{HEAD ~ The power source} may be interchanged between an AC powered sealed unit and {TAIL ~ a DC powered battery pack} 
interchanged between
{ HEAD ~ The power source } may be interchanged between an AC powered sealed unit and { TAIL ~ a DC powered battery pack }
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL