The IC thermal specification is the junction temperature Tj, and Tj(max) is 150°C. Data sheet P17 Fig.1 shows the efficiency curve when VBUS=12V, so you can calculate the total circuit loss from the input voltage, charging current, and battery voltage.

However, not all losses occur in ICs, and losses also occur in inductors and input/output capacitors. Assuming that loss is the loss of the entire IC, the increase in Tj can be calculated from the thermal resistance RθJA (P8 8.4).

See datasheet for details.
http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=BQ25892&&fileType=pdf