Adding a separate MPPT to an inverter system

Hi There,

In my setup, I two two strings of panels on east and west side of my roof. Each string has 6 approximately 400W panels serially connected. I would like to ensure that each string gets its own MPPT for optimal performance. I have previously found out that when I directly connected these strings in parallel, I ran into a malfunction in one of the strings (most likely bypass diodes in the strings). So I now want to do things right to avoid a similar future problem.

The issue is that my solar hybrid inverter has a single MPPT module. How can I best add the second MPPT to my system? It seems to be suggested that the second MPPT output should be directly connected to the battery. Doesn't it create a weird configuration where one string goes to an inverter and then to a battery and the other string directly goes to the battery (I mean after the MPPT). I guess if I make this connection, the inverter would be fully unaware of my second string which goes to the battery directly. Wouldn't it cause a problem?

Or would you suggest that using blocking diodes in a combiner box a better option for my system?

Thanks for any insights.

Hi Ahmet,

If I understand your description correctly, your first string doesn't actually go into an inverter,  it goes into an MPPT charger that happens to be packaged with an inverter. It's the same as if it were an independent charger.

You can,  therefore connect a second charger (I have two chargers on my system) but you want to make sure the charging parameter settings are identical to the charger in the inverter system.

Hi Nathanael,

Yes, the inverter is an all-in-one type inverter which includes MPPT. I do not see this MPPT but it must be inside the inverter box.  The description of the inverter states "Mppt 90-500v Pv Input 100a" , which I understand to contain an MPPT with the given specs.

If I now buy a second MPPT with the same voltage and current settings (I guess it doesn't have to be the same, but it must match my panel specs) AND program the MPPT to use the same charging parameters as the built-in MPPT that comes with the inverter, I should be good to go, right?

The only thing that I do not understand is that, the inverter would be unaware of this second MPPT. But I guess it is not a problem.

Thanks,
Oguz

You also need to figure out what the "bulk", and "float" settings are on your first charger so that the second charger can operate synchronously.

You are correct that the second charge does not effect the inverter; the inverter is "ouput" from the batteries (unless you charge with a generator or grid power) and the chargers are "input".

What's the brand/model of your All in One inverter?  With some brands, they make more than one model of either inverter or a stand alone MPPT charger.  Some brands are designed so that they can communicate with each other via a serial connection, and if your brand supports that, it might be able to slave a second controller to the AiO?  Some controllers, such as my Midnite 200, have this function, and controllers can be placed in parallel, but without knowing your brand/model, that's just guessing.

The second option, already suggested, is to just wire in a second MPPT controller in parallel, with the bulk/absorb/float setting identical to those of the AiO.  That is likely to be good enough.