the following detailed description , which references and incorporates fig1 - 6 , describes and illustrates one or more specific embodiments of the invention . these embodiments , offered not to limit but only to exemplify and teach , are shown and described in sufficient detail to enable those skilled in the art to implement or practice the invention . thus , where appropriate to avoid obscuring the invention , the description may omit certain information known to those of skill in the art . fig1 shows a perspective view of an exemplary 360 - volt operating , 390 - volt surge , 190 - microfarad , 15 . 9 - joule ( stored ) electrolytic capacitor 10 which incorporates various space - saving features of the present invention . capacitor 10 has a diameter 10 d of about 14 . 5 millimeters and a total height 10 h of about 30 millimeters , and a total volume of about five cubic - centimeters . thus , capacitor 10 has an energy density of about 3 . 2 joules per cubic - centimeter . in contrast , conventional electrolytic capacitors with comparable electrical characteristics and of about the same diameter have heights greater than or equal to about 40 millimeters and total volumes greater than or equal to about 6 . 6 cubic - centimeters , with energy densities around 2 . 4 joules per cubic - centimeter . thus , the exemplary capacitor is about 33 percent smaller than conventional capacitors with similar electrical traits . more specifically , capacitor 10 includes a cylindrical aluminum case 12 , a header ( or lid ) 14 , and two aluminum terminals 16 and 18 . two rivets 15 and 17 fasten terminals 16 and 18 to header 14 . aluminum case 12 , which houses an active element 20 ( not visible in this view ), includes a circumferential seating groove 12 a and a rolled lip 12 b , both of which secure header 14 to case 12 . fig2 , a cross - section taken along line 2 - 2 in fig1 , shows that case 12 has a thickness 12 t and that groove 12 a is spaced a distance 12 d from lip 12 b . in the exemplary embodiment , thickness 12 t is about 0 . 010 inches , and distance 12 d is about 0 . 145 inches . additionally , groove 12 a has a radius of about 0 . 035 inches , and lip 12 b , which is formed by rolling over the top edge of case 12 , has a radius of about 0 . 015 inches . groove 12 a and lip 12 b each have a smaller radius than the corresponding features of conventional capacitors . in another embodiment , case 12 is vertically compressed to completely flatten or reduce the height of groove 12 a and thus to further reduce the height and volume of capacitor 10 . fig2 also shows that header 14 comprises two bonded layers 14 a and 14 b and has a total thickness 14 t . layer 14 a consists of rubber , and layer 14 b consists of a phenolic resin . although thickness 14 t is about two millimeter in the exemplary embodiment , it ranges inclusively between 0 . 5 to 2 millimeters in other embodiments . in contrast , conventional aluminum electrolytic capacitors use headers that are about three to four millimeters thick . fig2 also shows that capacitor 10 includes an active element 20 comprising about 19 turns of a layered capacitive assembly 21 around mandrel region 28 and two pairs of insulative inserts 30 a - 30 b and 32 a - 32 b separating the top and bottom of active element 20 from interior surfaces of case 12 . for clarity , fig2 omits a plastic insulative sheath that surrounds the vertical surfaces of active element 20 . in the exemplary embodiment , this sheath is a piece of transparent tape having a width of 1 . 125 inches ( or 28 . 6 millimeters ). fig3 , a cross sectional view of layered capacitive assembly 21 , shows that it includes a cathode 22 , a three - foil anode 24 , and four electrolyte - impregnated separators 26 a , 26 b , 26 c , and 26 d . cathode 22 and anode 24 each have a width ( or height ) 22 w , which partly determines a minimum height of case 12 . though not shown in fig3 for clarity , cathode 22 and anode 24 also include insulative or dielectric coatings , for example aluminum or tantalum oxide , on at least their facing surfaces . in this exemplary embodiment , cathode 22 and three constituent foils 24 a , 24 b , and 24 c of anode 24 are about 24 millimeters wide and 100 micrometers thick . cathode 22 is about 422 millimeters long and anode 24 is about 410 millimeters long . although not shown in fig3 , anode foils 24 a , 24 b , and 24 c are connected to a single aluminum anode tab 25 ( which is shown in fig2 ). alternatively , individual anode tabs can be connected to each anode members , and to each other to form a joint or composite anode tab . for more details on these or other types of tabs incorporated in other embodiments of the invention , see u . s . pat . nos . 6 , 249 , 423 and 6 , 110 , 233 , which are respectively entitled electrolytic capacitor and multi - anodic attachment and wound multi - anode electrolytic capacitor with offset anodes and which are incorporated herein by reference . anode tab 25 , shown in fig2 , is ultrasonically welded to rivet 15 and thus electrically connected to terminal 16 . in this embodiment , anode tab 25 is folded over itself ; however , other embodiments omit this fold to reduce the space between header 14 and the top of active element 20 . though not visible in fig2 or fig3 , cathode 22 includes a cathode tab which is similarly connected via rivet 17 to terminal 18 . cathode 22 and anode foils 24 a , 24 b , and 24 c are made of an electrically conductive material , such as aluminum or tantalum foil , with the anode etched to enhance its effective surface area . examples of suitable etched foil structures include conventional core - etched and tunnel - etched foils , and a novel perforated - core - etched foil as well as various combinations of these foils . for instance , one embodiment forms anode 24 by stacking a core - etched or tunnel - etched foil with two perforated - core - etched foils . fig4 shows an example of a perforated - core - etched foil 33 . foil 33 includes two opposing surfaces 33 a and 33 b that define an average foil thickness 33 t and a number of perforations , or holes , 33 p that extend all the way through the foil . surfaces 33 a and 33 b include respective sets of surface cavities 34 a and 34 b , which have respective average maximum depths da and db and respective average cross - sectional areas sa and sb ( measured in a plane generally parallel to the foil ). in the exemplary embodiment , the perforations , which are formed using laser , etch , or mechanical means , have an average cross - sectional area that is 2 - 100 times larger than the average cross - sectional areas of the cavities . depths da and depths db are approximately equal to one third or one quarter of thickness 33 t , and cross - sectional areas sa and sb are equal and range inclusively between about 0 . 16 and 0 . 36 square - microns . the layout or arrangement of perforations can take any number of forms , including , for example , a random distribution or a specific pattern with each perforation having a predetermined position relative to other perforations . perforations 33 p , which can be any shape , for example , circular , have a cross - sectional area ranging between approximately 500 square - microns and 32 square - millimeters in the exemplary embodiment . additionally , the exemplary embodiment limits the total surface area of perforations 10 p to about 20 percent of the total area of foil 33 . the perforated - coil - etched foil can be made either by perforating a conventional core - etched foil or core - etching a perforated foil . further details of the perforated core - etched foil are disclosed u . s . pat . no . 6 , 556 , 863 , entitled high - energy density capacitors for implantable defibrillators . this patent has the same priority date as the present application and is incorporated herein by reference . in addition to cathode 22 and three - part anode 24 , fig3 shows that capacitive assembly 21 includes thin electrolyte - impregnated separators 26 , specifically 26 a , 26 b , 26 c , and 26 d . separators 26 a , 26 b , 26 c , and 26 d , each of which consists of kraft paper impregnated with an electrolyte , such as an ethylene - glycol base combined with polyphosphates or ammonium pentaborate , distinguish in at least two ways from separators used in conventional electrolytic capacitors . first , in contrast to conventional separators which are one - thousandth of an inch or more in thickness to improve fabrication yield and reduce electrolyte depletion , separators 26 a - 26 d are each less than one - thousandth of an inch in thickness . in the exemplary embodiment , each of the separators has one of the following thicknesses : 0 . 000787 , 0 . 0005 inches , and 0 . 00025 inches , with thicker papers preferably placed nearer the center of the active element to withstand the greater tensile stress that interior separators experience during winding . however , various other embodiments of the invention use combinations of these thicknesses , combinations of these thickness with other thicknesses , and combinations of other thicknesses . additionally , other embodiments of invention combine one or more thin separators with one or more conventional separators . ultimately , the use of one or more thinner separators reduces the diameter of the active element for a given length of separator ( assuming all other factors are equal ). second , in contrast to conventional separators which are about four to six millimeters wider than the anode and cathode foils to provide large two to three millimeter end margins , separators 26 have a width 26 w which is less than four millimeters wider than cathode 22 and anode 24 to provide smaller end margins 27 a and 27 b . for example , in the exemplary embodiment , width 26 w is about 27 millimeters , or three millimeters wider than cathode 22 and anode 24 , to provide end margins 27 a and 27 b of about 1 . 5 millimeters . other embodiments of the invention provide at least one end margins of about 1 . 75 , 1 . 25 , 1 , 0 . 75 , 0 . 5 , 0 . 25 , and even 0 . 0 millimeters . the large end margins of conventional separators are necessary to prevent damage to foil areas during high - speed fabrication and to insulate the cathode and anode foils from case 12 . however , the inventors recognized that they are not necessary in all applications , particularly defibrillator applications , where high - speed fabrication is of little concern or where the inventors have devised other ways of insulating the foils from the top and bottom of aluminum case 12 . in particular , fig2 shows that the exemplary embodiment provides two pairs of insulative inserts 30 a - 30 b and 32 a - 32 b , which prevent other conductive portions of capacitor 10 , specifically anode tab 25 and rivets 15 and 17 and the interior surface of case 12 , from shorting cathode 22 and anode 24 . in the exemplary embodiment , these inserts are two pairs of paper disks , with each disk having a thickness of one one - thousandth of an inch and a diameter of about 14 millimeters . however , other embodiments of the invention use not only thinner or thicker inserts , but also different insert materials and numbers of inserts . for example , in one alternative embodiment , one or both pairs of inserts 30 a - 30 b and 32 a - 32 b consist of a polymeric insulator , and in another embodiment , inserts 30 a and 30 b consist of different material combinations , such as paper and a polymeric insulator . as an alternative to insulative inserts , other embodiments enclose substantially all of active element 20 within an insulative bag . fig5 shows an exemplary embodiment of an insulative bag 40 enclosing substantially all of active element 20 , with the exception of the anode and cathode tabs . in this embodiment , bag 40 comprise materials similar to the insulative inserts . fig2 also shows that capacitive assembly 21 of active element 20 is wound around a mandrel ( not shown ), which has been removed after winding to leave an empty mandrel region or cavity 28 . in this exemplary embodiment , mandrel region 28 has a width or diameter of about 2 . 5 millimeters , or more generally less than about 3 . 5 millimeters . in contrast to conventional electrolytic capacitors which have mandrels or mandrel regions with 3 . 5 - millimeter diameters , the smaller mandrels of the present invention allow use of about 2 - 5 percent more aluminum foil without increasing the total volume of the capacitor . another embodiment of the invention uses about the same amount of foil as conventional capacitors with the smaller mandrel region , thereby reducing the diameter of the active element without reducing energy density . mandrels with diameters less than 3 . 5 millimeters are not used in manufacturing conventional electrolytic capacitors primarily because they increase the difficulty in rolling the cathodes , anodes , and separators around them . indeed , a smaller - diameter mandrel increases the tensile stress on the cathode , anode , and separators , leading them to break or tear during high - speed winding and thus to increase manufacturing waste . in addition , the smaller diameter mandrels tend to break and require replacement more often than larger mandrels . thus , conventional capacitor manufactures avoid smaller mandrels to increase manufacturing yield and to accelerate manufacturing . however , these conventional objectives are of lesser importance when making small numbers of capacitors for implantable medical devices , specifically defibrillators . fig6 shows one of the many applications for space - saving electrolytic capacitor 10 : a generic implantable defibrillator 50 . more specifically , defibrillator 50 includes a lead system 52 , which after implantation electrically contacts strategic portions of a patient &# 39 ; s heart , a monitoring circuit 54 for monitoring heart activity through one or more of the leads of lead system 52 , and a therapy circuit 56 which delivers electrical energy through lead system 52 to the patient &# 39 ; s heart . therapy circuit 56 includes an energy storage component 56 a which incorporates at least one capacitor having one or more of the novel features of capacitor 10 . defibrillator 50 operates according to well known and understood principles . in addition to implantable defibrillators , the innovations of capacitor 10 can be incorporated into other cardiac rhythm management systems , such as heart pacers , combination pacer - defibrillators , and drug - delivery devices for diagnosing or treating cardiac arrhythmias . they can be incorporated also into non - medical applications , for example , photographic flash equipment . indeed , the innovations of capacitor 10 are pertinent to any application where small , high energy , low equivalent - series - resistance ( ers ) capacitors are desirable . in furtherance of the art , the inventors have devised unique space - efficient packaging for aluminum electrolytic capacitors which allows either reduction of the actual size ( total volume ) of capacitors with specific electrical traits or improvement in the electrical traits of capacitors of a specific total volume . for example , in their exemplary embodiment , the inventors use thinner and narrower separators and top and bottom insulative inserts to achieve a capacitor which is about 33 percent smaller than conventional capacitors having similar electrical traits . the embodiments described above are intended only to illustrate and teach one or more ways of practicing or implementing the present invention , not to restrict its breadth or scope . the actual scope of the invention , which embraces all ways of practicing or implementing the concepts and principles of the invention , is defined only by the following claims and their equivalents .