NOVELTY - The battery pack (1) has battery cells (16a,16b), battery holders (18a,18b) with battery cell receiving sections (19a,19b,19i) that receive battery cells, and metal plates (23a-23e) joined to battery holders and formed with terminal contact sections. Rubber rings (21a-21h), each having an opening, are placed between end surfaces of battery cells and end sections of battery cell receiving sections. Portions of the rubber rings exposed from the end sections of battery cell receiving section are constricted by positive electrode terminal sections and metal plates near terminal contact sections.    USE - Battery pack used for e.g. electric instrument, electric vehicle, electric power system (all claimed), electric tool, power supply device for backup use and emergency use, and hybrid vehicle, including personal computer and domestic appliance such as vacuum cleaner and refrigerator.    ADVANTAGE - The battery pack attains improved waterproofing properties. Provides satisfactory heat dissipation properties of battery cells received in exterior upper case and exterior lower case since hole sections are formed in exterior upper case. Battery pack is prevented from breaking down and deteriorating due to moisture flooding in from the hole section since flooding into the battery cell is prevented. A thermally conductive material which contains metal powder or carbon and has high thermal conductivity can be adopted as materials of battery holders, and heat generation of battery cell can be effectively dissipated to the outside using the materials. Mechanical strength of battery holders against external impact is enhanced since reinforced plastic which contains fiberglass or carbon fiber and has excellent mechanical strength can be used as materials of battery holders. High stability can be obtained since it is possible to reliably insulate between the battery cells. Position of battery cell is prevented from deviating due to external impact since battery cell is stably fixed to the battery cell receiving section of the battery holder.    DETAILED DESCRIPTION - An exterior upper case (10a) is formed with hole sections (12). The positive electrode terminal section is formed on one end surface of battery cells (16a,16b), and a negative electrode terminal section is formed on other end surface of the battery cells. The ends of the battery cell receiving sections (19a,19b,19i) are opened, and end sections having openings are formed in the other ends of the battery cell receiving sections. Predetermined portions of the rubber rings (21a-21h) are constricted by an end surface near the positive electrode terminal section and the inner surface of the end section. INDEPENDENT CLAIMS are also included for:    (1) an electricity accumulation system in which battery pack is charged by power generator which performs power generation from renewable energy;    (2) an electronic instrument;    (3) an electric vehicle; and    (4) an electric power system.    DESCRIPTION OF DRAWING(S) - The drawing shows the exploded perspective view of the battery pack.    Battery pack (1)    Exterior upper case (10a)    Hole sections (12)    Battery cells (16a,16b)    Battery holders (18a,18b)    Battery cell receiving sections (19a,19b,19i)    Rubber rings (21a-21h)    Metal plates (23a-23e)