fig1 illustrates an exemplary power add - on device 100 useable for powering a manual wheelchair , in accordance with a preferred embodiment of the present invention . fig2 b illustrates the power add - on device 100 attached to a manual wheelchair 201 having a camber tube 202 . the power add - on device 100 includes a housing 102 . the housing 102 includes , therein , two conventional electric wheel - chair motors that are electrically coupled to a power source , and are arranged so that each drives a wheel 104 . the manual wheelchair 201 can be attached to the power add - on device 100 simply by backing up the manual wheelchair 201 until a pair of latches 103 holds the camber tube of the manual wheelchair 201 . advantageously , no modification is required to the manual wheelchair 201 . preferably , the latches 103 can be activated by an electric actuator so that when a user wants to de - couple the power add - on device 100 from the manual wheelchair 250 , the user employs remote control 108 to activate the electric actuator so that the latches 103 assume an open position , releasing the manual wheelchair 250 . preferably , the remote control 108 is a conventional joystick or other such user - friendly remote control device . preferably , the power add - on device 100 can also be released manually , for example , by pulling a lever . preferably , the power add - on device 100 has built in recline and anti - tip features so the user can recline and relieve pressure from their seat cushion safely which is very important in order to avoid pressure sores . preferably , the anti - tip feature is at least in part accomplished by employing front anti - tip castor wheel 107 and rear anti - tip castor wheel 106 . when the power add - on device 100 is attached to the manual wheelchair 250 , the front anti - tip castor wheel 107 can be lifted several inches off the ground when encountering obstacles ( preferably , as much as three inches ). a notable design feature of the present invention is that the latches 103 are not tightly clamped down on the camber tube ; instead , the latches 103 rather surround the camber tube , allowing the camber tube to rotate slightly as the manual wheelchair 250 tilts to allow the front wheels of the manual wheelchair to lift of the ground when encountering obstacles or when the user wishes to recline . preferably , the power add - on device 100 includes built - in armrests 109 on which the remote control 108 is mounted which operates the device . preferably , the armrests 109 are mounted to the housing 102 using brackets 111 , as shown . referring to fig3 , the framing structure 400 of the power add - on device 100 is shown . as depicted in fig4 , the framing structure 400 includes body frame 401 , motor mount 402 ( for securely holding the electric motors ), coil - over shock absorber 403 , rear anti - tip castor wheel 106 and front anti - tip castor wheel 107 ( to prevent tipping , as discussed above ), combination electric actuator 404 and latches 103 ( to open the latching system and release the manual wheelchair 201 , as discussed above ), armrest mounts 111 ( to secure the pair of armrests 109 ), and combination electric actuator 406 / swing arm 408 ( to allow the swing arm 408 to pivot upwardly , thereby raising the rear wheels of the manual wheelchair 250 off the ground and transferring the weight of the user from the rear wheels of the manual wheelchair through the shock - absorbed swing arm to provide traction to the power add - on &# 39 ; s drive wheels ). however , even when the rear wheels of the manual wheelchair 250 are positioned on the ground , relatively level terrain may still be traversed easily because the powered drive wheels 104 are lined up with the wheels of the manual wheelchair 250 . in general , the higher the rear wheels of the manual wheelchair 250 are raised , the larger the obstacles that can be traversed . an additional benefit of this design is that it enables the user the advantage of additional height and reach capabilities for different everyday tasks . fig4 shows an alternate and preferred latching mechanism wherein the user backs up to an upper clamshell 201 of the latch 203 and then employs the remote control 108 to activate an electric actuator to close the latch 204 by lifting a lower clamshell 204 , thereby securing the camber tube of the manual wheelchair to the power add - on device 100 . as illustrated , the upper clamshell 201 and a bottom clamshell 204 are attached by at least one hinge . as shown in fig4 , the latch 203 is in an open position . fig5 illustrates the latch 203 in a closed position . fig6 illustrates that the front castor wheel 107 can be lifted off the ground by this mechanism as well to provide extra clearance of obstacles . fig7 illustrates the framing structure 400 of the power add - on device 100 with the alternate preferred latching mechanism discussed above . as depicted in fig7 , the electric actuator 404 is coupled to a pivot member 405 , and the pivot member 405 is coupled to the coil - over shock absorber 403 . in operation , as shown in fig8 , when the electric actuator 404 is activated , the electric actuator 404 applies force to the pivot member 405 , and the pivot member 405 changes the direction of the force to upwardly apply the force to the coil - over shock absorber 403 . as shown , the coil - over shock absorber 403 is coupled to the bottom clamshell 204 , and the coil - over shock absorber 403 pushes the bottom clamshell 204 so as to close the latch 204 . fig9 shows a schematic view of the electrical components for the power add - on device 100 . as depicted in fig5 , the electrical system includes power source 501 , motors 502 , brakes 503 , linear actuators 504 , 505 , controller 506 , and charger 507 . the power source 501 comprises energy storage via batteries with charging and current limiting elements . the batteries are electrically connected in series , as shown , and provide all power for all functions . this configuration of battery power allows for use of standard batteries while providing the total output voltage needed for proper operation of the motors 502 , actuators 503 , 504 , and brakes 503 . since it is possible that either the motors 502 or the actuators 503 , 504 can experience states of operation , such as short circuiting during a failure mode , which draw excessive power from the batteries , the batteries are each protected with current limiting elements . these current limiting elements comprise left battery fuse 515 , right battery fuse 516 , and circuit breaker 518 . the circuit - opening characteristics of these current limiting elements preferably are selected based on allowing the circuit breaker first open - circuit followed by the fuses open - circuiting as the total current sourced from the batteries exceed the rated current discharge rate of the batteries . the motors 502 are preferably direct current motors , sized preferably for propelling the manual wheelchair 201 and an adult user up at least a twenty degree grade . similarly , the brakes are preferably direct - current - activated at the voltage of the two batteries when connected in series . the actuator 504 is preferably a direct - current - powered actuator sized and preferably mounted under the frame so as to raise the swing arm 408 , and thus lifting the rear of the manual wheelchair 201 . the actuator 505 is preferably a direct - current - powered actuator sized and preferably mounted so as to operate the latches 103 , thereby releasing the wheelchair 201 . the controller 506 preferably includes wired or wireless remote actuator switches attached to a joystick / controller . alternately , preferably , the actuator switches can be built into the joystick / controller . in either preferred configuration the actuator switches allow the operator to control the raising of the swing arm 408 ( thus lifting the rear of the manual wheelchair 201 ), and operating the release lever ( opening the latches 103 and releasing the manual chair 201 ), as shown . some embodiments of the power add - on device can be configured to minimize , dampen , and / or slow the rotation of the power add - on device relative to a manual chair coupled with the power add - on device . for example , with reference to fig1 and 11 , in some embodiments , the power add - on device 600 can be configured to have a rotation limiting or dampening element or mechanism 602 ( also referred to herein as an anti - tilt mechanism or a rotation resistance element ) configured to minimize , dampen , and / or slow rotation between the power add - on device 600 and a manual wheelchair coupled with the power add - on device . note that the manual wheelchair is not illustrated in either of fig1 or 11 . as discussed above , at least some of the power add - on device embodiments disclosed herein can be coupled with a camber tube or similar component of the wheelchair , for example , by engaging the camber tube of the wheelchair with the engagement mechanism ( which can be a clamshell engagement mechanism , latch mechanism , or other similar or suitable mechanism ). engaging the camber tube of the wheelchair can permit a desirable amount of rotation of the manual chair relative to the power add - on device . such relative rotation can be beneficial , for example , for traversing changes in the slope or height of the ground surface . for example , when the power add - on device and wheelchair advance up or down a ramp , whether an upward sloping ramp or downward sloping ramp , there will likely be a point when the drive wheels of the power add - on device are at a different height or level than the front wheels of the wheelchair . without permitting the rotation of the wheelchair relative to the power add - on device , such mismatch in the height of the drive wheels of the power add - on device relative to the front and / or the rear wheels of wheelchair can result in decreased stability and control of the wheelchair , and increased stress on the connection between the power add - on device and the wheelchair . permitting such relative rotation therefore increases the stability , comfort , and overall performance of the power add - on device and wheelchair ( i . e ., the powered wheelchair system ) and the ability of the powered wheelchair system to traverse uneven ground surfaces , bumps , and other changes in the ground surface . however , there are also advantages to softening or resisting the amount of rotation between the power add - on device and the wheelchair , as too much relative rotation or quick rotation between the power add - on device and the wheelchair can affect the comfort level of the user , the control of the power add - on device , and the control of the remote control , such as the remote control 108 . for example , in embodiments where the armrests 109 are secured to the power add - on device , and where the remote control is secured to the armrests , such as armrests 109 , rapid or sudden , undampened rotation between the power add - on device and the wheelchair can cause relative rotation and movement between a user seated in the wheelchair and the armrests and remote control . this can reduce the comfort level of the user and the level of control the user has over the remote control . in some embodiments , softening or dampening the amount of rotation between the power add - on device and the wheelchair can be achieved with the rotation resisting element 602 that can engage a rear facing surface of the wheelchair and prevent the power add - on device from over - tipping or over - tilting in a forward direction relative to the wheelchair and / or prevent the wheelchair from over - tipping or over - tilting in a rearward direction relative to the power add - on device . with reference to fig1 , some embodiments of the rotation resistance element 602 can have a bracket assembly 606 and roller member 610 secured to a distal end portion 606 a of the bracket assembly . as will be discussed , the roller member 610 can contact a rear portion of the wheelchair and provide at least one of the contact points between the rotation resistance element 602 and the wheelchair . the roller member 610 can rotate in response to the contact between the wheelchair and the rotation resistance element 602 , as the wheelchair rotates relative to the rotation resistance element and / or the rotation resistance element rotates relative to the wheelchair . in some embodiments , the bracket assembly 606 can have a first bracket element 612 and a second bracket element 614 . the first bracket element 612 can support the roller member 610 . as will be discussed , the first and second bracket elements 612 , 614 can be fastened to one another and / or to the power add - on device using any suitable fastening mechanisms , including without limitation threaded fasteners such as bolts and nuts . for example , a fastener can be used to fasten the first bracket member 612 to the second bracket member 614 through opening 616 . one example of a fastener that can be used to attach the first bracket member 612 to the second bracket member 614 through opening 616 is the threaded fastener 618 shown in fig1 . a similar threaded fastener 618 can be used to fasten the first end 620 a of the spring member 620 to the first bracket member 612 , as illustrated in fig1 . similarly , similar fasteners such as threaded fasteners 619 can be used to secure the bracket assembly 606 to the power add - on device , as shown in fig1 . with reference to fig1 , some embodiments of the rotation resistance element can have a spring mechanism 620 , such as but not limited to a gas spring component , secured to the bracket assembly 606 . for example and without limitation , a first end portion of the spring mechanism 620 a can be secured to the first bracket member 612 , and a second end portion of the spring mechanism 620 b can be secured to the second bracket mechanism 614 . openings 622 can be used to secure the bracket assembly 606 to a frame portion or other support member on the power add - on device . any embodiments of the power add - on device disclosed herein can have one or more rotation resistance elements 602 to resist the relative rotation of the power add - on device relative to the wheelchair . in some embodiments , as in the embodiments illustrated in fig1 , 11 , and 16 , inter alia , the power add - on device can have two rotation resistance elements 602 , one on each lateral side of the power add - on device . additionally , though not required , the first rotation resistance element 602 a and the second rotation resistance element 602 b can be interconnected with a lateral support member 630 that can be , but is not required to be , secured to the bracket assembly 606 of each of the first rotation resistance element 602 a and the second rotation resistance element 602 b . as shown in fig1 , some embodiments of the power add - on device can be configured such that the rotation resistance element 602 can be aligned with a frame portion of the wheelchair , designated with “ w ” in fig1 . for example , the rotation resistance element 602 can be positioned such that a centerline 634 of the roller member 610 can be approximately aligned with a centerline 634 of a vertical tube member of the wheelchair . in some embodiments , the roller member 610 can be aligned with any suitable member or surface on the wheelchair , whether it be the frame or otherwise . fig2 shows a side view of a portion of the rotation resistance element 602 in a first or relaxed position relative to a wheelchair . fig2 shows a side view of a portion of the rotation resistance element 602 in a second or rotated position relative to a wheelchair . as can be seen with reference to fig2 and 21 , rotating the wheelchair 610 in the direction designated by arrow a 1 relative to the power add - on device can cause the first bracket 612 of the rotation resistance element 602 to rotate in the direction a 1 about the fastener or axle positioned in opening 616 relative to the second bracket 614 . as a result , the second bracket 612 can compress the spring member 620 , which can be configured to resist the rotation of the first bracket member 612 about the fastener or axle positioned in opening 616 . in this arrangement , the rotation resistance element 602 can resist and slow the rotation of the wheelchair relative to the power add - on device 600 . any of the features of the power add - on device 600 and / or the rotation resistance element 602 can be used with any of the other features or components of any of the other power add - on device embodiments disclosed herein . while this invention has been described in conjunction with the various exemplary embodiments outlined above , it is evident that many alternatives , modifications and variations will be apparent to those skilled in the art . accordingly , the exemplary embodiments of the invention , as set forth above , are intended to be illustrative , not limiting . various changes may be made without departing from the spirit and scope of the invention .