Borrowdale, 24, joins on emergency terms after the Addicks suffered injuries to Kelly Youga, Grant Basey and loanee Johnnie Jackson.
BBC: Charlton Athletic snap up QPR's Gary Borrowdale on loan
2.
But, despite Grant Basey driving in a third just after half-time and veteran Chris Powell adding a fourth, Coventry survived thanks to results elsewhere.
BBC: Charlton 4-1 Coventry
3.
Three minutes after the interval Basey struck after another Schmeichel error, his mis-cued punch falling to Basey who slotted the ball home with ease.