Hello Juan,
Good day. Thank you for your question.
If the "ON NEED" charging scenario is selected, battery charging will start only after battery voltage drops below 3.9V (close to 90% battery charge) and this scenario saves a bit of external power source (in case if it is an accumulator that is not being recharged constantly). Whereas if the "Always" mode is selected, the battery will charge all the time.
Kind regards.