diy solar

diy solar

Do I need a separate MPPT controller if I buy Renogy DC to DC Charger w/ MPPT?

Roy T

New Member
Joined
Dec 4, 2021
Messages
2
I'm building the basic 400 watt system that will have three 100 AH SOK lithium batteries. I watched the Long Long Honeymoon solar install video where they installed a stand alone Renogy DC to DC charger and a separate Victron MPPT controller. I'm looking at the suggested MPPT controllers you have listed but it doesn't include the new 50 amp Renogy DC to DC charger with MPPT that lists for $299.99. As I understand it, when the vehicle is running the charger will pull any available power up to 25 amps from the solar panels and limit the alternator to 25 amps. Reducing the alternator output should also improve fuel efficiency because the engine won't have to work as hard. But if the panels are turned off, the alternator can provide 50 amps.

Question 1: Will the MPPT controller work if the vehicle is turned off and the panels are turned on?

Question 2: Is there an advantage to having a separate MPPT controller if I buy the Renogy DC to DC charger with MPPT?

Question 3: The Renolgy DC to DC chargers require a separate wire connected to the ignition circuit if the tow vehicle is equipped with a smart alternator. I can see where there may be times when you want to turn the charger off to conserve engine power. Some installs show connecting the wire to a fuse that is only powered when the ignition is turned on. In the "All About RV's" video the wire is connected so it can be turned on manually with head light switch. But the video shows that you can turn it off by turning the headlight switch to "Auto". Wouldn't the charger turn back on when the headlights automatically turn on? In an "RV with Tito" video he installed a separate toggle switch. Is this a better solution?
 
400W / 12V = 33.3A

With the renogy DC to DC + MPPT, you can ONLY put those panels in parallel - must not exceed 25Voc.

If using BOTH alternator and solar, each is limited to 25A.

1) Yes.

2) Yes. The MPPT is very limited. IMHO, separate DC-DC for alternator and true MPPT for solar.

3) 600W of charging from the - that's less than 1hp. The only time I would care is if I happen to need to replace my chassis 12V because it has failed. Furthermore, once the battery is full, it uses negligible current. IMHO, this is a solution inventing a problem.
 
Check out the Maximum PV Input Voltage on that DC-DC with MPPT device. If your panels have too high of a Voc then they are incompatible with the device.
 
I just installed a Renogy 50 amp DC-DC charger with MPPT (note it was on sale for $230 for much of November on Amazon), overall I think it works well, but has a couple of limitation, also if you get one I strongly suggest you get the BT-2 bluetooth dongle, as it adds a lot of functionality.

The 2 big downsides to this is limiting alternator to 25 amps if there is any solar input, so you might have a cloudy day with 1-2 amps of solar input and 25 amp limited draw from the alternator, resulting in only 27 amps going into the battery. Some people work around this by adding an inline relay with remote switch so they can turn off solar on cloudy days, allowing for full 50 amps to charge from the alternator. (some variant of this is on my to do list)

The other big limitation is the 25V max VOC for solar panels, my 100W Grape Solar panels have a VOC of 21.9, which in effect limits you to 12V panels wired in parallel, this was not an issue for me as I already had 400 watts of panels wired in parallel which were feeding a PWM controller installed about 6 year ago.

Note if you are concerned about drawing 50 amps from the alternator, the bluetooth software lets you set max amp draw in 10 amp increments.

Also note some people have issues with this unit and low amp output when using Renogy's suggested 4 awg and 6 awg cable size, I suspect due to voltage drop, running larger .cable seems to solve this problem, I ran 1/0 15 feet to the alternator, which may be overkill, but I had it on hand.

p.s. there are a couple of good youtube videos that show bench testing this unit, unfortunately youtube will not load for me at the moment so I can't post a link.
 
Last edited:
Isaac - Thanks for the feedback. You've convinced me to go with a stand alone MPPT controller. Do you have a smart alternator and if so how did you run the wire that lets the charger know the vehicle is turned on?
 
No smart alternator here, I have a 20 year old class A motorhome with 8.1L gas engine. For me the combined unit made sense, as I already had the parallel solar panels in place, I just need to add a solar disconnect relay for cloudy days, so it will do full 50 amp charge from the alternator. My only debate is should it be a simple manual toggle switch on the dashboard, or something more automatic, like a voltage sensing relay controller.
 
Chiming in on alternator charging: I saw a video where a user added the smart alternator wires with a standard alternator, because it would then cut out solar charging and allow full 50 amp charging from the alternator.

As far as solar input voltage, what’s the best way to reduce incoming voltage so it can be used with this unit? Say I have a large panel that has 50V+ open circuit. Could I just add a buck convertor to change it down to 20v? Or a cheap MPPT controller?

I know it’s less efficient but a single high quality panel has more power in less space which would make up for it. And the higher voltage is usable for other things I have when parked at home.
 
Back
Top