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jonathanr avatar image
jonathanr asked

MPPT as a DC to DC converter

I have a large battery with a nominal voltage of slightly over 48v. On the solar side, I have 2x MPPT 100/20 and 1x MPPT 150/35, with 48v loads made up of a 48v Phoenix 1200w inverter and a 48v to 12v DC to DC converter buck converter (non-Victron). I have all of the Victron devices monitored via Venus OS

I'm hoping to use a spare SmartSolar MPPT75/15 as a replacement to the DC to DC buck converter, where I would connect the 48v battery to the PV input and use either the load output or battery output of the MPPT75/15 as a 12v power source to my 12vDC system. I have three questions.

  • First, is it better to use the battery output or load output of the MPPT75/15 as the DC output to my 12v system?
  • Second, is there anything I need to configure beyond setting the battery voltage on the MPPT75/15 to 12v in order for this to work and to have Venus OS understand this as a DC to DC converter, not a PV charger?
  • Third, generally, my 12v DC sources draw less than 15 amps; however, given that I already have the buck converter, I plan to wire it in parallel with the MPPT75/15, and have both the input and output connections of the buck converter switched so it will normally be disconnected but can be manually connected in the event of a higher current need. Does anyone see any issues with this setup?
MPPT ControllersVenus OSdc system
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1 Answer
nickdb avatar image
nickdb answered ·

Just use the search function on the site, plenty of others having the same idea and then learning the wonders of physics.

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jonathanr avatar image jonathanr commented ·

Thanks, I have done some searching.

I found a bunch of threads looking to use an MPPT as a step-down battery charger. The primary risk described was related to the PV connections being shorted on some error conditions, which I planned to manage by fusing the PV (48v battery) input.

I also found some threads about using a DC charger as a PV input to an MPPT to charge a battery, with the primary risks being around overloading the DC charger.

In both of the above use cases, there is a 12v battery involved which is being charged; however, in my use case, there would be no other voltage source on the 12v side.

Some hints as to which physics you are referring to would allow me to consider that issue.

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