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Old 26-01-2021, 09:01   #1
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DIY alternator throttle?

I'm looking for a simple circuit that will measure the alternator temp and restrict current if it gets hot. I've been told adding a resistor to the excitor will reduce the output and help it stay cool, but while I get the theoretical reason, I don't understand the practical way to do it or the risks of messing with it.


I know there are commercial gadgets that do this, but they do more than I need and cost more than I want to spend.



Background. I've got 2 engines, same hitachi 80amp alternators on each. Each engine connected to it's own starter battery. And each one can switch to being connected to the newly installed lithium house bank. Problem is when I connect the house bank to the stb alternator it kicks out 20amps and stays relatively cool. But when I connect the port alternator kicks out 60amps and gets really hot.


So I'd like a little circuit with a thermocouple and a relay or something that throttles the output of the alternator when it gets hot.



Any info on acceptible working temps, and alternator excitor resistance values would be great.



Also can anyone explain why they pump out two different currents? Is it do with the state of charge of the starter, or it's state of life. I think they are both pretty abused (new to me boat).



Thanks in advance.
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Old 26-01-2021, 09:56   #2
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Re: DIY alternator throttle?

The alternator’s “S” terminal senses the battery voltage.
Inserting a diode in series (twixt “S” & Battery), will result in a 0.6V drop, resulting in “fooling” the regulator into increasing the output (thinking the battery voltage is 0.6V lower, than it actually is).
The diagram below indicates a fuse in the Sense circuit.


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Old 26-01-2021, 10:09   #3
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Re: DIY alternator throttle?

Causes... either one of the diodes is shot in the starboard alternator or there's a loose connection somewhere between it and the battery.


Circuit to reduce output using a resistor in the field circuit... not straightforward, I would try 1, 2, 4.7, and 10 ohm resistors and see if they do what you want. They will dissipate quite a bit of power, I would start with a 10 watt rating and then measure the actual voltage across them in circuit while charging and adjust as necessary.

You might need to put a cap or MOV or something in parallel to keep the relay contacts from burning up. Or not. I would measure the voltage spike with a scope or a peak-reading DMM and see how bad it is.


If you're using an internally regulated alternator then as GordMay says you can fool the sense line in various ways. A diode will work or you can put a pair of resistors to make a voltage divider between power and ground. Start with 5 watt resistors and have the one to ground 47 ohms and the one to power at 2 ohms. Increase the resistance to power to reduce output. They will draw power all the time and have to be switched off when not in use. With one resistor, it might work or not depending how the alternator is wired internally. You could start with 1K and vary up or down and see if you get results you like.


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Old 26-01-2021, 10:25   #4
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Re: DIY alternator throttle?

For the control of the field current you might want to look at this thread to see what is involved for just a manual control.

Synopsis - you need a variable resistance from 0-5 Ohm and capable of dissipating ~25W in order to use a resistor, and then you need a temperature control to vary that resistance.

In this day and age, probably much simpler (and cheaper) to use a PWM controller driven by a temperature probe, but that will be very (electrically) noisy.

In the end, the amount of time and money you spend on trying to create the "simple" circuit will be more than the cost of a reasonable external regulator that has these features.

For the differing outputs I would suspect that the alternators are sensing different voltages. Could be different length cable runs, but I would suspect a bad connection somewhere on one side that is causing a sense error.
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Old 27-01-2021, 09:59   #5
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Re: DIY alternator throttle?

Thanks everyone. These are really helpful answers.
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