Moderated Discussion Areas
ContinuousWave: Small Boat Electrical
|Author||Topic: Alternator Output|
posted 04-06-2009 11:08 PM ET (US)
On a friend's boat the engine alternator was putting out 12.84-volts with all of his electrical components on, and 13.1 volts with all the electrical components off; engine running at 1500-1800 RPM. [All] readings done at the battery. He states that he has new batteries, new alternator, all the connections are clean, and all the cables look excellant.
My experience has been that the alternator should put out additional voltage [in the presence of] electrical load, not less. In addition I believe the alternator should be putting out at least 14-volts not 13-volts. I feel he needs to take this new alternator back. But, after doing some research I have found that 13-volts might be okay. Any opinions?
posted 04-07-2009 12:12 PM ET (US)
Sounds wrong to me. Full charge is 12.7 volts on a standard lead-acid battery, I believe. The alternator should be at least a volt or more above that (something like 14+ seems typical). Are you measuring this with a DVM? - I guess you would have to be if you are quoting readings like "12.84".
posted 04-07-2009 09:45 PM ET (US)
Agree with swist. My voltage reads 12.7-12.8V with the
engine off. 13.8V at idle. 14.7V hauling ass after a
long run home. All of the above after time to recharge.
My Evinrude manual says 14.5V at the redline. That's within
Did the battery have plenty of time to recharge after starting?
And is the DVM known to be correct? I have two Fluke 77s
posted 04-07-2009 09:52 PM ET (US)
MMM, what are the electrical components?
What's the voltage with the engine off, and the electrical
What's the voltage with the engine off, and the electrical
Measure the voltages on the battery, that is, on the actual
posted 04-08-2009 01:25 AM ET (US)
The boat had been sitting for 2 months so before we launched the boat I tested the batteries and the batteries each read 12.2 volts at rest a little low but ok. I grabbed my battery jumper pack and we got the boat in the water and fired it up no problem. On the way out I noticed that the on-dash volt meter never went above 13 volts no matter what the engine speed. We ran for about 3 hours and just before we got back to the marina I checked again. Still the same so I pulled out the Digital volt meter to double check and that is where I got the 12.84 volt reading. I asked him to turn off all his electronics but leave the engine running but rev it up to 1500 RPM or so and I got the 13.1 volt reading.
I was pondering if his 1-2-all switch could be causing a voltage drop as it is the only item in the circuit he did not replace. By the way all tests were done with the switch in the ALL position. I should probably also check the water in the batteries and do a load test on them even though he claims and they look like they are new.
posted 04-08-2009 07:42 AM ET (US)
12.2 volts is only about 50% charged. To get the voltage up to 14 volts the battery has to be just about fully charged. To fully recharge a half charged battery takes more than 3-hours. Also, sitting for 2-months in a discharged state is very bad for lead acid batteries.
Due to variables, such as the type of batteries and their condition, don’t expect 100% accuracy. However, the approximate SOC value will be helpful in determining when conservation measures are needed. Keep the chart and a good DVM near your battery bank, and check the SOC often.
State of Charge (SOC) Chart:
% of Charge - - - - Charging - - - At Rest - - - Discharging
Tip: When batteries are deeply discharged it’s important to recharge them within 24 hours in order to prevent permanent damage.
posted 04-08-2009 10:58 AM ET (US)
Tom, I always thought that the alternator would put out top voltage regardless of battery state. i.e. the regulator would control the amount of voltage going to the battery?
posted 04-08-2009 01:31 PM ET (US)
Fishmore - the battery is, by itself, a very good regulator.
The switch should not be causing a voltage drop - unless the contacts are very corroded. But - check it out - by with the circuit open, but the switch closed - measure the resistance across the switch. ---- Jerry/Idaho
posted 04-09-2009 07:51 AM ET (US)
Fish, the battery "absorbs" the amps when it is discharged, therefore the voltage stays low. Compare to filling a tall bucket from the bottom, when the bucket is empty, the pressure (volts) will be low, when the bucket is almost full, the pressure will be high.
posted 04-09-2009 08:33 AM ET (US)
Is the alternator being discussed here a permanent magnet alternator? Or is it a automotive type alternator?
posted 04-09-2009 12:14 PM ET (US)
posted 04-12-2009 06:53 PM ET (US)
The voltage readings on a panel meter are generally not particularly precise or accurate, so I would not take much stock in them in diagnosing the status of the alternator. So far I see the following data as applicable:
DVM VOLTAGE READING and Condition
DVM =12.2 Vdc, battery at rest following long lay-up
DVM = 13.1 Vdc, battery being charged by alternator at 1,500-RPM engine speed
I interpret this data as follows:
The battery was in a state of about 50-percent discharged when its terminal voltage was 12.2-Vdc per the chart shown in the REFERENCE article at
The battery was receiving a charge from the alternator when its terminal voltage was 13.1-Vdc. This voltage is greater than the normal terminal voltage of a fully charged "12-volt" battery, nominally 12.9-Vdc.
The reading of 12.84-Vdc while the battery was under its normal load is indicative of its state of charge being about 10-percent below FULL CHARGE.
With an automotive type alternator there is generally an integral voltage regulator which tries to maintain the voltage output of the alternator below about 14-Vdc. The precise voltage at which the regulator operates is hard to say, as it may depend on the temperature, amount of current being delivered to the battery, and the speed of the engine. Usually with an automotive type alternator the regulator can push the output voltage even at lower engine speeds to reach a situation where charging current is available. The alternator is often turned faster than the engine crankshaft speed by the arrangement of the drive belt and pulley, so you get good charging current even at low speeds.
posted 04-19-2009 11:49 PM ET (US)
Well the answer is....
Once the batteries had a full charge the alternator put out a full 14.3 volts. Thanks everyone for your recommendations and knowledge. I never knew that a battery that was drained down would cause the alternator to put out less than 14 volts. Now I know.
posted 04-20-2009 10:07 AM ET (US)
The alternator regulator probably monitors the voltage at its output terminal. The electrical system in the boat owned by your friend may have some resistance in the wiring between the alternator output and the battery. Some output voltage is lost in that wiring.
The alternator is also current limited--it can only make as much current as its coils and windings are able to produce. The amount of current that flows from the alternator is proportional to the difference in voltage between the battery terminals and the alternator output. If the alternator was supplying its maximum current, its output voltage may fall below the usual 14.3-volt target.
If you have a large battery and a small alternator you will be more likely to see this situation where the alternator current limit comes into consideration.
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