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
one.
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
basis.

FINE.
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
reason.
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 three VOM's to read incorrectly?