I asked my contact there and she confirmed they do use minus Delta charging. Here is the answer to my question.
Hi Victor,
For charging NIMH batteries, MX4 adopts pulse charging, and enable 0ΔV/-ΔV termination plus the CVSA tech (NiMH Charging voltage slope analysis), and other multiple schemes to intelligently determine when the batteries get fully charged. These charge algorithms ensure faster charging, higher accuracy of full-charge detection, and lower battery temperatures on NIMH batteries. Besides, the charger has multiple protections, such as reverse polarity protection, Over-current protection, Over-voltage protection, over-charge protectioin, Short-circuit protection, overtime charging protection…
And yes, you are correct, NIMH batteries only get 500 mA per slot regardless how many is used or which slot
As far as your usage with brownouts, I’m not sure. It does not have its own internal battery or anything, so I presume it’s going to follow how well your incoming voltages. So you may want to run your solar panel through some type of small power bank or UPS