NOVELTY - An insulator for a battery pack (10), comprises a lower sheet (18) component; a side wall sheet(s) (22); and an upper sheet (20). Each sheet comprises a sheet or mat of inorganic fibers. The sheets combine with one another to form an insulated cavity (29) bounded by the lower sheet, side wall sheet(s), and upper sheet.    USE - For a battery pack for a battery pack assembly (8) for providing power for a vehicle, which is a hybrid vehicle, which runs on gas or diesel and electricity (claimed).    ADVANTAGE - The insulator is desirably formed from inorganic fibers that have been constrained to control the bulkiness of the fibers, i.e. to minimize the thickness of the insulator, while providing insulation to desirably allow a temperature of the battery pack to be controlled, within a narrow temperature range during operation. Minimizing the space for the insulating material can allow more space in the air passages for circulating air, to perform the heating and cooling functions in the battery pack. This can reduce the number of heating and/or cooling cycles and cycle times required to maintain the battery pack within the desired temperature range. Thus, the life of the air-moving device (e.g. the fan or blower motors) can be extended, the efficiency of the batteries can be improved, and the battery life can be prolonged..    DETAILED DESCRIPTION - An insulator for a battery pack, comprises a lower sheet component, a side wall sheet(s), and an upper sheet. Each sheet comprises a sheet or mat of inorganic fibers. The sheets combine with one another to form an insulated cavity bounded by the lower sheet, side wall sheet(s), and upper sheet. The sheet(s) comprises an attaching component on the sheet opposite the insulating cavity, and comprising a pressure-sensitive adhesive layer, a hot melt adhesive layer, a structural adhesive layer, a hook and loop type fastener, and/or a headed fastener.    INDEPENDENT CLAIMS are also included for the following:    (a) an insulator assembly comprising: the insulator; and a housing comprising a lower tray (14), tray side wall(s), and a removable lid (16), the insulator sized to be positioned within a tray cavity (30) formed by the lower tray, tray side wall(s), and removable lid; and    (b) a method of insulating a battery pack in a vehicle and maintaining the battery pack within a temperature range, comprising: providing a battery pack; and at least partially surrounding the battery pack with the insulator.    DESCRIPTION OF DRAWING(S) - The figure is a partial cross-sectional view of a battery pack assembly or power supply apparatus insulated and mounted, in a vehicle.    Battery pack assembly (8)    Battery pack (10)    Tray (14)    Lid (16)    Lower sheet (18)    Upper sheet (20)    Side wall sheet (22)    Insulated cavity (29)    Tray cavity (30)