question

Hey Tim,

mppt controlers are sweeping the pv voltage to find the point were you get the most power out of the module. The mpp is dependent on module specifics, solar input, temperatures and some more factors.

Simply put: The controller will load the panel with different amps during the sweep, wich in turn will result in different module voltages. Itll then note the v/a combination where it got the most power out of the module and stay there. It will periodically resweep to adjust to changing solar conditions. If you put a second controller in parallel on the same solar array one controller can not trust the measurements during the sweep anymore, because the other controller is pulling down the panel voltage. If it starts the sweep the other controller will think that the conditions changed (e.g. clouds) and will also try to start to find the new mpp. Measurements for both controllers are now complete garbage as they are both affecting each other.

What actually does work is to put both controllers on the same panel, but only using one at a time, e.g. by disconnecting the battery. According to your post the system doenst need a lot of solar input. So charging one bank and then the other, by manually switching the banks on and of, or by using a relay and some automation would be a working solution.

What also might work, but ive never tried that, is to put the 24v from one bank into the solar input of the second controller to just use it as charger. Not tested though and no expierience with it.