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?
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.
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)
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:
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.
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?
MPPT 75/10 Efficiency Curve 1 Answer