To add my 2c.

TL:DR buy a deep discharge 12V 250 amphour battery(wt or agm) and an applicable mains charge, a suitable solar regulator and a suitable mains convert(12V to mains if you need it). Gain experience with those first. Practically, I buy 2x6V 250amp hour batteries as I can(could) easily lift those if I needed to move them.

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A battery is made up of cells, e.g your 12volt car battery is made up of 6 2.2V cells to give 12volts nominal for car use.

According to a quick web search, your 18650 are cells with a nominal voltage of 3.7volts, so you are going to need 4 of these in series(joined positive to negative) to give 12v (nominal)

Now you have a nominal 2,600 milliamphour of capacity. Note the milliamphour. and convert to straight amphours of 2.6 amphour.

Now to work out the number of cells, it is a simple matter of dividing 200 by 2.6 is 77 strings of cells, then multiplied by 4 of get the total number of cells you need. About 308 cells.

This figure comes from tying everything to a 12volt battery (and 12volt solar panel?), because there is a mass of stuff out there(RV/camping field) using 12 volt batteries.

Now to make this work, you need a regulator(jargon) between your solar panel and a string of cells. This regulator basically takes whatever voltage and current the solar panel is outputting and converts it up/down to the ideal voltage to charge your string(s) of cells. Note, each cell, depending on its type (wet, agm,sla, nicad, lithium, etc) has a required/optimum charging voltage or current. For the first three it is voltage, and for the others it is current and both optimally vary according to the state of charge.

The major problem you are going to have is those 77 strings of cells are not going to perform equally and if you are using second hand batteries will almost certain fail continually if you try to deal with them in bulk.

Cells/batteries in parallel failing are usually not a major problem. The total voltage drops and the internal cell resistance increases and stuff just stops working.

If a cell in a parallel set up fails, it can slowly<->disastrously bring down the whole array.

If the cell fails and its voltage drops, then it can slowly/rapidly bring down all the other parallel strings. Being the lowest voltage in the array it draws power from the rest of the array and gradually discharges the whole array.

Now, a lot of people will tell you all sorts of batteries can be run flat. This is only at a great cost and it will shorten the life of the cell in stuff like capacity, life cycles, charge/discharge rates.

Now that you know about only some of the pitfalls, you are just a little bit wiser if you want o go ahead, scounge a few cells and stat experimenting.