The short answer: if your panels are the same type/brand (or same wattage and very similar Vmax), connect in series up to what your MPPT controller can handle. I have four 100W 19Vmax panels - 76V - on my 100/30 Vic, which can handle up to 100V input. Some controllers are limited to 30V or 50V.
Your panel voltage will vary by about 2V to 4V depending on how much sun and how much current you are asking the panel to supply.
The higher voltage from two panels in series does not pull more current out of the panels - but the higher voltage (38V instead of 19V - it varies by a few volts) means that the voltage is always more than 5V above your battery voltage and the MPPT controller will always be adjusting the voltage (up and down) to get the most watts (current X voltage). The MPPT controller actually sets the output voltage of the panels to get the maximum watts.
You can see this with your AGM batteries. Charged and with almost no load, they should be about 12.6V. Now add load by turning on the overhead fan and the furnace (about 7A) and watch the battery voltage drop. The amount of current drawn out of the batteries changes the battery voltage. The MPPT, by controlling the current drawn out of the panels changes the panel voltage. What's different about solar panels is that the change is non linear. Lets say, for the same amount of sun, your two panels in series are producing 40V at 5A. That's 200W charging your battery at 14V X 14.29A. (The MPPT takes in whatever voltage and current the panels supply and outputs what is needed for the battery.) But if the MPPT finds (generally by trial and error with small changes) that if it lets 5.4A flow from your panel, the panel voltage will drop to 39V. 39V X 5.4A is 210.6W. Your MPPT just got you an additional 10.6W so your battery will now charge a bit faster at 14V and 15.04A. As the amount of energy from the sun coming into your panels changes, this sweet spot changes. Your MPPT is always hunting for it when the panel voltage is 5V more than what is needed to charge the battery or meet your load.
This is a good explanation, but it would be good to clarify on battery voltage + 5V point. The requirement that the panel voltage be 5V above the battery voltage is somewhat artificially implemented by Victron and only applies at sunrise - once the MPPT is active it may track panel voltages below Vbattery + 5V. The reason it is done this way is that there is almost no power available from a solar panel when the open circuit voltage is below 17V. If the controller tries to load the panel down the voltage will immediately drop below the battery voltage and the MPP search will fail, wasting power. Solar panels are current sources, not voltages sources (which is a hard thing to wrap your head around), and as a result the Voc will rise with very little illumination, and is almost constant, it is the current that increases with increasing sunlight:
As a result, there is actually very little power to be gained by putting the panels in series from the Vbat + 5V limit. There are two other advantages to series panels, you can use smaller wires to the panels as the current is lower, and you can safely 'over panel' your controller - that is you can put more watts of solar panel than your controller is rated for based on the assumption that the panels will almost never put out their rated power in real world situations (unless you live at high altitude near the equator). The disadvantage is that it makes it much harder to add a portable panel to your system, and it is harder to mix different sized panels in series.
Edited by rando, 04 March 2021 - 02:57 PM.