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High Charging Voltage on 86 Model Merc

leachy

Contributing Member
Hi All,

After some advice here. Im getting close to launching a 2 year restoration project and checking things over. I was checking the voltage of the battery after starting the motor (86 Mod Merc 2 Stroke 75hp 4 cylinder) and the reading was 15.1 V at 1200RPM to 3000RPM. I have replace the stator, trigger plate, rectifier and switch box. All that was replaced after the switch box died for seemly no reason.

Is the high voltage caused by the rectifier or the stator pushing out to high an AC current? from my experience this is too high a charging voltage.

Thanks
Leachy
 

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FishinLite

Regular Contributor
I would say that is about normal. the Mercury charging system/(stator-rectifier) uses the internal resistance of the battery to control the voltage.

If you are using an AGM or other battery design it will not work properly. If I remember correctly it will be a bout 17 volts at 5000 rpm.
 

leachy

Contributing Member
Ok Thanks for the information, Its a brand new Century brand Marine battery (meant to be good quality here in Australia), is a seal lead acid type with extra internal bracing for the plates to cope with the extra vibrations. I was worried it had some kind of issue but still a little concerned about the battery life charging at that rate. I think pretty much all outboards have a similar setup but but my Yamaha 40hp would sit at about 14.2V, but it was a much newer engine.

Thanks for the information.
 

FishinLite

Regular Contributor
Your Yamaha probably has a rectifer/voltage regulator combination... Later model Mercury's also have recifier/voltage regulators...

If you do some research, you can probably find a how to update your charging circuit with a late model rectifer/regulator.
I beleive I have seen it here in the past, but haven't been interested enough to go through the effort...

Take a look at this..
 

leachy

Contributing Member
So just reading into this a little further and from what I can understand the motor only has a rectifier which does not have a built in regulator hence the voltage over 15V? or if its meant to regulate the quality of the rectifier / regulator is in the toilet. Its connected to a sealed lead acid battery so it is getting hot there and loosing water there is no way to add water to it.

Looking at the amazon link to part its substantially more, obviously there are a few more wires but just wondering how this will connect, obviously would need to be fitted externally which isn't an issue, but don't want to damage a switch box. one of those is worth three batteries.

815279T Regulator Rectifier Mercury 883072T 6 Wire for Mercury Voltage Regulator 40HP 50HP 55HP 60HP 75HP 90HP 100HP 115HP 125HP 135HP 150HP 175HP 200HP 250HP 275HP 854515 815279-3​


Do others here have this kind of problem, cant think its just me, or am I over thinking this too much.
 

makomark

Silver Medal Contributor
a flooded lead-acid battery is pretty tolerant to 'sloppy' charging conditions....a sealed battery typically requires higher standards...check with your battery manufacturer and get the data sheet for the one you have...then you will have enough data to start a viable search for a stand alone regulator....
 

leachy

Contributing Member
Yeah it appears that this motor only has a rectifier and no regulator and that using a sealed maintenance free battery is not recommended. I have found a part from CDI #193-5114 that is a regulated rectifier which will fit straight in according to CDI, just need to understand the wiring connections. I will investigate this further but Im surprised that Mercury were happy to manufacture stuff like this.
 

jimn

Advanced Contributor
How old is the battery in your meter? I have seen high Voltage readings when batteries needed replacement- if you test a different battery and the voltage is higher than expected, it's likely that the meter needs a new battery.
 

jimn

Advanced Contributor
Yeah it appears that this motor only has a rectifier and no regulator and that using a sealed maintenance free battery is not recommended. I have found a part from CDI #193-5114 that is a regulated rectifier which will fit straight in according to CDI, just need to understand the wiring connections. I will investigate this further but Im surprised that Mercury were happy to manufacture stuff like this.
Mercury and Yamaha worked together on several engines/models in the past.
 

leachy

Contributing Member
The meter is fine it reads in line with the actual meter in the boat, actually the boat reads a little high. after doing a bit of reading it seems that back then they didn't think batteries and electronics needed to have a regulated voltage, and that it was fine to just use the internal resistance of the battery to regulate the charge voltage. the new rectifier / regulator from CDI should help things. Also hoping it will provide additional protection to the rest of the electronics in the motor seeing it will be a stable voltage now.
 

jimn

Advanced Contributor
The meter is fine it reads in line with the actual meter in the boat, actually the boat reads a little high. after doing a bit of reading it seems that back then they didn't think batteries and electronics needed to have a regulated voltage, and that it was fine to just use the internal resistance of the battery to regulate the charge voltage. the new rectifier / regulator from CDI should help things. Also hoping it will provide additional protection to the rest of the electronics in the motor seeing it will be a stable voltage now.
Voltage has always been regulated and if it was OK before, it would point to the regulator as the cause of the problem. The Stator doesn't output DC voltage/current, it's AC and part of the job in the system is rectification to DC, with a bit of filtering, to make it usable for the other electronics. While the battery does a lot, its changes over time would have been a variable that should have made any engineer said "We need to make this reliable".

The Mercury/Mercruiser Electronic Parts Catalog shows 5772A7 as the part number for all '86 Merc 2 stroke 75HP OB motors, for US, Canada and Belgium. The Amazon listing shows the 815279T is for motors that start in 1989 and it looks nothing like the one for your year.

Yours looks like this, right?

61B7wNRy3XL._AC_SL1050_.jpg
 

leachy

Contributing Member
From what I can see and have read the older merc had a very simple design charging system and no regulator only a rectifier and relied on the battery to cope with the higher voltage, just meant they would get warmer and evaporate the water in the cells. This is perfectly fine for the older design batteries that you could access the cells to top up with water. The newer maintenance free ones it will kill them as there is no way to add water. This item here upgrades from the old rectifier only part to having a regulator as well so should limit the voltage to 14.6V

https://www.cdielectronics.com/product/193-5114

193-5114-scaled.jpg


The pic you posted is the switch box, don't want to ever replace another one of those very expensive,
 

leachy

Contributing Member
Ok so still trying to work this. I fitted the above part and that didn't, all it seems to have done is blown up my switch box, not impressed. Just wondering how others deal with the high voltage charging (17.1V) with the older Merc motors. I am 100% certain charging at the high a voltage will kill the battery.

Just wondering what others do in this situation. Im thinking of just making some kind of making some kind of "Black Box" with a few big resistors and heat sink to just dump the excess power.
 

leachy

Contributing Member
Thanks quicksilver,

At the moment mine only has the rectifier off the stator, and from what I cant see and determine only has the rectifier with the three point connection. It has a direct connect where you have the yellow wire, in my case red to the starter solenoid which is then off to the the battery, it doesn't have the part 88825A7in the loom, hence the high voltage output.

Thanks for your help
 

leachy

Contributing Member
Hi Quicksliver,

I used this part as per CDI's recommendations. That part you have used is no longer made by Mercury.


However it seems to have fried my Switchbox so I am nervous about fitting things inline. The switchboxes on the Mercury motors seem very fragile and such a weak point in the system. This will be my third so as you can see its getting expensive.
 
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