Limiting current from alternator

Hi everybody,

This is the first time I post on the forum, but i have been reading you a lot in the last few years, and found you incredibly interesting and useful for my projects.

I am now asking advice about if and how a certain project I have in mind can be realized.

I have an alternator and two battery banks (both 12V, but different capacities). At the moment there is an electronic device between the alternator and the batteries that split the charge between the two banks.

There exists a problem with this set up: it doesn't take into account that the alternator cannot run at maximum power for more than some minute. Even if it is rated 120A, an output of 50/60A already causes overheating and belt slipping. This happens when big loads are sucking energy from the big battery bank, causing the voltage to drop and the alternator to push more current there.

Solar panel have been added to the system, but their charge regulator cut them off at a lower voltage than the alternator regulator does, so this way only the alternator charges while the solar panels are disconnected.

So what I would like to do is:

  • continuously charging the small bank (that is the engine battery), by connecting it directly to the alternator output. This arrangement would be like the circuit on a car, and it would be suit the start battery fine;

  • connecting the big bank in parallel with the start battery, electronically controlling how much current passes to it.

This way, monitoring the current at the alternator output, I can reduce the flow of current between the two battery banks and keep the charging current below 30A. If there is load bigger then 30A on that bank, then the voltage will drop and the solar panels will kick in, supplying the rest.

My idea was to control some MOSFETs (P-channel) by PWM them from Arduino, allowing wider or stricter pulses of current to pass from one bank to the other, so keeping the average voltage at the alternator output at a level that makes the alternator internal voltage regulator supply a maximum of 30A.
I think PWM was the system used until a few years ago in solar panel charge regulators, before the coming of the MPPT technology. So I imagine that the pulses shouldn't cause any damage to electronic devices connected to the charging battery, since this system has worked for decades...

But I am not sure about how fast the alternator voltage regulator is at sensing the voltage at the output and consequently setting the outgoing current. If it is slow enough, I imagine that everything should be fine, since the MOSFETs keep the average voltage at the alternator output high enough to avoid a charge of more than 30A. But if the alternator voltage regulator senses the pulses and starts doing the same with the charging current, I am not sure if that could be a problem.

I even thought of gradually "closing" the MOSFETs, and use them as variable resistors, with a DAC and an op amplifier, but I have never heard of anything similar, so I am not sure if it can be done. The MOSFET curves are a bit steep at the beginning, so it might be difficult to use them this way.

Does any of you have any idea about it? Is there a way to gradually reducing the current flowing from one battery to the other? Thank everybody.

The problem you have is that lead acid batteries can supply very large currents and if you connect 2 of them in parallel the one with the higher charge will charge the other one, with not much limit on the current, potentially 100s of amps. You also have the problem that you charge a lead acid battery by applying 2.2V per cell (or very close to that) and letting the battery take the current it needs. The charger has to have current limiting to make sure it is not damaged by a flat battery drawing too much current.

If you have 2 batteries in parallel, as I understand you have, then both are going to supply the load, not just the big one.

From your description the problem is, in my opinion, the control of the alternator. I would have thought that its own internal controls would limit its output to whatever it is safe for it to provide. From your description this is not happening. You control the alternator output by controlling the current fed to the field magnet, which is the winding in the rotor. If the belt slips, and I assume the belt is in good condition and properly tensioned, you could try driving it faster (if this is practical) as higher RPM means less torque.

I suggest the proper solution is 2 alternators, one for each battery. Watch this space for different, possibly contradictory, suggestions from other people!

Many thanks Perry, for answering and for your suggestion. Maybe I will give the PWM way a try, and see what happens. In the worst case, I could follow the example of a friend of mine, who removed the voltage regulator from the alternator and uses a manual rheostat to regulate the output charging current...
If I find something out, I will update the post.
Thanks again

First off, it sounds like you have problems with your alternator installation. The belt needs to be tighter, or maybe to a different specification. And perhaps there needs to be better ventilation. What temperature does it reach when you consider it to be overheating?

I have two alternators connected together and charging my batteries through an earlier version of one of these alternator to battery chargers. However the purpose of the ABC is to draw a bigger current out of the alternators so as to get the charging done faster and use less diesel. I can't recall if my ABC has over-current protection, but it probably has.

Note, also that the ABC is a 3 or 4-stage charger that varies the charge voltage to match the state of charge of the battery.

The ABC works perfectly well from a single alternator and it has a mode for ensuring that the engine-start battery is kept properly charged.


PS ... the design of electronic circuits that can handle the amount of current that an alternator can produce is well beyond my pay grade.

I agree with Robin. Poor installation. His 12 volt, 120 amp alternator will produce 1.9 horse power. A single "V" belt is good for 10 horse power. OP is probably using an old. glazed-over belt from who know where.

As for running too hot, automobile alternators are NOT built to operate at maximum load for very long. He need to provide cooling air.


I offer some alternatives (!), probably not better but .... You could put low value resistors between the batteries to limit the current flow , not ideal but can be effective .

Voltage difference between batteries ( max) , say 2v . Current , say 100A . R= 0.02ohms .
The belt slipping is only because it’s unsuitable for the load - look at belt properties and /or go for a toothed belt.
You could look at better cooling or shutting down the alternator ( turn field off )when it gets hot - but it sounds as though it is not big enough for the task and won’t last long anyway as it’s being run above it’s rating.

The battery capacity may not be big enough if the voltage quickly drops when the load comes on . You need specialist batteries to withstand constant heavy charge / discharge systems .
Think I would look at the whole system and decide what it needs to work properly .

Many thanks Perry, for answering and for your suggestion. Maybe I will give the PWM way a try, and see what happens. In the worst case, I could follow the example of a friend of mine, who removed the voltage regulator from the alternator and uses a manual rheostat to regulate the output charging current...
If I find something out, I will update the post.
Thanks again

Using a rheostat to control the alternator output worries me as you then have a completely manual system and I can imagine either not charging the battery or, in the blink of an eye, over charging it. You could make your own controller, with inputs to monitor output voltage, current and alternator temperature.

How do you know it is overheating?

This is pure speculation (because I don't actually know) but I would have thought that the regulators built into alternators included over temperature protection. Assuming they do then it getting too hot shouldn't be a worry.

thanks everybody for replying.

the alternator overheating problem arises because the alternator is a car/truck type, so it is designed to handle just the starter battery charge. That usually means a few minutes of hard charge, and then just the minimum in order to compensate the consumption of the loads attached to the battery.

In a boat the set up is the same, plus a domestic battery bank, usually much bigger than the starter one. To give you an idea of the numbers, the starter battery is rated 100Ah, the domestic batteries are 3x180Ah.

Usually, the two battery banks are disconnected from each other, in one of these ways:

  • in the easiest case by using diodes installed between the alternator and the batteries
  • by using a voltage sensitive relay between the starter and the domestic battery. It closes the circuit and put the batteries in parallel when a specific voltage is reached, usually 13.5V
  • the more complex and expensive solution, the one I have found on my boat, is a smart charge regulator (mine is a Sterling Power).

The last one should be the best solution, since the smart regulator has got sensors all over the circuit to prevent overheating, overcharging and over_everything, but the result is that it still charges the batteries for hours at more than 14V, that it does not allow the solar panels to charge ('cause their regulator cut them off at a lower voltage than the alternator regulator does), and that the alternator must work with heavy loads, that reduce its working life.

I found the idea of a voltage sensitive and microcontroller-driven relay interposed between the two battery banks (and the alternator output connected to the starter battery only) a good solution, since it would allow the domestic batteries to be connected to the 14.5v circuit of the alternator only when needed, avoiding prolonged charging periods over 14V and allowing the solar system to work even with the engine running. But this way the alternator would still output a big amount of current when the domestic battery is connected.

The best solution would be to somehow limit the current going to the domestic batteries. This way the alternator regulator would detect an higher voltage in the circuit and limit the output current.

I prefer avoiding tampering with the alternator voltage regulator, even if I know that controlling the field current would be the best solution. But removing the voltage regulator from an alternator is not always that easy, and since I am not the only one with this kind of problems (a lot of friends, boat-owners as well, have encountered the same issues), it would be useful to find a way to achieve it without removing the alternator regulator, so that others could use it.

I am trying to figure out how the alternator voltage regulator works, technically, whether it just cut off the field current or regulate it, and how. That could be a useful piece of knowledge...

Please re-write Reply #7 with paragraphs and white-space to make it readable.

A good test of readability is to read your text aloud as part of your proof-reading.


I have the same problem but with Lifepo4 house bank. Alternator overheat then go off. Lifepo4 are even worse as they have a very small internal resistance and take all current you send them.
My plan is to add a diode to go from 14.5V to 13.8 V then maybee if needed a low value resistor in serial.
Of course the sense wire will still be connected at the engine battery.

I also looked in other options if this one is unsatisfactory.
This project even if the owner has remove many info as it is now commercial can give you some ideas

A great approach would be to have a mppt for alternator. A alternator can generate much higher tension, some people in the US even use it to power 120V resistive charges.
For example running a 100A at 50 A 48 V will not overheat it and give 200A available at a 12V battery.
Of course the bearings and belts should be up to the task.

I think the cleanest economical and future proof aproach would be to swap your regulator with a LIN regulator.
For energy efficiency and better battery management recent cars ACU communicate with the alternator that send info, voltage current temperature errors, and recieve tension information like 14.2V, 13.8 V
To keep cost low it does not use canbus but a single wire slower serial protocol.(BSS, LIN).
Your arduino will just have to manage the voltage by sending info to the LIN according to the temerature of the alternator, the battery temperature, the status of charge of the battery etc.

Many alternators have different regulators available as it is used in different cars, if lucky you will find one with LIN that can fit your existing alternator, otherwise it is possible to modify an old style with a specific IC.