Alternator Output voltage

Chris-380

Chris-380

Well-Known Member
Jan 20, 2019
2,435
Pontoon Beach IL
Boat Info
2000 380 Sundancer
Engines
7.4L Horizon w/V drives
My analog gauges have always showed a voltage difference between the two engines. I added the fox marine gateways, now i see the digital difference, depending on what rpm i am at, its .5-.9 volts difference....

Curious what peoples thoughts are, do i have an alternator going south and this is the symptoms?

Thanks in advance!
Chris
 
I have the same thing...my house battery (starboard side) is always a little lower. I attribute this to higher load/worse battery condition on that side.
 
I’ve had this issue off and on for awhile. I would see 12.7 on house/port side, and 13.9 on stbd. Especially after being out on the water all day. However, after I spend the night at the marina with battery charger on, I see 13.9 on both. Similar if I go on a 2 hour cruise, the port would slowly get closer to normal, like 13.4 or 5. Also, during this time, my galley fridge was going bad and my cockpit fridge was set at 5. Now, both are new and set at 3, and I didn’t have battery drain issues during the day like I have had before and I’m seeing the same reading on my vessel view mobile.
 
this is when i am running, which (I believe) is showing the output voltage which is charging the banks for each. On shore power or genny, they are the same when i check the gauge in the salon.... this is off the engines.....
 
I brought it up to my tech and I believe he said that if one of the banks is low, or bad, and it was low in my case, that reading 12.5 or similar is fine, no issue with alternator. Once I have both banks fully charged, then when I leave the dock, SC/VvM is reading 13.5-.9 for both sides depending upon rpm.
 
I attribute this issue to the crappy gages Sea Ray supplied. One of my tachs went out, both of my fuel gages read 18 - 20% less than what is actually in the tanks, and the starboard voltage gage always reads higher than the port gage.

Before troubleshooting anything else, troubleshoot the gage first.
 
bobeast said:
I attribute this issue to the crappy gages Sea Ray supplied. One of my tachs went out, both of my fuel gages read 18 - 20% less than what is actually in the tanks, and the starboard voltage gage always reads higher than the port gage.

Before troubleshooting anything else, troubleshoot the gage first.
Click to expand...
Thanks Bobeast, but these voltage readings are also verified through the digital readings from fox marine gateways.
 
Todd320 said:
I brought it up to my tech and I believe he said that if one of the banks is low, or bad, and it was low in my case, that reading 12.5 or similar is fine, no issue with alternator. Once I have both banks fully charged, then when I leave the dock, SC/VvM is reading 13.5-.9 for both sides depending upon rpm.
Click to expand...

Thank you Todd320

I pulled the low bank batteries and had them tested, They tested good....
 
bobeast said:
Interesting. l'll have to see what my smart-craft gateway says.
Click to expand...

Its def odd. When on shore power, they are the same. When off shore power and not running, still the same.... once I start the engines, that’s when they change....
 
Chris-380 said:
Its def odd. When on shore power, they are the same. When off shore power and not running, still the same.... once I start the engines, that’s when they change....
Click to expand...

No it’s not odd. Dtfeld pegged it. You have two different batteries With two different charging requirements: a single in one back and a pair in the other. Even if they are brand new identical batteries, they will be slightly different. You also have two different power sources; you may even have two different capacity alternators. Throw in a little mechanical variation, belt tension, different engine RPM, possibly different belt pulley configuration between engines, etc.

The shore power battery charger is one source of voltage that may also have more output than the output of your alternators.

Certainly note the difference. If it changes, more importantly if the output history of a battery/alternator set changes, then more investigation is in order. What you are more likely experiencing is access to new more detailed information.
 
Batteries are fully charged, it’s def the output of the alternators.... same voltages with all DC off....
 
Chris-380 said:
Batteries are fully charged, it’s def the output of the alternators.... same voltages with all DC off....
Click to expand...

Keep in mind the voltage output of the alternator changes based on the perceived need of the batteries. At start up for example, the voltage can spike to over 15 volts in some circumstances, as the battery recharges the voltage drops. Once the battery reaches the "charged" set point, the alternator shuts off and you see the voltage in the battery itself based on current load. Right there is reason for discrepancy; one system is just providing DC to an engine, the other side is not only powering another engine, but also any DC loads like nav electronics, dc fridge, stereo, etc.
 
Henry Boyd said:
Keep in mind the voltage output of the alternator changes based on the perceived need of the batteries. At start up for example, the voltage can spike to over 15 volts in some circumstances, as the battery recharges the voltage drops. Once the battery reaches the "charged" set point, the alternator shuts off and you see the voltage in the battery itself based on current load. Right there is reason for discrepancy; one system is just providing DC to an engine, the other side is not only powering another engine, but also any DC loads like nav electronics, dc fridge, stereo, etc.
Click to expand...
i understand your point... but as i stated previously "all dc loads off" so refriges, stereo, electronics are off.... its just the engines.... it can be 100 things, i am betting its as simple as the difference in alternators... maybe the PO swapped one of them out.... betting this one is original and getting near the end of its life
 
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