# Can I use an MPPT as a dc to dc charger for an alternator?

Can I use my MPPT 75/15 solar controller as a dc/dc charger to charge from my alternator to my secondary deep cycle battery in my 4x4?

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Hi Luke,

that is not going to work.

A vehicle alternator has its own voltage control mechanism, making it impossible for the mppt to work properly.

You can use the Victron DC Buck Booster for this. That should work.

Best Regards,

Markus

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@Markus, are you omitting the vehicle battery in this scenario? (your other comment on this thread re. alternator excitation seems to imply an absence of a battery).

In other words, the impedance of the vehicle's starter battery is very low so the MPPT controller will pretty much see an immovable voltage point on the input. The MPPT will no doubt hunt around a little bit in terms of current draw, but given the other loads that come and go on the vehicle I can't imagine the vehicle charging system won't be able to cope. So long as the MPPT controller doesn't attempt to draw excessive input current, i.e. use a lower-rated MPPT controller, then I'm puzzled as to why it's "impossible to work properly".

Hi @BenL
this thread is very old now.

In the meantime Victron has expanded their range of DC-DC chargers, so there is no real need of using an MPPT for Battery to Battery charging.

But I tell you anyways why it wont work:

A Victron MPPT needs PV Voltage to be +5V higher than Battery Voltage to start operating. If you want to charge a 12V battery from a 12V battery, there will (should) never be 5V difference so it will not start charging.

The next question you may come up with would probably be: I want to charge a 12V battery from a 24V battery, so we have a 5V difference here?!

There is a chance that the MPPT will short circuit the PV array under certain circumstances (discussed here in community in some other threads) this short is ok for a PV array, not for a battery.

You might now say: ok, lets put a quick blowing proper rated fuse between the first battery and the PV input of the MPPT.

I say: have a good and stable and supported solution and so just buy a DC-DC charger instead of an MPPT to do proper battery to battery charging.

Cheers,

Markus

markus ♦♦

Thanks for the quick reply and confirmation re. minimum input voltage requirements; as to "why" - it's partly curiosity and partly as I have a spare MPPT controller but need a DC-DC charger :-). The Orion-Tr does look to fit the bill.

benl

Ok, it's a moot point - attempting to power a MPPT 75/15 controller or any similarly designed buck regulator indeed "won't work", but only for the simple reason that the input voltage needs to be somewhat higher than the output. The manual for the 75/15 notes at least 5V higher to start, and 1V higher to keep running. i.e. the nominal 13.8V car battery has no hope of getting going with a 12V house battery, let alone a 24 V system.

If somehow you have 48V vehicle power, then it'd probably work fine :-)

Please don't ignore that the MPPT charger can short the PV input. If it does that when connected to a battery, bad things may happen.

See Error codes 38 and 39 for some situations that can cause it:

I have to add, there is a way to get this done.

I experimented with various vehicle alternators some time ago, to use them as a generator for my MPPT hydro power installation.

A vehicle alternator is separately excited, so the voltage control mechanism increases and decreases the excitation voltage to match the constant output voltage needed in your vehicle and to counteract to the changing speeds of the alternator.

To get rid of the voltage controI mechanism, I tried to connect the rectified output directly to the excitation coil. The result was that the alternator was going to self excitation and producing maximum output voltage possible, because of the rest magnetism in the main coil.

But I have to say that if this works or not, depends on, if your alternator has enough rest magnetism in the main coil to build a electric field.

If not, you will have to apply a short pulse of dc current to the excitation coil to start self excitation.

When you use a second(!) alternator in your 4x4, and "reconfigure" it for self excitation and the voltage at max engine speed stays safe below 75 Volts, then there is a chance to get this to work.

But I do not think that this would be a reasonable solution, I just wanted to add it, since I absolutely don't like guys telling me there is no way to do something.

There is always a way ;o)

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Thanks for the above.

Abit above my pay grade unfortunately.

Is it an option to use an Orian tr dc-dc converter. 12/24. Connect the converter to alternator, output on the converter will feed supply to Mppt. I just need to switch between solar or alternator when needed.

@Markus We are using "48V" alternators and rectifiers that result in a flat 58vDC output. We need to bring that into a CANbus controlled Lithium battery environment. There are few choices. One option is to experiment using a Smart Solar 150/75 or similar. The solar controllers become slave control under the GX with CANbus and so I can "control" the charging of the batteries. Your thoughts?

That doesn't sound like a good idea to me.

Why do you need to switch?

Connect the mppt and the dc-dc converter in parallel to your battery.

When there is pv power your battery will get charged, when you drive, the dc-dc converter will load your battery.

Why not this way?

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So the mppt is connected up as per normal.

The Orian is connected between alternator/primary battery and connects to secondary battery.

But I presume I would need a 12/12 converter as the alternator won’t produce the required 18 volts that the 24/12 unit requires.

Which converter would you suggest in terms of producing enough amps.

Will the Orian also be able to safely reduce the 80amps that the alternator produces?

Sorry for all the questions.

I think this is the right product for you if you have a 12V Primary Battery:

https://www.victronenergy.com/dc-dc-converters/buck-boost-dc-dc-converter-25a-50a

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I did something very similar to this and it works well. I used a 12V to 48V DCDC converter to boost the alternator output to 48V and fed that voltage to the MPPT. I used this DCDC converter:

https://smile.amazon.com/gp/product/B01LYVSL53/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

I used four units in parallel which required using balancing resisters and diodes to make the converters work together. I charge my 24V camper system from the 12V tow vehicle.

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Hi @Markus

A vehicle alternator has its own voltage control mechanism

Is this in reference to all alternators or only smart alternators?

https://www.redarc.com.au/alternator-vs-fixed-alternator

Also, in relation to this suggestion:

Connect the mppt and the dc-dc converter in parallel to your battery.

Can the DC-DC converter be swapped out with a VSR? ie VSR and MPPT in parallel?

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My experience is that the source must deliver more amperage then the mppt input requirements for it's maximum output.

I was testing my setup with an external supply and it overloaded the supply unless I used the app to turn down the mppt charge rate.

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