The Bees fell behind to a Jose Angel goal but levelled shortly after through Japhet Sery Larsen. Angel got his second of the game just moments later and the Spanish side added another in the second period through Jovanovic.
Gustav Mogensen then pulled another goal back for The Bees but they were unable to gain another to make it a draw in an entertaining affair.
Neil MacFarlane made two changes to the side that started Tuesday’s 4-0 win over Albacete Under-19s as Fredrik Hammar made his first appearance since suffering injury, and there was also a start for Dru Yearwood.
It was the Spanish side who would start the brighter as they maintained possession well in the opening stages with a great deal of experience amongst their starting XI.
But for much of the first quarter of an hour chances were few and far between with both sides forced to make important blocks from wide crosses.
In fact, it was the first real clear-cut opportunity created that led to the first goal, and it fell to the Spaniards. After good work down the left, the ball was pulled back to Angel who calmly swept an effort home.
But The Bees managed to wrestle back some control of the game as they began to see more of the ball. A David Titov cross was well blocked for a corner which then led to a Brentford leveller as birthday boy Jaakko Oksanen delivered excellently for Larsen to thump home a header from close range to make it 1-1.
Having gained more of a foothold in the game, The Bees were then disappointed to fall behind once again as just a minute later Angel was on hand to claim his second, again after good play down the FC Cartagena left.
Speculative shots from Joe Adams and Hammar then followed but for a lack of accuracy and a good Cartagena block as the Spanish side led the way at the break.
Cartagena then made significant changes to their side for the second half, but they continued to dominate possession, despite the first chance after the break falling to Hammar but his low effort was stopped by Martinez in the opposition goal.
Chances were then hard to come by as the match ticked past the hour mark, as Neil MacFarlane brought on the likes of Arthur Read, Julien Carré, Aubrel Koutsimouka and Kane O’Connor.
But it would be the Spanish side who would get the next goal when Fornies did well down the left before slotting the ball back for the fellow substitute Jovanovic who had a simple finish into the net to make it 3-1.
The Bees were still battling to get back into the game though, and they gave themselves a real chance of that when Adams tricked his way into the box before having his shot blocked into the path of Mogensen, who slotted in from close range.
More half chances fell for The Bees, as Arthur Read dribbled his way into the box but was unable to get a clean shot away at goal as the match finished 3-2.
Brentford B: Shepperd, Tsaroulla, Yearwood (Read, 64), Larsen (O’Connor, 64), Racic (Koutsimouka, 64), Adams, Hammar (Mitchell, 78), Majka (Carré, 64) Mogensen, Titov, Oksanen Subs not used: AnderssonBooked: Titov, Larsen, Adams
FC Cartagena: Esteve, Andújar, Fucile, Ayala, Verza, Caballero, Jara, Elady, José Ángel, De Vega, IsmaelSubs: Martinez, Forniés, Cordero, Quim Araujo, Rodrigo, Carrillo, Martín, Jovanovic, Carlos, Uri, WilliamBooked: Elady