If there is a BMS, why do I need a solar panel controller?
Attached directly to a hypothetical 9S battery (thus, minimum voltage 3.0V x 9 = 27.0V ; max 4.2V x 9 = 37.8V ) The panel has a short circuit current of 8.44Amps. That''s the maximum it can ever deliver. When the battery is empty (27.0V), the charging power will thus be limited to just 27.0V x 8.44A = 227.88 Watt.