining a few microamps when turned off (to keep the microprocessor memory alive), the fleet owner was blaming us for their truck batteries losing charge over the weekends a few months after he got all new trucks. We told him to take out the radio fuses over the weekends, but that didn't stop the problem and he still blamed us. One of our engineers had an engineer friend that worked for the Eagle-Pitcher battery Co so he called him for hints about what could cause that. Well, the battery engineer told him 'You didn't hear this from me, but the chloride batteries only work if they're used in passenger cars that run for only an hour or two a day. They can't take the constant charging that service vehicles produce.'
We told our customer he had bad batteries and he should yell at the truck maker and why. He got all new batteries from the mfr, no argument, and the problem went away. He was still doign great three eyars later with the same batteries. We noticed in the next year's car models the chloride types were gone and the old heavy battery types were back.
What I believe is happening to you is the standard battery design has changed, the care-free types are designed to take a constant charge by developing a high internal resistance with further charging once they've reached full charge. The internal resistance doesn't drop back low until the terminal voltage is brought back down to the 14v and below by a heavy load. This reduces the electrolysis water losses.
Just because they also make batteries with cell caps for refilling, doesn't mean it's not the same design internally. I think it would cost them more to go back to the old design for just the refillable sorts.
As a test, you might try using a deep discharge cycle battery, maybe borrow one from someone who uses it for their trolling moter. Those would still use the old design, they just have heavier plates and extra sump capacity in each cell.
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If I understand you correctly, the charging voltage was "correct" until you changed the battery for one with ~ 3 times the CCA.
If that is the case, there is no mystery. Think of this as a circuit with two elements: an unregulated source and a load. The source did not change, but the load (the battery) did. It is not at all unreasonable to expect a change in the charging voltage. The new battery is simply drawing less current than the old battery did when charging, thus the voltage is higher. That would be consistent with the increase in the size of the battery. Bear in mind that both the voltage and current drawn during charging change as the battery becomes more and more charged - the load that the battery presents to the charger depends upon the state of charge of the battery. Using rough numbers compare the state of charge of the old 360 CCA battery to the new 100 CCA battery: If 10 percent of the charge was drawn from the old battery prior to charging, then only a little more than 3 percent would be drawn from the new one. So with the old battery, you would be charing a battery that was 10 percent discharged, while with the new battery, you would be charging a battery that was only 3 percent discharged. That would cause the difference in the charging voltage.
16 volts when charging may or may not be a problem. It may be that in the boating field it is a good compromize, all things considered, to apply an "equalizing" charge of about 16 volts to 12 volt batteries. From what you posted, I gather that 16 volts while charging is fairly common in boating.
It is certainly not the best way to go based soley on technological reasoning. From the technological standpoint, the best treatment for a battery would be to have it continually on an intelligent charger, keeping it fully charged at all times. That is clearly impractical for boats that must rely on the engine to generate a charging source.
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