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Author Topic:   Unregulated Charging Output Too High
smudge posted 11-10-2006 05:49 PM ET (US)   Profile for smudge   Send Email to smudge  
I am having problems with my 1992 Mercury 50-HP. It has 16-Vdc output to the battery at idle, which is way too high. This engine does not have a regulator, just a rectifier which I replaced, even though it tested ok. The tachometer doesn't work either. but it may have fried with the high voltage. It has 8-Vdc on the sender wire (gray) and reads 18-Vac. The AC output on the yellow wires is 18-Vac, which sounds about right to me.

I was thionking of fitting a Mercury regulator. What is involved in doing this? The million-dollar question, though, is why should I have to? The only thing I can think of of is a battery problem, but the starter whizzes around pretty fast!

Any ideas? Thanks.

jimh posted 11-10-2006 09:05 PM ET (US)     Profile for jimh  Send Email to jimh     
Battery charging systems with relatively low output current are often not regulated. These systems depend on the battery itself to act as a regulator. If the battery is fully charged, the terminal voltage can become rather high. A terminal voltage of 16-volts would be typical.

I think Mercury has a bolt-on voltage regulator module. You may be able to add it to your engine. Maybe one of our many Mercury expert technicians will comment.

jimh posted 11-10-2006 09:08 PM ET (US)     Profile for jimh  Send Email to jimh     
The tachometer output is usually a half-wave rectified output of the stator. It consists of a pulsating DC voltage that goes from close to 0-Vdc to about the battery voltage. If you just put a voltmeter on it, there is no telling what it will read. It would depend on the characteristics of the meter.
smudge posted 11-10-2006 10:40 PM ET (US)     Profile for smudge  Send Email to smudge     
Thanks for your reply Jim. The voltage creeps up with time - after 5 minutes it is at 17 volts.

I am using an old analogue meter to put a little load on it, but even with the navigation lights on it is still just as high.

The rectifier failed several trips ago, I replaced it and the voltage is high, prior to this the voltage never exceded 14.6v.

How is this for a theory? When the rectifier failed the high AC voltage overcharged the battery, possibly lowering the batteries internal resistance. It would be that resistance that would 'regulate' the voltage I think (I am a sparky but I'm stumped on this!). I just tried another battery (it is flat and needs a recharge)in parallel with the original and the voltage doesnt go over 14.5 even reving high, so maybe it is the battery. I must admit to not knowing the technical side of batteries.

jimh posted 11-11-2006 12:29 AM ET (US)     Profile for jimh  Send Email to jimh     
A battery that has a high internal resistance can probably be pulled up to a greater terminal voltage than a really strong battery that has a very low internal resistance. Thus you observation that the charging current can reach 16-volts may be related to the battery.

Simple voltage regulators tend to regulate the voltage by getting rid of any excess electrical power as heat. Therefore they need a heat sink to drain off their heat, or else they will get very hot and burn up. On a bigger motor with a high-current charging system, a simple regulator has to shed a lot of energy as heat in some situations, so they are often water cooled. Smaller charging systems can get by with air-cooled heat sinks on the voltage regulator. Often the heat sink is bonded to the engine block so the heat can flow off into the block, too.

smudge posted 11-11-2006 06:02 PM ET (US)     Profile for smudge  Send Email to smudge     
Thanks Jim, I have found someone here in NZ that stocks regulators for the Merc stators, so I am going to fit it for peace of mind, wiring is very straightforward, finding somewhere to mount it may be tricky.

With an old discharged battery connected across the good one the volotages are spot on.

Chuck Tribolet posted 11-11-2006 07:57 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
At least on an automobile, the voltage regulator doesn't work
by resistance. It works by pulse width modulating the alternator field coils. But then outboards seem to use
permanent magnets for the field coils.

And modern electronic voltage regulators don't work by
resistance either, they are switching regulators that pulse
width modulate the the input voltage. It all comes on a chip
today. If one of these were used to regulate an outboard,
it could use an air-cooled heat sink.


jimh posted 11-11-2006 11:03 PM ET (US)     Profile for jimh  Send Email to jimh     
I explain in detail how a permanent magnet alternator works in an article in the Reference section:

The once nice thing about old-fashioned linear voltage regulators is their simplicity. They do waste a lot of energy as heat.

Having all the electrical components as separate assemblies is also nice, at least in terms of the repair and replacement. It is much less expensive than an entire engine module which could cost a small fortune these days.

smudge posted 11-12-2006 01:53 PM ET (US)     Profile for smudge  Send Email to smudge     
Thanks guys, obviously you fellas know your stuff and I understand all that stuff about linear regulators dissipating energy as heat, so on something as simple as a full wave rectified AC output from a simple permanent magnet alternator, how can the voltage on my boat all of a sudden be 16 to 17 volts after years of working well? Mercs all over the place run like this perfectly well, but the voltage on mine has now killed two fishfinders, a gps and blown the nav lights (I can live with the nav lights :))

I remain puzzled.

What should I do? Try another battery? Fit a regulator? How can a battery fail to make the rectified stator output do this? So many questions ......
Once again, Thanks!

Chuck Tribolet posted 11-12-2006 02:13 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Switchers are as simple to build as a linear these days.
One chip, a couple of caps. That's about it. And the
nice thing about switchers is you can feed them anything up
to breakdown voltage, and they can still regulate it.
Linears get hotter them higher the input voltage.

Smudge: I wonder of you have a bad connection some somewhere.
have you measured the voltage across the battery terminals
(lead to lead) while at high voltage charging? If there's
some resistance somewhere, the lead to lead may read something
sensible while measurement at the alternator may read 16V.


minitauk85 posted 11-12-2006 05:32 PM ET (US)     Profile for minitauk85  Send Email to minitauk85     
smudge- I went through a similar situation with my 1987 70 hp Johnson. I hope this helps:
minitauk85 posted 11-12-2006 05:36 PM ET (US)     Profile for minitauk85  Send Email to minitauk85     
Also this may help with your depthfinder, not sure if you use humminbird or not, but I hope your brand is as good with their customer service as mine was:
Chuck Tribolet posted 11-12-2006 08:23 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
My Humminbird depth finder shuts down if the voltage goes too

Humminbird used to have a product called "Sure-Volt". It
would regulate the voltage down to 13.8 OR UP to 13.8 (it
had some magic microchip). I installed one to keep the
electronics from shutting down every time I cranked the
engine (and the voltage dropped to about 8V). The one
downside is that it can only put out 1.5A, so it could only
run the GPS and fishfinder, not the VHF (about 6A on transmit).


smudge posted 11-12-2006 09:19 PM ET (US)     Profile for smudge  Send Email to smudge     
Well I have bought a regulator , now I am going to go and fit it. Good thinking on the bad connection, but I have used a voltmeter across each wire run and it does not show any voltage drop, which by the way is the best way to find a bad connection - for say a corroded battery clamp, one lead on the wire, the othe on the battery terminal. If there is any resistance there then a voltage will be dropped acrosss that connection, showing up on your voltmeter, this ONLY works however if there is a load connected.
I also used a lenght of wire to bypass things, I couldn't find any bad connections anywhere!

Minitauk, your post is describing exactly the problem I have and I am taking the same track. I do have a Humminbird but it is an old wide100 and I dont really want to fix it, my wife doesn't need to know it may be repairable but I can upgrade you see ;)

I will let you guys know how I get on, but I know that somewhere I am missing either a bad connection or the battery is a little kaput. The SG readings are low but it spins over fast and I had to addd water due to the overcharging so it needs a charge before retesting.

smudge posted 11-13-2006 02:14 AM ET (US)     Profile for smudge  Send Email to smudge     
Two hundred smackers and I got myself a Sierra Regulator, that's probably $120 in your money.

I put it in.....

I turned on the key...

It didn't go!!!!!!!

So I connected both red wires from the regulator to the solenoid connection and connected the fused supply to it as well and away she went, charging nicely at 14.6v no higher and gradually dropping as the battery charged up.

Thanks for your help on this guys. I am ALMOST sure a new battery would have fixed it too, but I dont like the idea of just a rectifier. The conversion was easy once I realised both wires needed connection to the supply, I assume its done right as it regulates... the mounting holes were all there and everything.

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