The battery pack includes a first battery, a second battery and {HEAD ~ a means} to allow switching between the first battery and {TAIL ~ the second battery} for powering the cell phone 
to allow switching between
The battery pack includes a first battery , a second battery and { HEAD ~ a means } to allow switching between the first battery and { TAIL ~ the second battery } for powering the cell phone
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH