Low charging voltage while running at idle/trolling speed

[Codger]

When I am running at 1000 rpm or slower, the voltage readout seems to be around 12.5 volts (not charging), after getting above 1000 rpms, the voltage will kick up to just over 13volts (charging). Verified with volt meter on batteries. (Haven't seen it get up to 14V, makes me wonder if my alternator or regulator is starting to fade.)

Anyway,
If I want to troll for several hours at less than 1000 rpm, it seems I run the risk of draining the batteries.

Is it really normal for the alternator output to not kick in until 1000 rpm?

Anybody else run their boat for extended periods at less than 1000 rpm and see the same voltage condition? Any issues with batteries draining?

Thanks.

[Heefus]

do you have a single wire alternator?

[tk416]

That's interesting... I've noticed that on mine as well.. so would like to hear some opinions...

75 F25 with 318

[prowlersfish]

What is the meter saying with the engine off ? I have seen many low readings at the dash and its fine at the battery . And thats where it counts , so check it at the battery . Heefus ask if it was a 1 wire alt , on those the won't charge until you rev them up just a little one time then the will charge at idle fine all day .

[Codger]

I will have to check of it a single wire or not when I'm back at the boat in a couple days.

I did check with voltmeter at the batteries (to take the accuracy of the dash gage out of the picture).

Engine off, voltage at batteries is around 12.5-12.7 volts. (Both batts pass on the load tester too).
Engine running at idle, volts are still around 12.5-12.7 volts.
When I rev the engine, the voltage will pop up to around 13 volts.

After revving engine and returning to idle, the voltage will stay at 13 volts for maybe 20-30 seconds, but then falls back to around 12.5-12.7 volts. Rev again and the voltage will kick up to around 13.

If I find that I have this "single wire" type and this is normal operation, is it possible to change to a different type of alternator that charges at all RPM? All the GM style engines seem to output around 14V at all RPMs. (Engine in my boat is a Chrysler 318)

[aweimer]

sounds like a possible bad altinator. You could take it off and go to your auto parts store, they can bench test it to see if its working properly.

Should be ~14 volts.

[g36]

mine is a single wire alternator and this is normal. if it picks up when you rev the engine this is a self exciting alternator. nothing to worry about unless is doesnt when you rev up or increase your rpms.
read on


What is a Self Exciting One-wire Alternator?
The Difference Between One-Wire, Two-Wire, and Three-Wire Self-Exciting Alternators
A self exciting or "One wire" alternator as it some times called, has only one wire running to it., that one wire is the main large battery wire. Don't confuse one wire because the plug going to the alternator only has one wire to it, this is a two wire system. The one wire, self exciting alternator uses a special voltage regulator that doesn't need an ignition wire to activate it.

This type alternator only requires a battery wire hooked to it. The voltage regulator contains circuitry that uses the residual magnetism in the alternators fields to determine when to turn the alternator on, the regulator does this by sensing the RPM the alternator is turning. When the alternator gets to a certain rpm the voltage regulator "turns on". Typically you start the vehicle, rev the motor slightly then the alternator starts charging. This type alternator is commonly used on custom cars, trucks, tractors and other non standard applications when wiring is a factor. In choosing this type alternator you must consider, do you want to rev your motor slightly to get the alternator to turn on. Also, when using the self-exciting alternator on tractors or other slow turning motors does the engine have enough RPM's to start the alternator charging. This can be overcome by using a smaller pulley or by adding an ignition wire. Is another name for the Self-Exciting Alternator mentioned above. You only need connect the battery wire (one-wire).

Two-wire Alternator:
Most standard and all self-exciting regulator alternators will work using the two wire setup. Two wire means that you use the battery wire and an ignition wire to activate the alternator. With this setup the alternator starts charging as soon as the engine is running.

[prowlersfish]

It is NOT normal for a single wire alt to stop charging at low rpm , Once you give it a little rpm to get it charging it should stay charging at idle until you turn it off .





May sure you belt is tight , #1 problem with poor charging .

[Stripermann2]

Having just replaced mine, single wire unit on Saturday, I can tell you that off idle and rpm increased, it starts charging and will continue to charge even at idle, albeit, at a lower rate but it's excited and charging. The alternator has to charge...even at idle! At a different amp output, dependent on load and rpm, you'll see the gauge fluctuate.

[davidsmith]

My chrysler engines have a oil pressure switch wired through the altenator circuit that wont allow the altenators to charge unless the oil pressure reaches a certain pressure. I am not sure the exact pressure the switch is set for but sometimes at low idle and hot engines they wont charge unless I idle up a little.

[Codger]

It is NOT normal for a single wire alt to stop charging at low rpm , Once you give it a little rpm to get it charging it should stay charging at idle until you turn it off .





May sure you belt is tight , #1 problem with poor charging .

Belt is brand new. It was changed by the shop at the marina this spring when the water circulating pump was replaced. Has nice tight tension.

[kallen]

How about a smaller alternator pulley?.

[Big D]

A smaller pulley will get the alternator's RPM up and charging at a more reasonable rate during idle. Considering your application (trolling), it may be a consideration. However, when you change the pulley to rev higher at idle, it will also spin higher at engine WOT. You must ensure that the alternator's max recommended rpm is not exceeded or you'll be replacing it anyway.

The alternator sounds to me like it's doing what it was designed to. Try this; next time, turn engine on, don't throttle past 1000 RPM. While idling, turn every possible electrical device on and watch the volt meter. It probably won't change. Turn electrical devices off, throttle up then back to idle, and repeat turning everything back on. It should increase output.

[Codger]

OK, looks like I have a 2 wire system. I have a larger gauge orange wire going to 1 post and a smaller gauge green wire going to another post on the alternator.

Checked with the voltmeter on the batteries again.

12.6 volts when engine off
12.5 volts when engine off and engine blower turned on
12.6 volts with engine running at idle (without revving to kick alt. in)
12.5 volts with engine running at idle and blower turned on
13.3 volts with engine revved over 1000 rpm with blower off
13.2 volts with engine revved over 1000 rpm with blower on
After revving and letting engine idle, voltage drops back to 12.5 with no load after about 20 seconds and seems to stay at 12.5 when loading with the blower on.

Still has me worried that I would be discharging while trolling and risk killing the battery after a couple hours.

Alternator has a sticker on it from a local rebuild shop. Previous owner must have had it rebuilt at some time.

[randyp]

It sounds like it's voltage regulator not kicking in the charge at low rpms. Mine has the same issue, but when I "goose" the throttle in neutral the charging rate kicks in and stays in at low rpms unless I put a load on it (like bilge blower). Then I "goose" it again, and things are fine. I don't think you have an alternator issue.