General Manager A.J. Smith has had an extremely busy offseason. The tally for Chargers free agent signings grew to 19 with the addition of QB Kevin O’Connell on Monday. While Smith’s hand may be cramping from inking contracts, many of the new faces including O’Connell will likely fall short of making the final 53-man roster. A few have the potential to make a significant impact.

The Best Free-Agent Additions:

OLB Jarret Johnson: Signed on March 14 from the Ravens. Johnson, 6-3, 260, is excellent at stopping outside runs. The Chargers defense was last in the league in 2011 in third down efficiency by allowing opponents to convert 49.2 percent of their opportunities. If Johnson can snuff out edge rushes, then the defense should do a better job of getting off the field.

FB Le’Ron McClain: Signed on March 14 from the Chiefs. San Diego is counting on Ryan Mathews to develop into a near every down back this season. Coach Norv Turner has even said Mathews is capable of leading the league in rushing. For that to happen, Mathews will need holes to run through. McClain, 6-0, 260, should be able to do the job while also touching the ball on occasion.

WR Robert Meachem: Signed on March 15 from the Saints. Meachem, 6-2, 215, has the potential to be a 1,000 yard receiver, according to coach Turner. Meachem’s career season high is 722 receiving yards in 2009. In 2011, he had 620 yards on just 40 receptions. This year Meachem will be afforded every opportunity to succeed with the departure of WR Vincent Jackson (Buccaneers). If Meachem is unable to rise to the occasion, then WR Vincent Brown (round three, 2011) may be the one who has a breakout season.

The Rest of the Pack:

RB Jackie Battle: Signed on July 20 from the Chiefs. Battle, 6-2, 240, is likely competing against Curtis Brinkley for the third RB spot. Battle excels on special teams and that may give him an edge over Brinkley.

SS Atari Bigby: Signed on March 20 from the Seahawks. Bigby, 5-11, 211, has the potential to make a huge impact if he can win the starting job over Brandon Taylor (round three, 73rd overall), but Bigby has to stay healthy and he has already missed practices.

QB Kyle Boller: Signed on July 27 from the Raiders. Boller retired after one practice.

RB Ronnie Brown: Signed on June 11 from the Eagles. Brown, 6-0, 230, will likely be the second-string RB and should provide more options to coach Turner’s already dynamic playbook. The Chargers may run the wildcat a little more frequently this season because of Brown

DT Aubrayo Franklin: Signed on July 20 from the Saints. Franklin, 6-1, 315, was brought in to provide competition for Cam Thomas (round five, 2010). Thomas, 6-4, 335, will probably stick around, but Aubrayo has seen snaps with the first-team.

C-G Rex Hadnot: Signed on April 30 from the Cardinals. Hadnot, 6-2, 310, was brought in to provide competition for C David Molk (round seven, 226th overall pick). Molk won the 2011 Rimington Trophy for best collegiate center and will likely make the final cut. Neither is expected to start.

OT Mario Henderson: Signed on April 13. Henderson, 6-7, 335, last played in the league for the Raiders on 2010, but has a realistic shot at making the team because of injuries to G-T Brandyn Dombrowski (foot, cyst) and LT Jared Gaither (cramps).

SS Corey Lynch: Signed on May 29 from the Buccaneers. Lynch, 6-0, 210, is a long shot to make the team. It would probably take a significant injury to Atari Bigby or Brandon Taylor for him to stick around.

QB Kevin O’Connell: Signed on July 30 from the Jets. O’Connell, 6-5, 225, is perceived to be a two week rental until second-string QB Charlie Whitehurst recovers from a sore MCL.

WR-KR Roscoe Parrish: Signed on April 11 from the Bills. The best shot Parrish, 5-9, 175, has at making the roster is if coach Turner decides to carry six WRs. WR Richard Goodman will likely retain the KR duties as the fifth receiver. Parrish is also competing against Micheal Spurlock.

TE Dante Rosario: Signed on March 19 from the Broncos. The stiffest competition is at the TE spot. Rosario, 6-4, 240, has seen a lot of reps and will likely make the team to provide depth behind Antonio Gates (foot) who appears to be healthy for the first time in two seasons.

WR-KR Eddie Royal: Signed on March 16 from the Broncos. Royal, 5-10, 185, could have easily been included at the top of this list. The Chargers are looking for him to make big contributions as a slot receiver.

WR-KR Micheal Spurlock: Signed on April 4 from the Buccaneers. Spurlock, 5-11, 210, is in the same boat as Parrish, but Parrish appears to currently have the edge over Spurlock.

QB Charlie Whitehurst: Signed on March 20 from the Seahawks. Whitehurst, 6-5, 226, will be the second-string QB, San Diego hopes he never plays.

OLB Demorrio Williams: Signed on March 8 from the Chiefs. Williams, 6-1, 235, provides excellent depth for the LB corps. He has the potential to make a huge impact in a defense that will frequently rotate LBs.

Follow Chargers reporter Dan McLellan on Twitter @CBSSportsNFLSD and@SanDiegoSports.