Solar panel with DC-DC Converter and Inverter connected in parallel

[ReWoP]

Hi all, first i would like to say that i'm a electronics hobbyist with no formal education on this subject so please bare with me 

I'm working on an idea that i had for some time that consists to attach a battery to a PV system composed by Grid-tie micro inverters. I have the system working but i'm having troubles on the "charging" side.

Here is a small "schematics" of that part.


Basically the charging part consists on a DC-DC Boost converter connected in parallel with the Grid-tie inverter so the the output of the solar panel could be used to both produce mains electricity and charge the battery.

What? Why?
My goal is when the battery is fully charged the Solar panel output is consumed by the inverter.

Here are some specs:

Solar panel

• Power: 250W
  Imp: 8A
  VOC: 37V
  VMP: 31V

DC-DC Boost Converter
150W 10A 10-30V to 12-35V Step Up CC CV DC DC Booster Converter
https://www.ebay.com/itm/CC-CV-Buck-Boost-Converter-80W-Booster-DC-DC-Step-Down-Step-Up-Adapter-Adjustabl/253702023584?hash=item3b11d1a9a0:g:YfIAAOSwrU1bK0P2:rk:1:pf:1&frcectupt=true

• Input: 10-30V
  Output: 12-35V
  CV (set to 31V)
  CC (about 3A)
  UVP (set to about 11V)

Battery Charge Controller
10A 12V 24V wincong SL03-10A solar Charge Controllers LCD Li Li-ion lithium LiFePO4 batteries
https://www.ebay.com/itm/SL03-10A-10A-12V-24V-Wincong-solar-Li-ion-battery-charge-controller-regulators/263341720180?hash=item3d5063c274:g:iNQAAOSw32lYvlJW:rk:1:pf:1&frcectupt=true

• Type: PWM

Here are the problems i'm having:

Sometimes there no AC output and only 0.1A DC output
I think that this happens when the solar panel output voltage is drawn down to about 10V (that is what i get on a volt meter on the solar panel output) by the DC-DC converter and/or the inverter.
I believe that there is no output because the DC-DC converter does not get enough voltage to work and 10V is too low to the inverter.
Also, i have some Clip On RFI EMI Noise Filters Ferrite Core on the input side of the inverter and when this happens they do a high pitch sound and if i apply some pressure one the ferrite cores the sound changes as if they where vibrating.
I've noticed that when this situation happens and if i disconnect the inverter, the DC-DC converter starts to output, some times about 3A, so there is power on the solar panel (and i believe that the solar panel voltage rises).
Sometime if i fiddle with the current limit on the DC-DC converter, without disconnecting the inverter, i start to get some output.

Sometimes few or none AC output and 3-4A DC output
I've noticed this on bright days, when the solar panel should be outputting 6-7A, i get some watts AC (5-20W oscillating) and the DC-DC converter output is 3-4A, oscillating. On this situations if i also disconnect the inverter the DC-DC converter output rises to 6-7A. Again, when the inverter is connected i can notice the high pitch sound on the Clip On Ferrite Cores.
So.. where is the power going?

I've tried a similar DC-DC converter with no UVP and no CC and the result is worst, there is no AC or DC output, only heat on the DC-DC converter and high pitch sound on the Clip On Ferrite Cores...


Is the problem at the DC-DC converter, on my setup or both?
Is this setup harmful to the inverter?
Is there a better way to achieve AC + DC output on this setup?
If not, is there a way to send the power to the inverter as soon as the battery gets full?


Big thanks in advance! 

[station240]

Yeah, the way MPPT works, means this isn't going to work, at least not in it's current form.
The grid tie inverter puts a load on the solar panels, adding another parallel load (the dc-dc converter) upsets the MPPT function.

What you need is a way to only power the inverter OR charge the battery.
You can determine if the battery is full, by measuring it's voltage, or the current passing in.
A full battery will stop at a certain voltage, and draw very little current.

[ReWoP]

I knew the MPPT would always try to suck all the power from the panel that's why i opted for a CC DC-DC converter.
The idea was if i limit the output current, say to 50% of the output, the MPPT point wold came down to the remaining 50% output thus splitting the solar panel output power between the inverter and battery charger.

Guess this is one of those situations that the practice does not met with the theory... 

Is there any "quick and dirty" way to check if the battery is fully charged (as you said by voltage / current drawn) without the use of a custom solution?
I'm thinking something on the line of a relay module that would switch from battery charger to the inverter that i could buy on ebay. 

[ReWoP]

Well.. I think i found the solution! 

Since i don't see a way to have the load in parallel i'm applying the battery first, AC second approach.

Found this module on ebay that i believe can take care of the switch of the solar panel between the battery and inverter when the battery is full with some nice features:
https://www.ebay.com/itm/1D86-Voltage-Monitoring-Module-Demoboard-Microcontroller-Relay-DIY-A613/202406009531?epid=19022747473&hash=item2f20569abb%3Ag%3AiWYAAOSwjKJbduU7&LH_BIN=1

I think it has a fighting chance

Yeah, the way MPPT works, means this isn't going to work, at least not in it's current form.
The grid tie inverter puts a load on the solar panels, adding another parallel load (the dc-dc converter) upsets the MPPT function.

What you need is a way to only power the inverter OR charge the battery.
You can determine if the battery is full, by measuring it's voltage, or the current passing in.
A full battery will stop at a certain voltage, and draw very little current.

Thanks station240 for you input 

[NorthGuy]

Why not to charge the battery using a charger connected to the AC side?

The efficiency of (inverter + AC charger) is likely to be better than (booster + DC charger).

[station240]

Why not to charge the battery using a charger connected to the AC side?

It's a grid tie inverter, meaning the battery charger would draw power from the grid at night.