MPPT controller for 24V alternator

[Pizzazz]

I am upgrading my charging system on a 12V boat. Would it make sense to get a cheap 24V alternator and feed it into an mppt solar charge controller?

My understanding is that 24V alternators are more efficient (less current losses, smaller percentage diode loss). Let's assume the efficiency goes up by 10-15%. Then, there will be losses in the MPPT which could be from 3% to 12%, unclear why. I can then use the mppt controller for the solar panels as well.

Does this make sense?

Thanks,
SV Pizzazz

[ozsailer]

Can I preface my response in saying that I am no expert when it comes to what you are talking about but a couple of things stand out to me.

1) MPPT units are just not designed for such inputs. The alternator will put out a relatively constant current that is already regulated as opposed to a solar/wind system where the current varies considerably due to outside influence such as wind and sun.
2) Why not just upgrade your alternator to a larger output to cover loss's, way cheaper than buying a 24 V alternator and MPPT
3) You will spend maximum money for little return

As stated I am no expert but these would be my concerns.

Greg H

[Pitchondesign]

Originally Posted by Pizzazz

I am upgrading my charging system on a 12V boat. Would it make sense to get a cheap 24V alternator and feed it into an mppt solar charge controller?

My understanding is that 24V alternators are more efficient (less current losses, smaller percentage diode loss). Let's assume the efficiency goes up by 10-15%. Then, there will be losses in the MPPT which could be from 3% to 12%, unclear why. I can then use the mppt controller for the solar panels as well.

Does this make sense?

Thanks,
SV Pizzazz

Several things come to mind. If the 24v a alternator is internally regulated, it will expect to see battery voltage. Instead it will see the mppt controller, which provides an unknown voltage, if any, so anyones guess as to what it would do. If you use an externally regulated alternator with an mppt controller, the alternator expects some type of field excitation, which an mppt controller does not provide. There are multiple other unknowns, but it would be an interesting science experiment.

[Pizzazz]

Fair points. I got an 80A internally regulated alternator, with a stated voltage of 14.7V. Sometimes it works well (65A at 14.4V), sometimes it just drops to 40A at 13.8V which is bad for the batteries. It depends on the temperature, batteries, etc. but basically Hitachi internal regulators are no good.

At some point I had a modified Hitachi alternator, with the field wire out. It worked well but it is not designed for continuous operation and I was always afraid I will burn it since the external regulator bypasses the temperature protection. Then we have the efficiency issue where the Yanmar engine belt cannot transfer that much power. In general it is wasteful to operate a 12V alternator at 55% efficiency and then remove the heat. A 24V alternator has much better efficiency I think, which would allow a higher output regulator (say 50A @ 24V) to work from the current belt. Doubling the belt is also an option but some people say small Yanmars do not like strong side forces, so I wanted to avoid that.

My thinking is that if the mppt regulator sees 24-27V, it will try to regulate it by changing the voltage and it will quickly figure out that the current does not change (because it is regulated by alternator internally). So, it will just reduce the voltage to what the battery can take. There would be a loss but it may even out with the higher efficiency of the alternator. I agree it is a non-standard way but I was getting the mppt controller anyway and wanted to avoid the hassle of disassembling my current alternator to get the field wire out.

[Pitchondesign]

Let's assume you got this to work, making the necessary modifications. Then you are out cruising somewhere and something goes wrong. Now you have a non standard installation and must perform troubleshooting and repair. How is that going to work? There are so many things to go wrong on a cruising boat, I for one, value maintainability. It hard enough with standard stuff. Custom designs can be marginally more efficient, and are fine when they are working. My sense is you want to use the same controller for solar and your alternator. Presumably, there would be a transfer switch somewhere. You only need to have that switch in the wrong position once and your alternator diodes are fried. If you don't use a transfer switch, you need some monster diodes, but they insert voltage drop, which affects the voltage the controller sees. By the time your done fixing these issues, you would be better of with a standard configuration.

[dkenny64]

a couple of thoughts.


alternator output..how are you going to control this?
the MPPT doesn't.. so full output all the time? not good.. over heating?


now if the alternator output can be controlled.. might work..


-dkenny64

[john61ct]

Check if Sterling makes a 24/24 DCDC charger, ideally in their BB series.

Otherwise, let us know the results of your experiments.

[guyrj33]

Then we have the efficiency issue where the Yanmar engine belt cannot transfer that much power. .... A 24V alternator has much better efficiency I think, which would allow a higher output regulator (say 50A @ 24V) to work from the current belt.

Sorry Pizzaz, but I don't think you should seriously pursue this idea. The limiting factor for you system is the input belt, you can not double output power by doubling the output voltage.
If you do want to upgrade to 24v I strongly suggest you read up on electricity.

[charris]

Nope. Makes no sense.

[Cpt Pat]

I have no idea why you would want to do that. Alternators don't have a "maximum power point" to "track." Solar panels do have an optimum voltage for delivering maximum power (P=IxE -- the product of voltage times current). Alternators do not. The output of an alternator varies linearly with rotational speed.

This seems to be a fundamental misunderstanding of what MPPT controllers do.

Please don't try that. You'll only lose power in the conversion process, and you may fry the controller by driving it with an inductive power source, for which it wasn't designed. Transistors are sensitive little creatures. And the loss in the alternator diodes is very small -- you won't gain much proportionally by using a 24 volt alternator on a 12 volt bus. Just run your engine 10 RPM higher to compensate.

An MPPT controller will only increase complexity (the enemy of reliability) and create a dependency where you can not charge your 12 volt bus without the MPPT controller. If the MPPT controller fails, the consequences could be catastrophic (exploding batteries).

I've heard of people using an MPPT controller on wind turbines, which may have an optimum rotational speed for any given wind velocity, governed to an extent by load. The MPPT on a wind turbine is a "poor man's variable-pitch impeller" - and probably doesn't cause much improvement in output. Overall, it might even be a loss.

[Pizzazz]

Thank you guys, may be I should scratch the idea.

Just some quick answers, mppt controllers do two things, 1) vary the voltage until they find the maximum power point and then 2) do dc-dc conversion to charge the battery according to a set profile. In my case, I would be using only the second function as the controller would figure out that varying the voltage does not change the voltage/current/power of the alternator. Technically, this can be done with a DC-DC converter only but then I would need a charge profile, etc.

A 24V alternator is more efficient, period. Not only the diode but the I2R losses, which are significantly lower as the current is lower. You can find cheap 12V alternators with 58% efficiency and similarly priced 24V alternators with 75% efficiency. The higher efficiency allows more current to be produced with the same tension on the belt. For example, a 55% efficient 12V alternator will take 13.5V x 80A /55% = 2kW (2.6 hp) from the belt and produce only 1.2kW of electricity, while a 75% efficient 24V alternator will take 1.8kW from the belt and produce 27V x 50A = 1.4 kW of electricity. This part makes sense.

I think I will end up with two alternators, one 12V modified for external control and one to play with the mppt controller and see how it goes. I agree with the comments regarding a single point of failure. Not sure if the inductive load is so important.

[Cpt Pat]

Originally Posted by Pizzazz

Thank you guys, may be I should scratch the idea.


Not only the diode but the I2R losses, which are significantly lower as the current is lower.

My friend, you're off on an odd tangent.

Diodes are not resistors. Resistors have a linear I2R power dissipation (loss) relationship. Diodes do not. Resistance (the "R" in the equation) isn't the governing loss factor with diodes -- voltage drop is. After a very small amount of current (milliamps) are drawn through a diode, it's voltage drop is nearly constant regardless of additional current (until it destructively saturates). Pass 20 amps with a 24 volt alternator, or 40 amps with a 12 volt unit -- the diode voltage drop will be the same (or just barely different). The voltage regulator will compensate - that's its job.

A typical silicon power diode has a forward voltage drop of about 0.8 volt (it varies by type - but is constant-versus-current for that type). So at 1 amp, the drop may be 0.6 volt, and at 100 amps, it may be 0.9 volts.

If you want a lower voltage drop, substitute Schottky diodes, if they are not already used in your alternator. You'll get about half the voltage drop. Not that it will matter much, it'll just reduce the already very small amount of power dissipated in the diodes.

If your alternator had an output of 100 amps, you'd see less than 10 watts lost as heat in a diode. If it's charge efficiency you're after, your time and money are better spent using heavier gauge wiring.


Finally: DC-to-DC converters are not 100% efficient. And don't believe the manufacture's claims about conversion efficiency. They probably have qualifying weasel words like "as high as" in the specifications. You'll have loss in that MPPT no matter what. That's why they get hot in operation.

[Cliffhanger]

This discussion highlights the fact that battery charging on a boat is far from ideal in most circumstances.

[Pizzazz]

Originally Posted by Cpt Pat

My friend, you're off on an odd tangent.

Diodes are not resistors. Resistors have a linear I2R power dissipation (loss) relationship. Diodes do not.

Just correcting inaccurate information:

Losses in an alternator include resistance losses (current flowing through stator and rotor), windage losses, friction losses, mostly in the bearings that support the rotor and diode drop losses in the rectifier.

The resistance losses account for about 65%. Resistive losses depend on current based on Ohm's law. The diode drop accounts for about 8% of the total loss and so on... It is a fact that 24V alternators are more efficient than 12V alternators, check any manufacturer specs.

[Cpt Pat]

My apples are correct, and your oranges are also correct. But we're talking apples and oranges.

Yes, all other things being equal, for the same power output, a 24 volt alternator will have about half the power loss due to internal wiring resistance. (Provided the internal wiring resistances are exactly identical.)

But I was discussing diode losses.


If you do use an MPPT controller intended for solar power applications, please consult the manufacturer of the MPPT unit so they can have an opportunity to dissuade you. Alternators are inductive devices that can generate voltage spikes, especially as the slip rings wear out, that may go well beyond what the MPPT unit can tolerate at its input.


Also, the entire design of the alternator/voltage regulator system assumes it is connected directly to a battery, and you may need a 24 volt source to energize the alternator at startup. I don't want to even think about the dynamics of how the MPPT unit will load the alternator at startup. You'd probably be wise to place a 24 volt battery at the alternator output/MPPT input. The whole schema is crazily complex.

If you look in the cockpit of many light airplanes, you'll see there is a separate circuit breaker for the alternator, and that the checklist calls for turning it on only after the engine is started. That's to keep alternator voltage transients ("spikes") from frying the radios during startup. I doubt an MPPT unit, designed for stable and steady solar panel voltages, will be any more tolerant of transients.